Safety, Tolerability, and Immunogenicity of mRNA-1345 in Adults at Increased Risk for Respiratory Syncytial Virus Disease Aged 18-59 Years

Respiratory syncytial virus (RSV) is a significant health risk for adults aged 18-59 years with chronic medical conditions.

This ongoing, randomized, double-blind phase 3 trial evaluates safety and immunogenicity of the RSV vaccine, mRNA-1345, in adults aged 18-59 years at increased risk for RSV-associated lower respiratory tract disease (LRTD). Participants received a single 50-µg (licensed dose) or 30-µg dose. Co-primary immunogenicity objectives were to demonstrate noninferiority of day 29 RSV-A/B neutralizing antibody (nAb) geometric mean titers (GMTs) for the 50-μg dose compared with those observed in adults aged ≥60 years from the phase 3 pivotal efficacy trial. The other primary objective was to evaluate safety and tolerability.

A total of 999 participants received mRNA-1345 (50 µg, n = 502; 30 µg, n = 497). Most solicited adverse reactions (ARs) were mild to moderate with a median duration of 2 days. Day 29 nAb GMTs in the 50-µg group met noninferiority criteria: RSV-A GMT ratio (GMR), 1.2 (95% confidence interval [CI], 1.1-1.3); RSV-B GMR, 1.1 (95% CI, 1.0-1.2). Noninferiority was also demonstrated for seroresponse rate differences: RSV-A, 11.8% (95% CI, 7.8-15.5); RSV-B, 10.8% (95% CI, 5.9-15.6). Immune responses were consistent across subgroups and remained above baseline through day 181.

In adults aged 18-59 years at increased risk for RSV-LRTD, a 50-µg dose of mRNA-1345 was well tolerated and elicited RSV-A and RSV-B nAb responses noninferior to those observed in older adults in the pivotal study, supporting inference of efficacy in this population.

NCT06067230.