Study of mRNA-1010 Seasonal Influenza Vaccine in Adults

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains. Antibody values reported as below lower limit of quantification (LLOQ) were replaced by 0.5\*LLOQ. Values greater than the upper limit of quantification (ULOQ) were converted to the ULOQ.

Time frame: Day 29

Population: The Per-Protocol Immunogenicity Set (PPIS) included all randomized participants who received the planned dose of study intervention, complied with the immunogenicity testing schedule for Baseline and Day 29, and had no significant protocol deviations that impacted key or critical data. Participants were analyzed according to the group to which they were randomized.

An MAAE was an AE that led to an unscheduled visit to a healthcare practitioner. An AESI was an AE (serious or nonserious) of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and immediate notification by the Investigator to the Sponsor was required. 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, was a congenital anomaly/birth defect, or was an important medical event. A summary of SAEs and all nonserious AEs (Other), regardless of causality, is located in the Reported Adverse Events section.

Time frame: Day 1 through Day 181

Population: Safety Set included all randomized participants who received any study intervention. Participants were included in the group corresponding to the study intervention that they actually received.

Solicited ARs, representative of vaccine reactogenicity, were collected in an electronic diary (eDiary). Local ARs: injection site pain, erythema (redness), swelling/induration (hardness); and axillary (underarm) swelling or tenderness ipsilateral to the side of injection. Systemic ARs: fever, headache, fatigue, myalgia, arthralgia, nausea/vomiting, and chills. Per prespecified analysis, only solicited ARs that were assessed and confirmed by the Investigator as both serious and related to IP were recorded as an adverse event. These adverse events were recorded as a serious AE (SAE) and are included in the summary of SAEs in the Reported Adverse Events section.

Time frame: Up to 7 days after study injection

Population: Solicited Safety Set included all randomized participants who received any study intervention and contributed any solicited AR data. Participants were included in the group corresponding to the study intervention that they actually received.

An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. Unsolicited AEs included any AE not collected as a solicited AR, and also included solicited ARs assessed and confirmed as both serious and related by the Investigator. A summary of SAEs and all nonserious AEs (Other), regardless of causality, is located in the Reported Adverse Events section.

Time frame: Up to 28 days after study injection

Population: Safety Set included all randomized participants who received any study intervention. Participants were included in the group corresponding to the study intervention that they actually received.

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains. Seroconversion rate was defined as the percentage of participants with either a Baseline HAI titer \<1:10 and a postbaseline titer ≥1:40 or a Baseline HAI titer ≥1:10 and a minimum 4-fold rise in postbaseline HAI antibody titer. Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were converted to the ULOQ.

Time frame: Day 29

Population: The PPIS included all randomized participants who received the planned dose of study intervention, complied with the immunogenicity testing schedule for Baseline and Day 29, and had no significant protocol deviations that impacted key or critical data. Participants were analyzed according to the group to which they were randomized.

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains. The GMFR measured the changes in immunogenicity titers or levels from Baseline within participants. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. Fold-rise was calculated by dividing post-vaccination results by the baseline value. 95% confidence interval (CI) for GMFR was calculated based on the t-distribution of the differences in the log-transformed values between analysis timepoint and baseline, then back transformed to the original scale for presentation.

Time frame: Baseline (Day 1) to Day 29

Population: The PPIS included all randomized participants who received the planned dose of study intervention, complied with the immunogenicity testing schedule for Baseline and Day 29, and had no significant protocol deviations that impacted key or critical data. Participants were analyzed according to the group to which they were randomized.

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains.

Time frame: Day 29

Population: The PPIS included all randomized participants who received the planned dose of study intervention, complied with the immunogenicity testing schedule for Baseline and Day 29, and had no significant protocol deviations that impacted key or critical data. Participants were analyzed according to the group to which they were randomized.

A death that occurred during the study or that came to the attention of the investigator during the study was reported to the Sponsor, whether or not it was considered related to study drug. The investigator assessed causality (that is, whether there is a reasonable possibility that the study drug caused the death). The relationship was characterized using the following classifications: Not related: There was not a reasonable possibility of a relationship to the study drug. The temporal sequence of the death relative to administration of the study drug was not reasonable AND/OR the death was more likely explained by a cause other than the study drug. Related: There was a reasonable possibility of a relationship to the study drug. There was evidence of exposure to the study drug. The temporal sequence of the death relative to the administration of the study drug was reasonable. The death was more likely explained by the study drug than by another cause.

Time frame: Day 1 through Day 181

Population: Randomization set included all participants who were randomly assigned to the study intervention, regardless of the participants' study intervention status in the study. Participants were analyzed according to the group to which they were randomized.