Safety and Immunogenicity of mRNA-1010, an Investigational Seasonal Influenza Vaccine, in Healthy Adults: Final Results From a Phase 1/2 Randomized Trial

Background. Seasonal influenza remains a global public health concern. A messenger RNA (mRNA)–based quadrivalent seasonal influenza vaccine, mRNA-1010, was investigated in a first-in-human, phase 1/2 clinical trial conducted in 3 parts. Methods. In parts 1 to 3 of this stratified observer-blind study, adults aged ≥18 years were randomly assigned to receive a single dose (6.25–200 µg) of mRNA-1010 or placebo (part 1) or an active comparator (Afluria; parts 2 and 3). Primary study objectives were assessment of safety, reactogenicity, and humoral immunogenicity of mRNA-1010, placebo (part 1), or active comparator (parts 2 and 3). Exploratory end points included assessment of cellular immunogenicity (part 1) and antigenic breadth against vaccine heterologous strains (A/H3N2; parts 1 and 2). Results. In all study parts, solicited adverse reactions were reported more frequently for mRNA-1010 than placebo or Afluria, and most were grade 1 or 2 in severity. No vaccine-related serious adverse events or deaths were reported. In parts 1 and 2, a single dose of mRNA-1010 (25–200 µg) elicited robust day 29 hemagglutination inhibition titers that persisted through 6 months. In part 3, lower doses of mRNA-1010 (6.25–25 µg) elicited day 29 hemagglutination inhibition titers that were higher or comparable to those of Afluria for influenza A strains. When compared with Afluria, mRNA-1010 (50 µg) elicited broader A/H3N2 antibody responses (part 2). mRNA-1010 induced greater T-cell responses than placebo at day 8 that were sustained or stronger at day 29 (part 1). Conclusions. Data support the continued development of mRNA-1010 as a seasonal influenza vaccine. Clinical Trials Registration. NCT04956575 (https://clinicaltrials.gov/study/NCT04956575).