Reactogenicity and adverse events were generally milder and less frequent in participants in the older group (≥55 years of age) compared with the younger group (18-55 years of age) and tended to increase after the second dose for the mRNA vaccines. Reactogenicity was mostly mild to moderate and short-lived after dosing for both adult age groups (median onset was 0-2 days after either dose for a median duration of 1 - 2 days). The vaccine’s adverse events profile did not suggest any specific safety concerns. The median onset of systemic adverse events was 1-2 days after either dose for a median duration of 1 day. Severe adverse reactions occurred in 0.0% - 4.6% of participants. The incidence of serious adverse events, deaths, and discontinuations due to AEs were low and comparable for both the vaccine and placebo groups. There were no specific safety concerns identified in subgroup analyses by age, race, ethnicity, medical comorbidities, or prior SARS-CoV-2 infection.