Human vaccines & immunotherapeuticsSep 15, 2025

Safety and immune response of an mRNA RSV vaccine in children aged 12-59 months with prior RSV exposure

mRNA Technology Weekly Brief ↗PubMed ↗DOI ↗OA ↗

Updated Jun 28, 2026

Abstract

Essence

In -seropositive children aged 12-59 months, was well tolerated and increased RSV antibody levels after one dose.

Evidence

This phase 1 randomized observer-blind trial assigned 46 seropositive children to mRNA-1345 15 microg, mRNA-1345 30 microg, or placebo with doses 56 days apart and 12 months of safety follow-up.

Caveat

The evidence is preliminary because the study was small, included only seropositive children, and measured immunogenicity rather than clinical protection, with later doses not further increasing antibody levels.

Simplified

Respiratory syncytial virus () is a common respiratory pathogen in children. After demonstrating safety and immunogenicity in adults, 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 ( = 15), mRNA-1345 30 μg ( = 16), or placebo ( = 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. n n n