Acute Adverse Events at a Mass Vaccination Site after the Third and Fourth COVID-19 Vaccinations in Japan.

The third and fourth doses of the vaccine against coronavirus disease 2019 (COVID-19) were widely administered in Japan since December 2021. Currently, however, data are scarce regarding acute adverse events with the third and fourth doses. The present study reports the profiles of acute adverse events after the third and fourth COVID-19 vaccine doses, seen at the site of a mass vaccination center in Japan. Between December 2021 and July 2022, 267,515 individuals received the third, and 32,934 received the fourth COVID-19 vaccine dose at the mass vaccination center, of whom 442 recipients of the third (0.19%), and 22 recipients of the fourth (0.07%) dose reported acute adverse events and were examined by doctors on site. The most common diagnosis was vasovagal syncope/presyncope (incidence: 0.01-0.10%), followed by other miscellaneous complaints, acute allergic reactions (0.05-0.005%), and anaphylaxis (< 0.005%). Vasovagal syncope/presyncope occurred most frequently in recipients in those in their 20s, whereas acute allergic reactions were most frequent in those in their 40s. Both reactions were more frequent in women than men. The peak occurrence of vasovagal syncope/presyncope was earlier than 15 min after the injection, whereas that of acute allergic reaction was later than 15 min after the injection. The incidence of acute allergic reactions appeared to differ between various vaccine manufacturers, whereas that of vasovagal syncope/presyncope did not. These real-world data may benefit the safe and efficient implementation of mass vaccination campaigns for citizens who want to receive COVID-19 vaccines now and in the future.