Reactogenicity and adverse events were generally milder and less frequent in clinical trial 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).

Safety and reactogenicity studies of booster doses are based on small-scale clinical trials and post-licensure data with limited follow-up. Overall, they show a similar safety profile to that observed after the second dose in the primary series.