Safety and immunogenicity of an mRNA-based RSV vaccine in seropositive children aged 12–59 months

Respiratory syncytial virus (RSV) is a common respiratory pathogen in children. After demonstrating safety and immunogenicity in adults, mRNA-1345 encoding the RSV prefusion (preF) stabilized F glycoprotein was investigated in children. This phase 1, randomized, observer-blind study assessed the safety and immunogenicity of mRNA-1345 (NCT04528719) in RSV-seropositive children (per microneutralization assay) aged 12-59 months and is part of a larger study in multiple populations. Participants received three doses of mRNA-1345 (15 μg or 30 μg) or placebo 56 days apart, with 12-month safety follow-up. Forty-six participants were randomized to receive mRNA-1345 15 μg (<i>n</i> = 15), mRNA-1345 30 μg (<i>n</i> = 16), or placebo (<i>n</i> = 15). mRNA-1345 was well tolerated at both dose levels. The most frequently reported solicited local adverse reaction (AR) was injection site tenderness (15 μg, 27.3%-50.0%; 30 μg, 53.3%-71.4%; placebo, 26.7%-33.3%). Most solicited systemic ARs were grade 1/2, with irritability/crying, loss of appetite, and sleepiness most frequently reported; reactogenicity did not increase with additional doses. Throughout the 12-month follow-up, no serious adverse events (AEs), deaths, AEs of special interest, or AEs leading to study discontinuation were reported. Three medically attended RSV infections were reported among placebo recipients. A single injection increased RSV-A and RSV-B neutralizing antibody titers (geometric mean fold rise [GMFR] over baseline: 15 μg, RSV-A = 18.9, RSV-B = 7.2; 30 μg, RSV-A = 34.9, RSV-B = 14.3) and RSV preF and postF binding antibody concentrations (GMFR: 15 μg, preF = 13.9, postF = 9.3; 30 μg, preF = 26.5, postF = 16.0); binding antibody responses were preF-biased. Subsequent doses did not further increase antibody levels. In conclusion, mRNA-1345 was well tolerated and boosted antibody levels in seropositive children aged 12-59 months.