Adverse events following COVID-19 virus vaccination in Japanese young population: The first cross-sectional study conducted by a questionnaire survey after the first-time-injection

A cross-sectional study was conducted to clarify the adverse events of COVID-19 vaccines in Japanese young population. The proportion of participants with adverse events at the vaccination site (immediately or within 30 minutes after vaccination) was 0.5%, and anaphylaxis occurred in one female student (0.03%). We analyzed 1,877 data obtained from a questionnaire survey of 1,993 vaccinated individuals. Eighty-two percent of participants complained of local adverse events. Injection site pain was the most common local adverse event (71%). Systemic adverse events occurred in 48% of participants. The most common adverse event was myalgia (34%). A multivariable logistic regression model was used to determine risk factors. Local adverse events were associated with sex (female) and allergy history, with odds ratios (ORs) (95% confidence interval [CI]) of 2.15 (1.69-2.73) and 1.73 (1.10-2.74), respectively. Systemic adverse events were associated with sex (female), age (<20 years old), allergy history, and history of adverse events with previous medications, with ORs (95% CI) of 2.49 (2.03-3.06), 1.80 (1.44-2.24), 1.39 (1.03-1.89), and 1.53 (1.02-2.29). The results of this study clarified for the first time that age less than 20 years is a risk factor for systemic adverse events from the COVID-19 Vaccine Moderna Intramuscular Injection. This information will give impacts on considering adverse events and its mechanisms in mRNA vaccination.