ABSTRACT
Mass vaccination against coronavirus disease 2019 (COVID-19) is ongoing in many countries worldwide. This study reports the occurrence of acute adverse events among vaccine recipients at a mass vaccination center in Japan. Between August and November 2021, approximately 130,000 individuals received two mRNA vaccine doses (mRNA-1273; Moderna) at the vaccination center. Acute adverse events at the site were observed in 1.1% of the recipients after the first dose and in 0.4% of the recipients after the second dose. The most common event was vasovagal syncope/presyncope, followed by acute allergic reactions. The occurrence rate of vasovagal syncope/presyncope was highest in the young population of those aged 16-29 years, but such age-dependency was not apparent in acute allergic reactions. Both symptoms were more prevalent in women than in men. Vasovagal syncope/presyncope occurred mainly within 20 min of the injection, whereas nearly half of the episodes of acute allergic reactions occurred after 20 min. The vaccine being injected while the recipient was in the supine position effectively reduced the occurrence of vasovagal syncope/presyncope. In summary, the suggested risk factors for vasovagal syncope/presyncope included a young age and female sex. The vaccine being injected while the recipient was in the supine position would reduce the risk of vasovagal syncope/presyncope.