Solicited ARs (local and systemic) were collected in the electronic diary (eDiary). Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. Note, not all solicited ARs were considered adverse events (AEs). The Investigator reviewed whether the solicited AR was also to be recorded as an AE. A Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section and presented by each dose group separately.

An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. A treatment-emergent AE (TEAE) was defined as any event not present before exposure to vaccine or any event already present that worsens in intensity or frequency after exposure. Any abnormal laboratory test result (hematology, clinical chemistry, or prothrombin time \[PT\]/partial thromboplastin time \[PTT\]) or other safety assessment (for example, electrocardiogram, radiological scan, vital sign measurement), including one that worsens from baseline and is considered clinically significant in the medical and scientific judgment of the Investigator. A summary of SAEs and all nonserious AEs ("Other") reported up to the end of the study, regardless of causality, is located in the "Reported Adverse Events" section and presented by each dose group separately.

An SAE was defined as any AE that resulted in death, is 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 leads to an unscheduled visit to an healthcare practitioner. This would include 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 (for example, urgent care, primary care physician). A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section and presented by each dose group separately.

Seasonal influenza A strains included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage.

Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage.

The GMFR measures 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% 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.

The GMFR measures 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% CI was calculated based on the difference in the log-transformed values for GMFR, then back transformed to the original scale for presentation.

Seroconversion at a participant level is defined as corresponding visit titer ≥ 4\*LLOQ (lower limit of quantification) if baseline is \< LLOQ or a 4-fold or greater rise if baseline is ≥ LLOQ in anti-HA antibodies. When LLOQ is 1:10, seroconversion is defined as a corresponding visit titer ≥ 1:40 if baseline is \< 1:10 or a 4-fold or greater rise if baseline is ≥ 1:10 in anti- HA antibodies measured by HAI assay. 95% CI was calculated using the Clopper-Pearson method.

≥ z-fold rise from baseline at participant level is defined as a ≥ z x LLOQ for participants with baseline antibody level \< LLOQ, or a z-times or higher antibody level ratio in participants with baseline antibody level ≥ LLOQ. 95% CI was calculated using the Clopper-Pearson method.