mRNA-1083

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria-lineage and Yamagata-lineage. Seroconversion was defined as a Day 29 postinjection level ≥1:40 if Baseline was \<1:10 or a 4-fold or greater rise if Baseline was ≥1:10 in anti-hemagglutinin (HA) antibodies measured by HAI assay. The PPIS included all randomized participants who received study intervention, complied with the timing of immunogenicity blood sample collections to have both Baseline and Day 29 assessments using a window of -7 to +14 days, had no major dosing error, had negative RT-PCR test for influenza and SARS-CoV-2 on Day 1, and had no major protocol deviations or conditions/medications that affected the immune response.

SARS-CoV-2 strain included Omicron XBB.1.5 antibody. Seroresponse was defined as a Day 29 postinjection level ≥4-fold rise if Baseline was ≥lower limit of quantification (LLOQ) or ≥4×LLOQ if Baseline value was \<LLOQ in the nAb values measured by PsVNA. LLOQ was 38 arbitrary unit (AU)/milliliter (mL). The PPIS included all randomized participants who received study intervention, complied with the timing of immunogenicity blood sample collections to have both Baseline and Day 29 assessments using a window of -7 to +14 days, had no major dosing error, had negative RT-PCR test for influenza and SARS-CoV-2 on Day 1, and had no major protocol deviations or conditions/medications that affected the immune response.

An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, was a congenital anomaly/birth defect, or was an important medical event. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. An MAAE is an AE that lead to an unscheduled visit to an healthcare practitioner. This included visits to a study site for unscheduled assessments (for example, abnormal laboratory follow-up, and/or COVID-19 and visits to healthcare practitioners external to the study site. Number of participants with SAEs, AESIs, MAAEs, and AEs leading to discontinuation up to the end of study (Day 181) are reported in this outcome measure. A Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.