Study Overview

The study will be conducted in 2 phases: Phase 2 and Phase 3. In the Phase 2 segment, up to 2,000 participants will be randomly assigned to receive a single injection of either mRNA-1345 vaccine at the selected dose or placebo in a 1:1 randomization ratio.

In the Phase 3 segment, approximately 35,000 participants will be randomly assigned to receive a single injection of either mRNA-1345 vaccine at the selected dose or placebo in a 1:1 randomization ratio.

Study Results

Overall, 35,541 participants were assigned to receive the mRNA-1345 vaccine (17,793 participants) or placebo (17,748). The median follow-up was 112 days (range, 1 to 379). The primary analyses were conducted when at least 50% of the anticipated RSV-associated lower respiratory tract disease cases had occurred. Vaccine efficacy was 83.7% (95.88% confidence interval [CI], 66.0 to 92.2) against RSV-associated lower respiratory tract disease with at least two signs or symptoms and 82.4% (96.36% CI, 34.8 to 95.3) against the disease with at least three signs or symptoms.

Vaccine efficacy was 68.4% (95% CI, 50.9 to 79.7) against RSV-associated acute respiratory disease. Protection was observed against both RSV subtypes (A and B) and was generally consistent across subgroups defined according to age and coexisting conditions.

Participants in the mRNA-1345 group had a higher incidence than those in the placebo group of solicited local adverse reactions (58.7% vs. 16.2%) and systemic adverse reactions (47.7% vs. 32.9%); most reactions were mild to moderate in severity and were transient.

Serious adverse events occurred in 2.8% of the participants in each trial group.