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Background: A 2-dose mRNA-1273 primary series in children aged 6 months-5 years (25 µg) and 6-11 years (50 µg) had an acceptable safety profile and was immunogenic in the phase 2/3 KidCOVE study. We present data from KidCOVE participants who received an mRNA-1273 booster dose., Methods: An mRNA-1273 booster dose (10 µg for children aged 6 months-5 years; 25 µg for children aged 6-11 years; age groups based on participant age at enrollment) was administered ≥6 months after primary series completion. The primary safety objective was the safety and reactogenicity of an mRNA-1273 booster dose. The primary immunogenicity objective was to infer efficacy of an mRNA-1273 booster dose by establishing noninferiority of neutralizing antibody (nAb) responses after a booster in children versus nAb responses observed after the mRNA-1273 primary series in young adults (18-25 years) from the pivotal efficacy study. Data were collected from March 2022 to June 2023., Results: Overall, 153 (6 months-5 years) and 2519 (6-11 years) participants received an mRNA-1273 booster dose (median age at receipt of booster: 2 and 10 years, respectively). The booster dose safety profile was generally consistent with that of the primary series in children; no new safety concerns were identified. An mRNA-1273 booster dose elicited robust nAb responses against ancestral SARS-CoV-2 among children and met prespecified noninferiority success criteria versus responses observed after the primary series in young adults., Conclusions: Safety and immunogenicity data support administration of an mRNA-1273 booster dose in children aged 6 months to 11 years., Clinical Trials Registration: NCT04796896 (Clinicaltrials.gov)., Competing Interests: Potential conflicts of interest. M. B., X. Z., W. D., W. X., H. Z., B. G., R. K., E. J. A., R. D., J. M., and S. S. G. are employees of Moderna, Inc, and hold stock/stock options in the company. A. Y. and K. S. are consultants and were contracted by Moderna, Inc, for this study. V. B. has received institutional research support from Moderna, Inc, GSK-ViiV, Gilead Sciences, Inc, and Novavax, Inc. C. B. C. has consulted for Pfizer, Inc, Moderna, Inc, GSK plc, Vir Biotechnology, CommenseBio, Cowen Investment Management LLC, Sanofi S.A., and Guidepoint Global. C. A. Rostad has received institutional support from Moderna, Inc, BioFire, Inc, GSK plc, MedImmune, Micron Technology, Inc, Janssen Pharmaceuticals, Merck & Co, Inc, Novavax, PaxVax, Pfizer, Inc, Regeneron, Sanofi Pasteur, and from the National Institutes of Health (NIH). She is coinventor of patented respiratory syncytial (RSV) vaccine technology, which has been licensed to Meissa Vaccines, Inc. E. J. A. has consulted for Pfizer, Inc, Sanofi Pasteur, GSK plc, Janssen Pharmaceuticals, Moderna, Inc, and Medscape, and his institution received funds to conduct clinical research unrelated to this manuscript from MedImmune LLC, Regeneron Pharmaceuticals, Inc, PaxVax, Pfizer, Inc, GSK plc, Merck & Co, Inc, Sanofi Pasteur, Janssen Pharmaceuticals, and Micron Technology, Inc. E. J. A. served on a safety monitoring board for Kentucky BioProcessing, Inc, and Sanofi Pasteur. E. J. A. served on a data adjudication board for WCG and ACI Clinical and his institution received funding from the NIH to conduct clinical trials of COVID-19 vaccines. K. L. has consulted for Gilead Sciences, Inc, has received research funding from the NIH and Moderna, Inc, and has received funding for clinical research from Gilead Sciences, Inc, Moderna, Inc, and Pfizer, Inc. C. A. Rodriguez has received institutional research support from Moderna, Inc, Novavax, Inc, and Gilead Sciences, Inc. M. D. has received clinical trial funding from Moderna, Inc, and Gilead Sciences, Inc. W. T. has received institutional research support from Moderna, Inc, Pfizer, Inc, Janssen Pharmaceuticals, GSK plc, ViiV Healthcare Limited, and AstraZeneca plc. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)

Background: Respiratory syncytial virus (RSV) presents a global health concern. A lipid nanoparticle-encapsulated mRNA-based RSV vaccine (mRNA-1345) that encodes the membrane-anchored RSV prefusion-stabilized F glycoprotein is under clinical investigation., Methods: This phase 1 dose escalation study was based on a randomized, observer-blind, placebo-controlled design, and it assessed the safety and immunogenicity of mRNA-1345 in healthy adults aged 18 to 49 years. Participants were randomized to receive 1 dose of mRNA-1345 (50, 100, or 200 µg) or placebo or 3 doses of mRNA-1345 (100 µg) or placebo 56 days apart., Results: mRNA-1345 was well tolerated at all dose levels. The most common solicited adverse reactions were pain, headache, fatigue, myalgia, or chills, which were all generally mild to moderate. At 1 month postinjection, a single injection of mRNA-1345 boosted RSV neutralizing antibody titers (geometric mean fold rise: RSV-A, 20.0-23.5; RSV-B, 11.7-16.0) and RSV prefusion binding antibody concentrations (geometric mean fold rise, 16.1-21.8), with no apparent dose response. Antibody levels remained above baseline through 6 months. Sequential doses of 100 µg were well tolerated but did not further boost antibody levels., Conclusions: A single mRNA-1345 injection demonstrated an acceptable safety profile in younger adults and induced a durable neutralizing antibody response, supporting its continued development., Clinical Trials Registration: ClinicalTrials.gov NCT04528719., Competing Interests: Potential conflicts of interest. C. A. S., R. M., A. K., R. D., L. W., L. E. A., S. S.-G., K. S., and S. K. S. are employees of Moderna, Inc, and may hold stock/stock options in the company. A. A. and G. L. C. were Moderna, Inc, employees at the time of study conduct. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)