Are adverse effects observed significantly higher following the second dose (or booster doses) as opposed to the first dose for the mRNA vaccines?

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.