Safety and efficacy of the monoclonal antibody L9LS for malaria prevention in

# Safety and efficacy of the monoclonal antibody L9LS for malaria prevention in children exposed to perennial malaria transmission in Kenya: a randomised, double-blind, placebo-controlled, phase 2 trial *The Lancet 2026* Articles Safety and efficacy of the monoclonal antibody L9LS for malaria prevention in children exposed to perennial malaria transmission in Kenya: a randomised, double-blind, placebo- controlled, phase 2 trial Laura C Steinhardt*, Titus K Kwambai*, Martina Oneko, Eunice Ouma, Ruth Njoroge, Viviane Callier, Zonghui Hu, Julie R Gutman, Reuben Yego, Kephas Otieno, Kelvin Onoka, Lilian Otieno, Kennedy Oduol, Leonid Serebryannyy, Bob C Lin, Will Adams, Somia Hickman, Anne C Preston, Kevin Carlton, Michael Holdsworth, Yan Xiao, Feiko O ter Kuile, Wycliffe Odongo, Sean C Murphy, Tuan M Tran, Simon Kariuki, Peter D Crompton†, Robert A Seder†, and the Kenya Malaria mAb Trials Team‡ Summary Lancet 2026; 407: 1614–25 Background Malaria remains a major cause of mortality globally, especially among young children in sub-Saharan Africa. See Comment page 1575 The long-acting monoclonal antibody L9LS has shown high ecacy in preventing malaria in children aged 6–10 years exposed to seasonal transmission but remains untested in perennial transmission settings and younger children. We *Contributed equally as first authors assessed the safety, tolerability, and ecacy of L9LS in infants and children in a high perennial malaria transmission †Contributed equally as senior setting. authors ‡A full list of contributors to the Methods This double-blind, two-part, randomised, placebo-controlled, phase 2 trial was done in Siaya county in Kenya Malaria mAb Trials Team is western Kenya. In parts 1a and 1b, we tested the safety and tolerability of L9LS using an age de-escalation and dose in the appendix (pp 1–2) escalation approach and randomly assigned (3:1) cohorts of healthy children (three cohorts aged 5–10 years, Malaria Branch, National three cohorts aged 5–59 months, and two cohorts aged 5–71 months) to L9LS at doses of 5, 10, 20, 30, or 40 mg/kg Center for Emerging and Zoonotic Infectious Diseases, subcutaneously or to placebo (normal saline). In part 2, healthy children aged 5–59 months were randomly assigned US Centers for Disease Control (1:1:1) by use of centralised computer-generated lists to receive two doses of L9LS at 10–20 mg/kg at baseline and and Prevention, Atlanta, GA, month 6, one dose of L9LS at baseline and placebo at month 6, or placebo at both timepoints. Children were followed USA (L C Steinhardt PhD, up for 12 months with monthly clinic visits and blood smear collections. Primary safety outcomes were incidence and J R Gutman MD, W Odongo MSc); Malaria Branch, National severity of local and systemic solicited adverse events within 7 days of dosing and serious adverse events throughout Center for Emerging and follow-up. The primary ecacy endpoint was Plasmodium falciparum infection detected by blood smear over Zoonotic Infectious Diseases, 12 months. Primary analyses were done in the modified intention-to-treat population, consisting of all randomly US Centers for Disease Control assigned participants who received the study intervention. This trial is registered with ClinicalTrials.gov and Prevention, Kisumu, Kenya (T K Kwambai MD); Kenya (NCT05400655) and is complete. Medical Research Institute, Global Health Research Centre, Findings In parts 1a and 1b, 96 children were enrolled and randomly assigned between Oct 1, 2022, and Jan 16, 2024; Kisumu, Kenya (M Oneko MD, 72 participants were assigned to L9LS and 24 were assigned to placebo. In part 2, 324 children aged 5–59 months E Ouma MD, R Njoroge BPharm, R Yego MSc, K Otieno MSc, were enrolled and randomly assigned between Jan 26 and June 2, 2023; 108 children were assigned to one-dose L9LS, K Onoka MSc, L Otieno BA, 106 to two-dose L9LS, and 110 to placebo. Across all study parts, grade 3 or worse treatment-related adverse events K Oduol BA, S Kariuki MD); occurred after four (1%) of 384 L9LS injections and two (1%) of 338 placebo injections; these events all resolved by Clinical Monitoring Research study end. The proportion of solicited and unsolicited adverse events was similar across all L9LS dose groups. There Program Directorate, Frederick National Laboratory, Frederick, were no serious adverse events related to the trial. In part 2, 70 (66%) of 106 children in the two-dose L9LS group had MD, USA (V Callier PhD); Office at least one P falciparum infection during the 12-month follow-up versus 91 (83%) of 110 children in the placebo group of Biostatistics Research, (protective ecacy 42·7%, 95% CI 22·5–57·7; p=0·0003). Division of Clinical Research, National Institute of Allergy and Infectious Diseases, Interpretation L9LS was protective against malaria in young children in western Kenya without evident safety National Institutes of Health, concerns over 6–12 months. A higher dose of L9LS might be needed to achieve high-level ecacy against malaria in Rockville, MD, USA (Z Hu PhD); young children exposed to intense perennial P falciparum transmission. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, Funding Gates Foundation. National Institutes of Health, Bethesda, MD, USA Copyright Published by Elsevier Ltd. (L Serebryannyy PhD, B C Lin BS, W Adams MPH, S Hickman PhD, K Carlton PhD, R A Seder MD); Introduction the burden of malaria between 2000 and 2015, progress Malaria Infection Biology and Malaria caused 282 million clinical cases and has stalled during the past decade, and eorts to reverse Immunity Section, Laboratory 610 000 deaths globally in 2024, with children younger this trend are threatened by the emergence and spread of of Immunogenetics, Division of Intramural Research, than 5 years accounting for approximately three-quarters insecticide-resistant mosquitoes2 and drug-resistant National Institute of Allergy of all malaria deaths.1 Despite substantial reductions in Plasmodium falciparum parasites.3 Although recently 1614 Articles and Infectious Diseases, Research in context National Institutes of Health, Rockville, MD, USA Evidence before this study 10–20 mg/kg, for prevention of malaria in children aged (A C Preston RN, We searched PubMed and the Cochrane Library without 5–59 months over 12 months of follow-up. The trial took place P D Crompton MD); Office of language restrictions from Jan 1, 2017, when the first in western Kenya, a site of intense, perennial malaria Cyber Infrastructure and Computational Biology, antimalaria monoclonal antibodies were in initial clinical transmission. L9LS was safe and well tolerated in children and National Institute of Allergy development, until Dec 1, 2025, using the search terms infants at all doses tested. Two doses of L9LS, given 6 months and Infectious Diseases, “monoclonal antibodies” or “monoclonal antibody” and apart, achieved 43% protective efficacy against infection National Institutes of Health, “malaria”. Three long-acting antimalaria monoclonal antibodies (48% against clinical malaria), and one dose provided Rockville, MD, USA (M Holdsworth, Y Xiao BS); for mediating protection that are specific for the 46% protection over 6 months (48% against clinical malaria). Department of Clinical Plasmodium falciparum circumsporozoite protein—namely, Pharmacokinetics of L9LS showed the expected dose-linear Sciences, Liverpool School of CIS43LS, L9LS, and MAM01—are currently in clinical relation. Additionally, at the 5 mg/kg dose, L9LS serum Tropical Medicine, Liverpool, development. These monoclonal antibodies were isolated from concentrations decreased to lower levels by 3 months in UK (Prof F O ter Kuile MD); Malaria Molecular Diagnostic blood samples of study participants immunised with either a Kenyan children when compared with US adults, a finding Laboratory, Department of radiation-attenuated whole sporozoite vaccine (CIS43LS and previously observed with other monoclonal antibodies, such as Laboratory Medicine and L9LS) or the RTS,S/AS01 vaccine (MAM01) and engineered to nirsevimab for respiratory syncytial virus prevention. Pathology, and the Center for extend their half-lives. CIS43LS and L9LS were shown to be safe Emerging and Re-emerging Implications of all the available evidence Infectious Diseases, University and efficacious against controlled human malaria infection in Building on evidence from phase 1 and 2 studies in the USA and of Washington, Seattle, WA, the USA and natural infection in endemic settings. In Mali, Mali, findings from this trial suggest that L9LS is safe and USA (S C Murphy MD); Division where intense P falciparum transmission occurs during the of Infectious Diseases, efficacious in infants and young children, paving the way for 6-month rainy season, a phase 2 study in healthy adults Department of Medicine and future implementation in this age group that is particularly Ryan White Center for Pediatric showed that a single intravenous dose of CIS43LS was safe and vulnerable to malaria. Although protection against infection Infectious Disease and Global 88% efficacious against infection over 6 months at a dose of Health, Department of was significant (for both one dose at 6 months and two doses 40 mg/kg and 75% efficacious at a dose of 10 mg/kg, compared Pediatrics, Indiana University at 12 months), efficacy was approximately 35% lower than that with placebo. Another phase 2 study at the same site in Mali in School of Medicine, seen in older children in Mali (aged 6–10 years) who received Indianapolis, IN, USA children aged 6–10 years showed that a single subcutaneous L9LS subcutaneously at a similar dose range. Age-dependent (T M Tran MD) dose of L9LS was safe and 77% efficacious against clinical differences in L9LS pharmacokinetics, potential differences in Correspondence to: malaria over 6 months at a dose of 10–20 mg/kg and malaria transmission between Mali and Kenya, and population- Dr Laura C Steinhardt, Malaria 67% efficacious at a dose of 5–10 mg/kg, compared with Branch, National Center for specific immunological profiles, along with other factors, might placebo. Efficacy of MAM01 against controlled human malaria Emerging and Zoonotic explain the lower efficacy in young children in western Kenya. Infectious Diseases, US Centers infection was assessed in US adults (NCT05891236), and its Because the previous studies have shown dose-dependent for Disease Control and safety and pharmacokinetics are being assessed in an ongoing protection by CIS43LS and L9LS, higher dosing might Prevention, Atlanta, GA 30329, phase 1b trial in adults and children in Uganda USA contribute to achieving optimal efficacy, especially in young (NCT06408857). However, to date, no published studies have LSteinhardt@cdc.gov children and infants in perennial transmission sites. On the assessed the safety and efficacy of antimalaria monoclonal or basis of these findings, planned and continuing studies antibodies in children younger than 6 years, who are most Dr Peter D Crompton, Malaria (NCT06461026) will evaluate a dose of about 30 mg/kg of Infection Biology and Immunity vulnerable to severe malaria, or in a perennial transmission L9LS across different clinical use cases, which include an Section, Laboratory of setting. ongoing study in vulnerable children discharged from hospital Immunogenetics, Division of Intramural Research, National Added value of this study with severe malaria or severe anaemia in Kenya Institute of Allergy and This two-part, placebo-controlled, randomised, phase 2 clinical (NCT07082205). Additional studies are needed to understand Infectious Diseases, National trial tested the safety of a range of L9LS doses (5 mg/kg potential differences in pharmacokinetics of L9LS in infants and Institutes of Health, Rockville, bodyweight, 10 mg/kg, 20 mg/kg, 30 mg/kg, and 40 mg/kg) young children and to elucidate other possible factors affecting MD 20852, USA pcrompton@niaid.nih.gov administered subcutaneously to children aged 5–59 months protective efficacy of antimalarial monoclonal antibodies in See Online for appendix and 5–10 years, and the safety and efficacy of one or two doses sites of intense, perennial transmission. of L9LS, administered 6 months apart at a dose of approved malaria vaccines oer important advances, directly neutralise sporozoites, the infectious form of the they require multiple doses to generate and maintain malaria parasite that mosquitoes inject into the skin and protection, and their ecacy varies with the age of blood, preventing malaria infection before liver-stage recipient. Thus, there is an urgent need for development.4–6 Unlike vaccines, which require the host complementary, long-acting interventions that provide to induce an eective immune response after multiple immediate, high-level protection to populations of any immunisations, monoclonal antibodies can provide age who are at high risk of malaria for defined periods. immediate malaria prevention with a single Antimalaria monoclonal antibodies represent a administration for up to 6 months.7,8 As a result, potentially transformative new intervention that can monoclonal antibodies could be particularly valuable as a Articles potential replacement for seasonal malaria de-escalation, dose escalation safety and tolerability chemoprevention in children, which requires monthly evaluation of L9LS administered subcutaneously at a dosing; for protecting infants before they become eligible single dose of 5, 10, or 20 mg/kg in children aged for multidose malaria vaccines at 5–6 months of age; and 5–10 years followed by children aged 5–59 months. In for other high-risk groups such as immunocompromised part 2, we recruited additional children to assess ecacy individuals, pregnant women, and children discharged of a single dose or two doses of L9LS at 10–20 mg/kg after hospital admission for severe anaemia or severe administered 6 months apart in children aged malaria, who have an increased risk of fatal reinfection.9 5–59 months during 12 months of follow-up. The dose Three antimalaria monoclonal antibodies (MAM01, range for this trial was based on data from previous CIS43LS, and L9LS) targeting dierent regions of the phase 1 and 2 trials. In a phase 1 trial, 15 (88%) of P falciparum circumsporozoite protein, the most 17 malaria-naive, healthy adults who received L9LS either abundant antigen expressed on sporozoites, are in intravenously or subcutaneously at a dose of 1 mg/kg, clinical development.10–12 Phase 1 and 2 field trials in 5 mg/kg, or 20 mg/kg were protected against P falciparum Africa have shown that CIS43LS and L9LS are safe and infection after controlled human malaria infection.12 In a highly ecacious against P falciparum infection. In a phase 2 trial in children aged 6–10 years in Mali, ecacy highly seasonal transmission setting in Mali, CIS43LS of L9LS against clinical malaria at doses of 150 mg or provided 75% ecacy against infection at a dose of 300 mg subcutaneously (weight-based dose range 10 mg/kg and 88% ecacy at a dose of 40 mg/kg when 5–20 mg/kg) was 67% and 77%, respectively, over administered intravenously to Malian adults,7 while L9LS 6 months of follow-up; exploratory analyses suggested showed 70% ecacy against infection and 77% ecacy that approximately 12 mg/kg of L9LS corresponded to against clinical malaria following a single 300 mg dose protective ecacy of approximately 75% over 6 months.8 administered subcutaneously to Malian children aged On the basis of these clinical data, a dose of 10–20 mg/kg 6–10 years.8 Because L9LS demonstrated superior was chosen for the part 2 assessment of ecacy against potency compared with CIS43LS in mouse models and malaria over 6 months. Part 1b was added through a showed more favourable characteristics for product protocol amendment to evaluate the safety, tolerability, development,13 it was prioritised for further clinical and pharmacokinetics of 30 mg/kg and 40 mg/kg of development and evaluation in additional settings and L9LS in children aged 5–71 months to inform future age groups. trials that would potentially assess ecacy endpoints up This trial was designed to address several questions in to 12 months of follow-up with a single dose. the development of monoclonal antibodies for key at-risk This study took place in Siaya county in western Kenya, populations: first, whether monoclonal antibodies can which has a high entomological inoculation rate maintain ecacy in areas of intense perennial estimated in 2019 to be 16–30 infectious bites per person transmission, where continuous P falciparum exposure per month.14 Previous studies in the area have shown an might overwhelm protection or potentially accelerate incidence of two to five episodes of clinical malaria per monoclonal antibody clearance; second, whether child per year in children aged 5–17 months15 and incident protective ecacy can be achieved in infants and young parasitaemia of 66·2% in children aged 5–12 months children—the population bearing the highest malaria during 6 months of follow-up.16 burden; and third, whether repeated monoclonal antibody The study protocol was approved by the institutional dosing can extend protection to 12 months. To address review boards of the Kenya Medical Research Institute these crucial knowledge gaps, this phase 2 trial assessed (Scientific and Ethics Research Unit; number 4413), the the safety of L9LS in children aged 5 months to 10 years, US Centers for Disease Control and Prevention, and the and the 12-month ecacy of one or two doses of L9LS Liverpool School of Tropical Medicine, with regulatory given subcutaneously to infants and young children aged review by the Kenya Pharmacy and Poisons Board 5–59 months in western Kenya, where intense perennial (ECCT/22/05/03). A data and safety monitoring board P falciparum transmission provides a stringent test of reviewed the trial protocol, consent documents, and monoclonal antibody ecacy. adverse event summaries, and conducted an interim safety review after day 7 data were available for all Methods participants in part 1a before recommending progression Study design to part 2, and then twice yearly thereafter. The study We did a double-blind, two-part, randomised, placebo- design was also shared with community advisory boards controlled, phase 2 trial in children aged 5 months to for their input. The full study protocol is available in the 10 years to assess the safety and ecacy of the antimalarial appendix (pp 64–193). This study is registered with monoclonal antibody L9LS in an area of western Kenya ClinicalTrials.gov (NCT05400655) and is complete. that has intense perennial malaria transmission with seasonal peaks during the long rains (April–July) and Participants short rains (November–December). The trial comprised Healthy children were recruited from the catchment three sequential components. In part 1a, we did an age area of Siaya County Referral Hospital, the main tertiary 1616 Articles hospital in Siaya county, western Kenya. For part 1a, masked to treatment assignment until database lock in three cohorts of 12 children each in the older age group January, 2025. (aged 5–10 years at the time of dosing) and three cohorts of 12 children each in the younger age group (aged Procedures 5–59 months) were randomly assigned to receive L9LS L9LS is a human IgG1 monoclonal antibody produced in (at doses of 5, 10, or 20 mg/kg) or placebo (normal a recombinant Chinese hamster ovary cell line.12,13 The saline) in a dose escalating, age de-escalating design Vaccine Production Program (Vaccine Research Center, (appendix p 16), with subsequent cohorts enrolled after National Institute of Allergy and Infectious Diseases confirming that no concerning safety signals, including [NIAID], National Institutes of Health [NIH], Bethesda, abnormal laboratory results, had occurred within 7 days MD, USA) developed the manufacturing processes for after dosing in the previous cohort(s). For part 1b, L9LS and then transferred them to the Vaccine Clinical two cohorts of 12 children aged 5–71 months were Materials Program for Good Manufacturing Practice- randomly assigned to 30 mg/kg L9LS, 40 mg/kg L9LS, compliant production. L9LS was supplied at a or placebo. For part 2, children aged 5–59 months were concentration of 150 mg/mL, with 2·2 mL per vial. recruited from the catchment areas of Siaya County Clinicians assessed participant eligibility through Referral Hospital and Kogelo Dispensary, a satellite medical history, physical examination, anthropometry, clinic located 17 km away. and laboratory assessments for complete blood counts, Eligibility criteria for all study parts included HIV- liver and kidney function, haemoglobin typing, and negative status, absence of sickle cell disease, no wasting HIV testing. All eligible participants received or stunting, no killed or live vaccine within 14 days dihydroartemisinin–piperaquine to clear any baseline (changed from 28 days in a protocol amendment in parasitaemia. Participants returned 2–3 weeks after October, 2023) before study agent administration, no dihydroartemisinin–piperaquine treatment for receipt of malaria vaccine, and normal blood counts and randomisation and dosing of L9LS or placebo. chemistry values. A full list of inclusion and exclusion In parts 1a and 1b, dosing was based on bodyweight, with criteria is in the appendix (pp 2–3). Parents or guardians up to 1·5 mL per injection for doses of 5 mg and 10 mg per provided written informed consent for their child’s kg of bodyweight and up to 2 mL for the higher doses. For participation. volumes exceeding these limits, L9LS was divided equally between two injections, up to a maximum of 2 mL in each Randomisation and masking of two syringes. In part 2, bodyweight-tiered dosing was In parts 1a and 1b, within each age-dose cohort, used: 75 mg of L9LS (0·5 mL) for participants weighing participants were randomly assigned (3:1) to L9LS or 5·0 kg to 7·5 kg, 150 mg (1 mL) for participants weighing placebo by use of permuted block randomisation in R more than 7·5 kg to 15·0 kg, and 225 mg (1·5 mL) for with block sizes of four. In part 2, randomisation was those weighing more than 15·0 kg to 22·5 kg, the upper stratified according to participant age, so that there was weight limit in part 2, resulting in an overall dose range of one cohort for children aged 5–17 months and one cohort 10–20 mg/kg. This bodyweight-tiered dosing strategy was for children aged 18–59 months. Within each age stratum used to model an easy-to-deploy approach while enabling and site, participants were randomly assigned (1:1:1) to pharmacokinetic and pharmacodynamic analysis across a two doses of L9LS (L9LS at baseline and 6 months), dosing range to better define the protective dose. All one dose of L9LS (L9LS at baseline and placebo at subcutaneous injections were administered in the 6 months), or placebo at baseline and 6 months, by use of posterior upper arm, although the protocol also allowed for permuted block randomisation with block sizes subcutaneous injections in the abdomen and inner thigh of six and nine. area. An unmasked trial statistician (VC) provided After dosing (day 0), study field workers visited computer-generated randomisation lists that were participants at home on day 1 and day 3 to inspect shared only with unmasked pharmacy sta, who injection sites and collect solicited adverse event data (see sequentially assigned eligible participants at their dosing appendix pp 6–8 for more details on assessment of visit. All other study sta, participants, and parents or solicited adverse events). Participants returned to the guardians remained masked to study assignment clinic on day 7 for a physical examination, adverse event throughout the study. Randomisation lists were secured assessment, and safety laboratory tests (complete blood in a locked pharmacy cabinet. To maintain blinding, count, liver, and kidney function). Participants in parts 1a syringes were covered with transparent yellow tape to and 1b were followed up on days 14, 21, 28, 56, and 84. mask the yellowish tint of L9LS versus the colourless For part 2, active surveillance included monthly clinic placebo. Additionally, since L9LS is more viscous than visits from day 28 until day 364. Home visits occurred normal saline, dedicated nurses administered study 2 weeks after each scheduled monthly clinic visit to product separately from the masked team conducting assess health status and facilitate clinic referral when follow-up assessments. All study sta, apart from the indicated. At 6 months, participants received their second unmasked statistician and pharmacy team, remained dose of L9LS or placebo, with home visits 1 day and Articles 3 days later and a clinic visit on day 7, and monthly visits presented here included assessment of IgG1 allotypes and thereafter. allotype-specific eects on L9LS pharmacokinetics, Parents or guardians were encouraged to seek care for whether L9LS ecacy is specific to certain parasite any illness between scheduled visits, with transport genotypes, eect of pre-existing circumsporozoite protein reimbursement provided. Study visits in parts 1a and 1b antibodies on ecacy and pharmacokinetics of L9LS, occurred at Siaya County Referral Hospital and those in eect of L9LS on measles antibodies in participants in the part 2 either at Siaya County Referral Hospital or Kogelo 5–17-month age group (analyses ongoing), eect of L9LS Dispensary. on hospital admissions with malaria, and ecacy of Blood was collected, primarily by fingerprick, at all one versus two doses of L9LS over 12 months of follow-up scheduled clinic visits and, when indicated, at unscheduled (insucient statistical power). sick visits for blood smears and dried blood spots. Blood smears were read by two independent microscopists Statistical analysis (appendix p 9) within 48 h (or within 1 h for symptomatic For part 2, the sample size was calculated to detect a participants), with a third microscopist adjudicating target 60% protective ecacy of two doses of L9LS discordant results. Participants with any parasitaemia, against infection at 12 months in the younger age group regardless of symptoms, were treated with an antimalarial (5–17 months) with 80% power, assuming a 45% infection drug (typically artemether–lumefantrine) within 72 h. rate in the placebo group by 12 months, and 25% attrition, Venous blood samples (up to 4·5 mL) for yielding 324 participants (162 per age stratum). pharmacokinetic and antidrug antibody analyses were The primary analyses for safety and ecacy were done collected at baseline (2–3 weeks before L9LS or placebo in the modified intention-to-treat population, consisting dosing), on day 7, and either day 28 or day 84 (parts 1a of all randomly assigned participants who received the and 1b), or days 28, 196, and 336 (part 2). Samples were study intervention. A sensitivity per-protocol analysis centrifuged, aliquoted, and stored at –80°C before included participants who received both doses as shipment to the Vaccine Research Center (NIAID, NIH). randomly assigned, attended the close-out visit, and Dried blood spots were similarly stored and shipped to the missed fewer than two consecutive clinic visits. University of Washington (Seattle, WA, USA) for highly The primary ecacy analysis examined the incidence sensitive P falciparum 18S rRNA quantitative RT-PCR of P falciparum infection from 7 days post-initial dosing (qRT-PCR) analysis.17 Serum L9LS concentrations were (estimated peak L9LS concentration) up to 52 weeks. The measured as described elsewhere.8 Antidrug antibody protective ecacy was estimated using time-to-first- detection methods are detailed in the appendix (pp 9–10). infection analysis, and protective ecacy was defined as (1–hazard ratio) × 100% and estimated through a Cox Outcomes proportional hazards model accounting for interval Primary safety outcomes were incidence and severity of censoring (icenReg R package). Secondary analyses local and systemic solicited adverse events within 7 days included proportion infected using Kaplan–Meier of dosing and serious adverse events throughout follow- estimates (1–relative risk of infection) with melding up. Unsolicited adverse events were collected throughout method CIs17 and recurrent event analysis using the the study in parts 1a and 1b and up to 28 days after each Anderson–Gill method,18 as well as analyses that excluded dose in part 2; thereafter, they were recorded only if participant time when protected by antimalarial grade 3 or worse, or if deemed related to study product. treatment. No multiplicity adjustments were applied to The primary ecacy endpoint for part 2 was asexual the analyses. Post-hoc analyses included comparison of P falciparum infection detected by blood smear over proportions of serious adverse events in L9LS and 52 weeks of follow-up, comparing the two-dose L9LS group placebo groups, assessment of the eect of seasonality with the placebo group. Secondary ecacy endpoints on L9LS ecacy by including a covariate for month of included clinical malaria, defined as either: (1) parasitaemia enrolment in the Cox regression model, and comparisons of more than 5000 parasites per µL with axillary temperature of L9LS concentrations to protective ecacy. Participant at least 37·5°C; or (2) any parasitaemia with either data were entered directly into an electronic database temperature at least 37·5°C or history of fever within the (appendix p 10) during study visits. R statistical software past 24 h. The second definition aligns with Kenyan malaria (version 4.5.0) was used for all analyses. treatment guidelines and is emphasised here. Additional secondary endpoints included ecacy against Role of the funding source parasitaemia detected by qRT-PCR;17 ecacy of a single The funder of the study had no role in study design, data dose of L9LS versus placebo at 3, 6, and 12 months and by collection, data analysis, data interpretation, or writing of age stratum; and L9LS pharmacokinetics, overall and in the report. relation to P falciparum infection risk. Exploratory analyses included assessment of antidrug antibodies to L9LS and Results examination of whether pre-existing parasitaemia From September to October, 2022, 156 children had modified ecacy. Additional exploratory endpoints not consent given for part 1a, of whom 72 (46%) were enrolled 1618 Articles and randomly assigned from Oct 1 to Nov 7, 2022; (5–17 months and 18–59 months). 108 children were 79 children had consent given for part 1b in assigned to one dose of L9LS (L9LS at baseline and December, 2023, of whom 24 (30%) were enrolled and placebo at 6 months), 106 were assigned to two doses of randomly assigned from Jan 3 to 16, 2024 (appendix L9LS (L9LS at baseline and 6 months), and 110 were p 16). In total, 72 participants were assigned to L9LS and assigned to placebo at baseline and 6 months. 24 were assigned to placebo in part 1. In part 2, 152 (47%) of 324 participants were male, For part 2, 679 children had consent given from January 172 (53%) were female, 199 (61%) were enrolled at Siaya to May, 2023, of whom 324 (48%) were enrolled and Hospital, and the remaining 125 (39%) were enrolled at randomly assigned between Jan 26 and June 2, 2023 Kogelo Dispensary (table 1). At baseline, 2–3 weeks (figure 1), with 162 children in each age stratum before L9LS or placebo dosing when eligible participants 679 children with consent 375 aged 5–17 months 304 aged 18–59 months 355 excluded 155 ineligible on screening* 86 consent withdrawn 51 non-compliant 37 post-screening ineligibility 13 study group complete 11 eligible but not enrolled 2 other† 324 enrolled and randomly assigned 162 aged 5–17 months 162 aged 18–59 months 108 assigned to one-dose L9LS 106 assigned to two-dose L9LS 110 assigned to placebo 54 aged 5–17 months 54 aged 5–17 months 54 aged 5–17 months 54 aged 18–59 months 52 aged 18–59 months 56 aged 18–59 months 108 received first dose of L9LS 106 received first dose of L9LS 110 received first dose of placebo 54 aged 5–17 months 54 aged 5–17 months 54 aged 5–17 months 54 aged 18–59 months 52 aged 18–59 months 56 aged 18–59 months 2 consent withdrawn 6 lost to follow-up 3 lost to follow-up 2 had second dose withheld‡ 1 out of study area 3 out of study area 1 lost to follow-up 1 met withdrawal criteria§ 2 met withdrawal criteria¶ 1 had second dose withheld‡ 103 received placebo 98 received second dose of L9LS 101 received second dose of placebo 51 aged 5–17 months 51 aged 5–17 months 49 aged 5–17 months 52 aged 18–59 months 47 aged 18–59 months 52 aged 18–59 months 3 missed ≥2 clinic visits 3 lost to follow-up 2 lost to follow-up 2 lost to follow-up 2 met withdrawal criteria|| 1 consent withdrawn 1 consent withdrawn 1 missed ≥2 clinic visits 97 completed follow-up 93 completed follow-up 97 completed follow-up 48 aged 5–17 months 49 aged 5–17 months 46 aged 5–17 months 49 aged 18–59 months 44 aged 18–59 months 51 aged 18–59 months Figure 1: Trial profile for efficacy cohort *Reasons for ineligibility included weight-for-age or height-for-age Z score less than –2 (n=68); underlying illness (n=34); abnormal laboratory test (n=23); receipt of vaccine within prohibited window (n=14); participating in another study (n=13); or planning to move out of study area (n=3). †Other reasons were difficult veins for venepuncture and participant due for measles vaccine. ‡Two participants (one in the placebo group and one in the one-dose L9LS group) developed epilepsy and one (in the one-dose L9LS group) developed patent ductus arteriosus; second dose withheld but these three participants continued to be monitored up to 12 months. §Child received L9LS within 28 days of (live) rotavirus vaccine. ¶Both children received RTS,S vaccine. ||Both chidren received RTS,S vaccine. Articles received dihydroartemisinin–piperaquine to clear after the second dose; among those receiving placebo, parasitaemia, 53 (16%) of 324 had a P falciparum-positive local reactions occurred in none of 110 participants after blood smear, and 145 (46%) of 315 had P falciparum the first dose, and in one (<1%) of 204 participants detected by qRT-PCR (table 1). All participants had receiving placebo for the second dose (table 2). Systemic negative blood smears on the day L9LS or placebo was events occurred in 20 (9%) of 214 participants after the administered. Baseline characteristics for participants in first L9LS dose and in 14 (14%) of 98 participants after the parts 1a and 1b are shown in the appendix (pp 14–15). second L9LS dose; among those receiving placebo, In parts 1a and 1b, solicited local reactions were systemic events occurred in six (5%) of 110 participants infrequent: in part 1a, injection site swelling occurred in after the first dose, and in 22 (11%) of 204 participants three (11%) of 27 participants aged 5–10 years and in five receiving placebo for the second dose (table 2). These (19%) of 27 participants aged 5–59 months in the L9LS events consisted of pain, swelling, or reaction at the group (all dose groups), compared with one (11%) of injection site, and nausea, malaise, pyrexia, and headache, nine participants aged 5–10 years and none of all of which were mild or moderate apart from nine participants aged 5–59 months in the placebo three grade 3 events: one injection site induration starting group; in part 1b, injection site swelling occurred in 1 day after the second L9LS dose (resolved by day 3), and three (17%) of 18 participants in the L9LS group two episodes of pyrexia after the second L9LS dose compared with none of six in the placebo group (one at 39·6°C on day 3, resolving within 48 h; another (appendix pp 17–19). Mild injection site pain occurred in with concurrent varicella [chickenpox] that began the day two (7%) of 27 participants aged 5–10 years and three of dosing [maximum fever of 39·8°C 7 days after dosing] (11%) of 27 children aged 5–59 months assigned to L9LS and resolved 11 days later). In part 2, unsolicited adverse in part 1a compared with none assigned to placebo events occurred in 252 (78%) of 324 participants within (appendix pp 17–18). The only grade 3 (severe) local 28 days after the first dose of L9LS or placebo and in 246 reaction was injection site swelling in one child assigned (81%) of 302 participants after the second dose, with to placebo in part 1a (5–10-year age group). Solicited similar rates across study groups (table 2, appendix systemic events were predominantly mild, and included pp 31–35). Across all study parts (parts 1a, 1b, and part 2), chills, headache, and pyrexia, with one case each of grade 3 or worse treatment-related adverse events severe pyrexia and moderate nausea in the 40 mg/kg occurred after four (1%) of 384 L9LS injections and two L9LS group in part 1b. No dose-related trends emerged (1%) of 338 placebo injections; these events all resolved by for solicited or unsolicited adverse events (appendix study end. No deaths occurred in part 2, while 28 serious pp 17–28). One child in the placebo group (in the adverse events occurred, all deemed unrelated to L9LS in 30 mg/kg cohort of part 1b) died 23 days post-dosing masked analysis (appendix pp 29–30). A post-hoc analysis from severe dehydration following thermal burns of serious adverse events (most commonly severe malaria (appendix pp 29–30). [three events in participants assigned to L9LS vs six in In part 2, solicited adverse events were also uncommon: participants assigned to placebo] and pneumonia local reactions occurred in four (2%) of 214 participants [two vs one]) showed that the proportion of serious after the first L9LS dose and in three (3%) of 98 participants adverse events was significantly higher in the placebo 18–59 months of age* 5–17 months of age* One-dose L9LS Two-dose L9LS Placebo One-dose L9LS Two-dose L9LS Placebo (n=54) (n=52) (n=56) (n=54) (n=54) (n=54) Sex Female 29 (54%) 27 (52%) 27 (48%) 28 (52%) 34 (63%) 27 (50%) Male 25 (46%) 25 (48%) 29 (52%) 26 (48%) 20 (37%) 27 (50%) Age, months 37 (25–45) 34 (27–49) 31 (24–41) 13 (9–15) 10 (6–14) 13 (7–15) Study site Kogelo 22 (41%) 21 (40%) 24 (43%) 19 (35%) 19 (35%) 20 (37%) Siaya 32 (59%) 31 (60%) 32 (57%) 35 (65%) 35 (65%) 34 (63%) Weight at first dose, kg 13·6 (2·4) 14·1 (2·4) 13·2 (2·2) 9·3 (1·3) 8·9 (1·3) 9·2 (1·4) L9LS dose, mg/kg 12·4 (1·5) 12·3 (1·5) 12·6 (1·5) 15·4 (2·4) 15·6 (2·8) 14·9 (2·5) Positive blood smear at baseline† 11 (20%) 14 (27%) 12 (21%) 5 (10%) 9 (17%) 2 (4%) Positive qRT-PCR at baseline†‡ 26 (49%) 31 (61%) 32 (58%) 23 (45%) 19 (37%) 14 (26%) Data are n (%), median (IQR), or mean (SD). qRT-PCR=quantitative RT-PCR. *Age at time of dosing. †Baseline refers to the pre-enrolment visit for giving dihydroartemisinin– piperaquine for parasite clearance, 2–3 weeks before antibody administration. Positivity is only for Plasmodium falciparum. ‡qRT-PCR unable to be run on nine baseline samples: four in the one-dose L9LS group, three in the two-dose L9LS group, and two in the placebo group. Other variables have no missing data. Table 1: Characteristics of participants enrolled in efficacy cohort 1620 Articles group than in the L9LS dose groups combined (χ² 7·4, the 5–17-month and 18–59-month age groups (appendix p=0·0067). pp 37–38). At 6 months, after one dose of L9LS (both Of the 324 children enrolled in part 2, 253 (78%) had L9LS groups) or placebo, infections occurred in 99 at least one P falciparum infection during 52 weeks of (46%) of 214 children in the L9LS group and 71 (65%) of follow-up: 92 (85%) of 108 in the one-dose L9LS group, 110 in the placebo group (protective ecacy 45·9% 70 (66%) of 106 in the two-dose L9LS group, and 91 [95% CI 26·5–60·1]; p=0·0001; figure 3, appendix p 36), (83%) of 110 in the placebo group (figure 2 , appendix p 36). and ecacy was higher in the 18–59-month age group Estimated by time to first infection and accounting for (50·9% [25·3–67·7]) than in the 5–17-month age group interval censoring, the protective ecacy of two doses (39·0% [1·9–62·0]), although this dierence was not of L9LS against P falciparum infection by blood smear statistically significant (appendix p 38). One dose of at 12 months, the primary endpoint, was 42·7% (95% CI L9LS provided no statistically significant protection 22·5–57·7; p=0·0003; figures 2, 3), and was similar for against P falciparum infection at 12 months (protective One-dose L9LS Two-dose L9LS Placebo After dose 1 After dose 2 After dose 1 After dose 2 After dose 1 After dose 2 (n=108) (n=103) (n=106) (n=98) (n=110) (n=101) Solicited adverse events Participants with at least one solicited local adverse event within 1 (1%) 0 3 (3%) 3 (3%) 0 1 (1%) 7 days of dosing Participants with at least one solicited systemic adverse event 12 (11%) 10 (10%) 8 (8%) 14 (14%) 6 (5%) 12 (12%) within 7 days of dosing Local reactogenicity adverse events Injection site induration* Severe 0 0 0 1 (1%) 0 0 Injection site pain Mild 0 0 1 (1%) 0 0 0 Injection site reaction† Mild 0 0 1 (1%) 0 0 0 Injection site swelling Mild 1 (1%) 0 1 (1%) 2 (2%) 0 1 (1%) Systemic solicited adverse events Nausea Moderate 2 (2%) 2 (2%) 3 (3%) 3 (3%) 2 (2%) 2 (2%) Mild 3 (3%) 1 (1%) 2 (2%) 4 (4%) 1 (1%) 4 (4%) Malaise Mild 0 1 (1%) 0 0 0 0 Pyrexia Severe‡ 0 0 0 2 (2%) 0 0 Moderate 1 (1%) 2 (2%) 1 (1%) 2 (2%) 0 3 (3%) Mild 7 (6%) 5 (5%) 3 (3%) 5 (5%) 3 (3%) 5 (5%) Headache Mild 1 (1%) 0 1 (1%) 0 0 1 (1%) Unsolicited adverse events Participants with at least one unsolicited adverse event within 83 (77%) 84 (82%) 83 (78%) 81 (83%) 86 (78%) 81 (80%) 28 days of dosing Participants with at least one related unsolicited adverse event 6 (6%) 9 (9%) 1 (1%) 6 (6%) 5 (5%) 7 (7%) within 28 days of dosing All adverse events Participants with a grade 3 related adverse event (solicited or 0 0 0 3 (3%) 0 1 (1%) unsolicited) Participants with a serious adverse event§ 3 (3%) 4 (4%) 3 (3%) 1 (1%) 6 (5%) 10 (10%) No participants had local reactions of injection site tenderness, redness, pruritus, or bruising. No participants had systemic solicited events of muscle aches, chills, or joint pain. All local events were deemed related to the study. All systemic events were deemed related to the study, apart from one case of nausea in a participant in the placebo group after dose 2, who had concomitant malaria and bacterial infection. *Injection site induration began 1 day after dosing (maximum swelling of 30 mm) and resolved by day 3. †Injection site reaction was a muscle tension under the injection site that appeared within 1 h of dosing and resolved by day 3. ‡Severe pyrexia defined as axillary temperature from 39·5°C to 41·9°C. §No deaths occurred in part 2, and none of the serious adverse events were deemed related to the study. Table 2: Summary of adverse events in efficacy cohort Articles ecacy 20·0% [95% CI –9·8 to 41·7]; figure 3, appendix per person-year [2·9–4·5]), although the incidence of p 36). clinical malaria was similar during this time (appendix Protective ecacy of two doses of L9LS against clinical pp 39, 43). Ecacy of one dose of L9LS was 41·8% malaria definition 2 (ie, any parasitaemia with either (95% CI 21·4–56·9) against all P falciparum infections temperature at least 37·5°C or history of fever within the and 42·3% (20·1–58·4) against all episodes of clinical past 24 h) at 12 months was 48·3% (95% CI 27·4–63·1; malaria over 6 months by recurrent event analysis figures 3, 4), without substantial dierences by age group (appendix p 42). (appendix pp 38–39). One dose of L9LS provided modest Analyses that excluded periods of time when protection against clinical malaria definition 2 at participants were protected after antimalarial treatment, 12 months (28·4% [95% CI 3·1 to 47·1]), but not against as well as per-protocol analyses, yielded similar results to clinical malaria definition 1 (ie, parasitaemia the primary modified intention-to-treat analyses >5000 parasites per µL with axillary temperature at (appendix pp 44–46). Participants positive for P falciparum least 37·5°C; protective ecacy 20·6% [–10·5 to 43·0]; by blood smear or qRT-PCR at baseline before figure 3). At 3 months, one dose of L9LS had a protective dihydroartemisinin–piperaquine administration were ecacy of 51·6% (95% CI 28·9–67·1) against P falciparum more likely to have infection during follow-up, a finding infection and 55·2% (30·2–71·2) against clinical malaria that was consistent across all study groups and both age (definition 2), with no meaningful dierences by age groups (appendix pp 47–50). One dose of L9LS showed group (appendix p 40). slightly higher protective ecacy at 6 months in As expected, protective ecacy estimates from Kaplan– participants who were qRT-PCR-negative versus qRT- Meier proportional analyses were consistently lower than PCR-positive at baseline, while two doses of L9LS showed those from time-to-first event analyses (appendix p 41). similar ecacy at 12 months in participants who were Protective ecacy estimates for recurrent events were qRT-PCR-negative or qRT-PCR-positive at baseline higher than those from proportional analyses, but (appendix p 51). A post-hoc analysis that included the slightly lower than estimates from time-to-first-event participants’ month of enrolment as a regressor analyses. For example, at 12 months, ecacy of two doses (surrogate for potential transmission seasonality) did not of L9LS was 31·4% (95% CI 7·1–49·4) against all change the ecacy estimates significantly (appendix P falciparum infections and 41·0% (20·2–56·4) against p 52). Ecacy against infection detected by qRT-PCR is all clinical malaria episodes (definition 2; appendix p 42). not presented here because qRT-PCR analyses were not Participants in the younger cohort (age 5–17 months) in complete at the time of manuscript submission. the one-dose L9LS group had modestly higher L9LS showed dose-proportional pharmacokinetics in P falciparum infection rates during months 7–12 this trial, with maximum serum concentrations of (5·6 infections per person-year [95% CI 4·6–6·6]) 52·5 µg/mL at 5 mg/kg, 104·8 µg/mL at 10 mg/kg, compared with those in the placebo group (3·7 infections 197·0 µg/mL at 20 m/kg, 417·1 µg/mL at 30 mg/kg, and 542.0 µg/mL at 40 mg/kg (appendix pp 53–54). Notably, 100 despite the 40 mg/kg dose group having a higher observed maximum concentration (C ) at day 7, L9LS max 80 serum concentrations by 28 days were similar between the 30 mg/kg and 40 mg/kg dose groups (appendix pp 53), as L9LS serum concentrations decreased at significantly faster rates from observed C to day 28 or max day 84 for 40 mg/kg than in the lower dose groups, with 40 the exception of the 30 mg/kg dose group, which had the smallest sample size (appendix pp 55–56). In part 2, in participants who received a single dose of L9LS, median terminal half-life estimated by non-compartmental analysis using sparse data was 39·9 days (95% CI 0 39·3–40·1) in participants aged 5–17 months and 0 60 120 180 240 300 360 420 41·6 days (40·9–42·5) in those aged 18–59 months Time since administration (days) Number at risk (appendix pp 58–59). Placebo 110 74 44 35 25 19 5 ·· One-dose L9LS 108 89 69 52 37 22 3 ·· No antidrug antibodies were detected among children Two-dose L9LS 106 89 66 48 43 35 14 ·· in the L9LS groups at any timepoints in parts 1a and 1b or in part 2 (measured at baseline and days 28, 196, and 336 Figure 2: Kaplan–Meier curve of time to first Plasmodium falciparum infection as detected by blood smear, by treatment group in part 2), although one participant in the placebo group Data for all participants (ie, 5–17-month and 18–59-month age groups combined). Surveillance for infection by had functional antidrug antibodies detected at blood smear began 7 days after the first dose of L9LS or placebo and continued until the close-out visit. Shaded three timepoints after the first dose (appendix p 60), areas represent 95% CIs. Log-rank test p=0·0009 for all participants comparing the two-dose L9LS group with the suggesting a likely non-specific interference leading to a placebo group. Median time to first infection for all participants was 176 days (95% CI 141–222) in the one-dose L9LS group, 169 days (139–274) in the two-dose L9LS group, and 101 days (77–140) in the placebo group. false-positive result. 1622 )%( noitcefni fo ecnedicni evitalumuC Placebo One-dose L9LS Two-dose L9LS p=0·0009 Articles In part 2, bodyweights ranged from 6·5 kg to 12·2 kg 100 in children aged 5–17 months and from 9·3 kg to 20·4 kg in children aged 18–59 months, resulting in weight-based L9LS dose ranges during the first dose of 10·0–19·7 mg/kg (mean 15·5 mg/kg) for participants aged 5–17-months and 10·0–16·1 mg/kg (mean 12·3 mg/kg) for those aged 60 18–59 months (appendix pp 61–62). Post-hoc analyses showed positive relations between weight-based dosing 40 and protective ecacy across all trial endpoints (appendix p 63). Formal analyses correlating pharmacokinetics with ecacy are in progress. Discussion 0 0 60 120 180 240 300 360 420 This phase 2 trial provides evidence that subcutaneous Time since administration (days) administration of L9LS is safe, well tolerated, and Number at risk Placebo 110 81 54 42 26 21 6 ·· ecacious against P falciparum infection and clinical One-dose L9LS 108 94 76 59 43 26 4 ·· malaria in infants and young children in an area of Two-dose L9LS 106 95 75 57 51 40 15 ·· intense perennial P falciparum transmission in western Figure 4: Kaplan–Meier curve of time to first clinical malaria, by treatment group Kenya. L9LS doses up to 40 mg/kg and subcutaneous Data for all participants (ie, 5–17-month and 18–59-month age groups combined). Clinical malaria defined as injections up to 2 mL were well tolerated in children as blood smear-detected Plasmodium falciparum infection plus high measured temperature (≥37·5°C) or history of young as 5 months. Two doses of L9LS at 10–20 mg/kg, fever in past 24 h. Surveillance for clinical malaria began 7 days after the first dose of L9LS or placebo and continued until the close-out visit. Shaded areas represent 95% CIs. Log-rank test p=0·0001 comparing the administered 6 months apart, showed comparable safety two-dose L9LS group with the placebo group. Median time to first clinical malaria for all participants was 199 days profiles and did not elicit antidrug antibodies. One dose (95% CI 174–252) in the one-dose L9LS group, 259 days (175–336) in the two-dose L9LS group, and 121 days of L9LS at 10–20 mg/kg in children aged 5–59 months (98–177) in the placebo group. provided 46% ecacy against P falciparum infection detected by blood smear and 48% ecacy against clinical than the lower doses assessed in part 1a. Ongoing trials in malaria over 6 months, while two doses administered Mali and Kenya are investigating L9LS doses of 30 mg/kg 6 months apart provided 43% ecacy against infection and higher in infants and children (NCT06461026 and and 48% ecacy against clinical malaria over 12 months. NCT07082205) and adults (NCT07060508). While multitrial pharmacokinetic and pharmacodynamic The favourable safety and tolerability profile of L9LS, modelling of L9LS is underway, this trial suggests that with only four (2%) of 214 participants having local higher doses of L9LS might be needed to achieve more reactions and 20 (9%) of 214 having solicited systemic than 70% ecacy in young children, particularly in areas events after their first L9LS dose, aligns with findings of intense perennial transmission. In part 1b of this study, from previous L9LS trials across dierent age groups.8,12 30 mg/kg yielded a higher serum concentration of L9LS By comparison, phase 3 trials of WHO-recommended )%( noitcefni fo ecnedicni evitalumuC Events/participants (%) Protective efficacy (95% CI) p value L9LS Placebo 6 months Blood smear 99/214 (46%) 71/110 (65%) 45·9 (26·5 to 60·1) 0·0001 Clinical malaria definition 2 84/214 (39%) 64/110 (58%) 47·5 (26·5 to 62·5) 0·0002 Clinical malaria definition 1 61/214 (29%) 47/110 (43%) 43·8 (16·5 to 62·1) 0·0043 12 months Blood smear, one dose 92/108 (85%) 91/110 (83%) 20·0 (−9·8 to 41·7) 0·17 Clinical malaria definition 2, one dose 84/108 (78%) 90/110 (82%) 28·4 (3·1 to 47·1) 0·030 Clinical malaria definition 1, one dose 70/108 (65%) 74/110 (67%) 20·6 (−10·5 to 43·0) 0·17 Blood smear, two doses 70/106 (66%) 91/110 (83%) 42·7 (22·5 to 57·7) 0·0003 Clinical malaria definition 2, two doses 64/106 (60%) 90/110 (82%) 48·3 (27·4 to 63·1) 0·0001 Clinical malaria definition 1, two doses 57/106 (54%) 74/110 (67%) 36·7 (11·1 to 54·9) 0·0083 –25 0 25 50 75 Protective efficacy (95% CI) Figure 3: Protective efficacy of one or two doses of L9LS against Plasmodium falciparum infection and clinical malaria at 6 months and 12 months compared with placebo, by time-to-event analysis Protective efficacy calculated using Cox proportional hazards model ([1–hazard ratio] × 100%) accounting for interval censoring. Lines around protective efficacy point estimates represent 95% CIs. Clinical malaria defined as either: (1) parasitaemia of more than 5000 parasites per µL with axillary temperature at least 37·5°C; or (2) any parasitaemia with either temperature at least 37·5°C or history of fever within the past 24 h. Placebo One-dose L9LS Two-dose L9LS p=0·0001 Articles malaria vaccines found that up to 47% of participants had settings will further refine dosing regimens across age fever with R21/Matrix-M,19 and up to 33% had fever with groups. Third, the study was done within a single area RTS,S/AS01.20 This dierence in adverse events between where P falciparum transmission is intense and perennial. monoclonal antibodies and vaccines probably reflects It will be of interest in future studies to investigate L9LS in immune-stimulating adjuvants required for vaccine- areas of low and moderate perennial transmission. Fourth, induced immune responses. Indeed, widespread use of all participants received dihydroartemisinin–piperaquine other monoclonal antibodies, including nirsevimab for before L9LS or placebo administration to clear possible respiratory syncytial virus prevention, show similarly low P falciparum infection so that the primary ecacy endpoint adverse event rates in infants.21 of infection could be assessed. Consequently, this The ecacy of two doses of L9LS over 12 months, and trial could not assess whether pretreatment with one dose over 6 months in this study, was lower than the dihydroartemisinin–piperaquine aected ecacy 70% ecacy against P falciparum infection and estimates of L9LS. Ongoing (NCT06461026) and future 77% ecacy against clinical malaria over 6 months trials of L9LS that have clinical malaria as an endpoint and observed in Malian children aged 6–10 years when do not include pretreatment with an antimalarial drug 300 mg of L9LS (approximate weight-based dose range could provide insight into this question. Finally, this trial 10–20 mg/kg) was administered just before the 6-month included healthy children only. A continuing trial is malaria season.8 Seasonal transmission patterns might addressing this limitation by evaluating the safety and result in higher ecacy estimates for interventions like ecacy of L9LS in children with severe anaemia or severe monoclonal antibodies that have waning ecacy over malaria to prevent malaria after hospital discharge time when administered at the start of the malaria (NCT07082205). season, compared with ecacy estimates in perennial The results of this study support conducting further settings.22 However, during the first 3 months of this trials to assess the safety and ecacy of L9LS in infants trial, when L9LS concentrations remained well above the and children in various transmission settings and for limit of detection, ecacy was only 52% against dierent use cases. For example, in perennial P falciparum infection and 55% against clinical malaria. transmission settings, L9LS could be assessed when Furthermore, 81% of participants in the placebo group administered at routine immunisation visits at 6 weeks in Mali8 versus 63% in Kenya had at least one P falciparum or 10 weeks of age, potentially complementing existing infection over 6 months, suggesting comparable countermeasures (eg, long-lasting insecticidal nets) to transmission intensities at both sites. Ongoing genotype enhance protection of infants before completion of the analyses of P falciparum infections detected in this trial three-dose malaria vaccine primary series several months and the Mali trials will provide additional transmission later. Importantly, a trial in Mali is investigating whether intensity measures23 at the two sites and might also L9LS aects subsequent R21/Matrix-M immunogenicity determine whether polymorphisms in the P falciparum in infants (NCT06461026). To protect children exposed to circumsporozoite protein locus are associated with seasonal malaria transmission, a single annual dose of dierential L9LS ecacy. The lower ecacy of L9LS in L9LS could be compared with monthly seasonal malaria this trial might also reflect dierent L9LS chemoprevention—the current standard of care that is pharmacokinetics in Kenyan children 5–59 months old limited by the challenge of delivering frequent treatment compared with Malian children 6–10 years old. L9LS courses.29 Finally, a trial in Kenya is assessing the safety serum concentrations in the 5–59-month and 5–10-year and ecacy of a single dose of L9LS in children with age groups from part 1a showed more rapid decreases severe anaemia or severe malaria to prevent malaria after compared with those reported in US adults who received hospital discharge over 6 months compared with the 5 mg/kg L9LS in the phase 1 trial (appendix p 57).12 WHO-recommended three courses of monthly post- Previous studies of other monoclonal antibodies have discharge malaria chemoprevention (NCT07082205). shown shorter half-lives in infants and young children Larger trials in the future can also explore the eect of than in adults.24–26 Population-specific factors, including L9LS on severe and fatal malaria, as well as the potential Fc receptor polymorphisms27 and expression levels as for increased malaria risk after a period of L9LS-mediated well as dierences in baseline immunological states,28 protection.30 could influence L9LS pharmacokinetics and ecacy; In conclusion, this phase 2 trial represents the first these factors are being investigated in continuing L9LS evaluation in children younger than 5 years and in studies. the setting of intense perennial P falciparum transmission. This study has limitations. First, pharmacokinetic The results suggest that L9LS was well tolerated and sampling was sparse because of community concerns moderately protective against malaria in infants and about paediatric phlebotomy. Second, the narrow range of young children at the doses used, without evident safety L9LS doses in part 2 (due to bodyweight-tiered dosing) concerns. A higher dose of L9LS might be necessary to limited exploration of dose–ecacy relations. Ongoing achieve more than 70% ecacy over 6 months in infants pharmacokinetic and pharmacodynamic modelling with and young children when exposed to intense perennial data from multiple L9LS trials in various transmission transmission. In the longer term, higher ecacy could 1624 Articles also be achieved through ongoing research to enhance 11 Williams KL, Guerrero S, Flores-Garcia Y, et al. A candidate the potency and durability of L9LS.4 antibody drug for prevention of malaria. Nat Med 2024; 30: 117–29. 12 Wu RL, Idris AH, Berkowitz NM, et al, and the VRC 614 Study Contributors Team. Low-dose subcutaneous or intravenous monoclonal antibody RAS, PDC, TKK, JRG, LCS, ZH, WA, SH, KC, MO, and SK conceived to prevent malaria. N Engl J Med 2022; 387: 397–407. the study and drafted the initial protocol. MO, EO, RN, RY, SK, LO, KOn, 13 Wang LT, Pereira LS, Flores-Garcia Y, et al. A potent anti-malarial KOt, and KOd carried out the clinical trial. LCS, TKK, JRG, ACP, WA, human monoclonal antibody targets circumsporozoite protein SH, WO, FOtK, KC, PDC, and RAS provided oversight and technical minor repeats and neutralizes sporozoites in the liver. Immunity support. LS, KOt, BCL, and SCM conducted laboratory analyses, and 2020; 53: 733–44.e8. MH and YX provided database and data management support. LS, TKK, 14 The PMI VectorLink Project. Kenya annual entomological YX, ZH, and VC accessed and verified the data. ZH, VC, LCS, TKK, YX, monitoring report, October 2018–September 2019. https://stacks. and TMT conducted the statistical analyses. All coauthors read and cdc.gov/view/cdc/146397/cdc_146397_DS1.pdf (accessed approved the final version of the manuscript. Feb 23, 2026). 15 Samuels AM, Ansong D, Kariuki SK, et al, and the RTS,S study Declaration of interests group. Ecacy of RTS,S/AS01 malaria vaccine administered E SCM has a contract with NIAID. RAS has a patent issued for L9LS. according to dierent full, fractional, and delayed third or early All other authors declare no competing interests. fourth dose regimens in children aged 5–17 months in Ghana and Kenya: an open-label, phase 2b, randomised controlled trial. Data sharing Lancet Infect Dis 2022; 22: 1329–42. Anonymised data and the accompanying data dictionary will be deposited 16 Oneko M, Steinhardt LC, Yego R, et al. Safety, immunogenicity and in a secure, access-controlled institutional repository following ecacy of PfSPZ Vaccine against malaria in infants in western publication of the trial results. Access may be granted to researchers Kenya: a double-blind, randomized, placebo-controlled phase 2 trial. submitting methodologically sound proposals through the corresponding Nat Med 2021; 27: 1636–45. authors, subject to approval by the ethics committee (Scientific and 17 Skinner J, Kayentao K, Ongoiba A, et al. Anti-sporozoite Ethics Research Unit) and compliance with Kenyan data protection laws monoclonal antibody for malaria prevention: secondary ecacy and regulations. Data access will require a formal data use agreement. outcome of a phase 2 randomized trial. Nat Med 2025; 31: 2682–90. Acknowledgments 18 Andersen PK, Gill RD. Cox’s regression model for counting processes: a large sample study. Ann Stat 1982; 10: 1100–20. We would like to thank the study participants and their caregivers for their willingness to participate in this study; without their time and 19 Datoo MS, Dicko A, Tinto H, et al, and the R21/Matrix-M Phase 3 Trial Group. Safety and ecacy of malaria vaccine candidate support this trial would not have been possible. We also thank the R21/Matrix-M in African children: a multicentre, double-blind, members of the data and safety monitoring board who helped ensure randomised, phase 3 trial. Lancet 2024; 403: 533–44. this trial adhered to the highest safety and ethical standards. This trial 20 Agnandji ST, Lell B, Soulanoudjingar SS, et al, and the RTS,S was funded by a grant from the Gates Foundation (INV-035091). TMT is Clinical Trials Partnership. First results of phase 3 trial of RTS,S/ supported by the Gates Foundation (INV-088153). This research was AS01 malaria vaccine in African children. N Engl J Med 2011; supported (in part) by the Intramural Research Program of the NIH, 365: 1863–75. including federal funds from the National Cancer Institute, NIH, under 21 Drysdale SB, Cathie K, Flamein F, et al, and the HARMONIE Study contract number 75N91019D00024. Support was also provided by NIAID, Group. Nirsevimab for prevention of hospitalizations due to RSV in NIH grant number 1R01AI192628-0. The contributions of the NIH and infants. N Engl J Med 2023; 389: 2425–35. CDC author(s) are considered Works of the US Government. The 22 Macià D, Pons-Salort M, Moncunill G, Dobaño C. The eect of findings and conclusions presented in this paper are those of the disease transmission on time-aggregated treatment ecacy author(s) and do not necessarily reflect the views of the NIH, the CDC, estimates: a critical analysis of factors influencing the RTS,S and or the US Department of Health and Human Services. R21 malaria vaccine phase 3 trials. Lancet Infect Dis 2025; 25: e516–26. 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