Adverse events following COVID-19 vaccination in young Japanese people: A case-control study of the risk of systemic adverse events by a questionnaire survey
Authors: Suehiro Marie , Okubo Shinya , Nakajima Kensuke , Kanda Kosuke , Hayakawa Masanobu , Oiso Shigeru , Kabashima Tsutomu , Fujita Hideaki , Ando Yukio , Muro Takahiro 
Published: October 2021

What is known and objective

Racial differences in adverse events following COVID-19 vaccines have not been sufficiently studied. Here, we aimed to study the adverse events of Moderna’s intramuscular COVID-19 vaccine in young Japanese people.

Methods

A case-control study was conducted using a questionnaire survey. Risk factors were determined using a multivariable logistic regression model. We also compared the occurrence of systemic adverse events in three pairs (minor and adult; male and female; and occurrence and non-occurrence of adverse events after the first dose). Propensity matching was used to balance variables.

Results

We analysed 3,369 data points (1,877 after the first dose and 1,492 after the second dose) obtained from a questionnaire survey of 7,965 vaccinated individuals. Comparing the results of the first and second doses, the incidence of local adverse events did not change significantly; however, the incidence of systemic adverse events increased significantly (p < 0.001). Eighty-three percent of the participants complained of local adverse events, and 65% of participants complained of systemic adverse events. Anaphylaxis occurred in one female student (0.03%). Even when an adverse event occurred, most of the symptoms improved within 3 days. Female sex was associated with systemic adverse events after the first and second doses with odds ratios (ORs) (95% confidence interval, CI) of 2.49 (2.03–3.06), and 1.83 (1.28–2.61), respectively. Age (<20 years: minor) was associated with systemic adverse events after the first dose with an OR of 1.80 (1.44–2.24).

The results of the analysis of six cohorts that were created using propensity score matching showed that the incidence of systemic adverse events at the first dose in females was significantly higher than that in males, and that of minors was significantly higher than that of adults.

What is new and conclusion

The results of this study clarified, for the first time, the risk factors for several adverse events from the injection of Moderna’s intramuscular COVID-19 vaccine in young Japanese people.

This study suggests that women, minors who experienced adverse events after the first dose, those who experienced adverse events after the first dose, and those who had adverse events after the second dose, should be aware of adverse events.

What is known and objective

The COVID-19 crisis has been spreading worldwide, and the number of infected people is increasing due to the emergence of mutant strains. The COVID-19 vaccine is expected to be effective in preventing coronavirus infection and disease aggravation.

In Japan, priority groups, such as healthcare workers and the older adults (aged 65 and over), were the first to be vaccinated. Workplace vaccinations began on 21 June 2021 at universities and businesses; university employees and students have also been vaccinated.

In June 2021, in Japan, Moderna’s intramuscular injection of the COVID-19 vaccine was used for vaccination in places associated with students and corresponding teaching staff. However, as of September 2021, the Comirnaty intramuscular injection vaccine had been used for healthcare workers and elderly people ahead of other COVID-19 vaccines.

Adverse events of the Comirnaty intramuscular injection vaccine in healthcare workers and elderly people and that of Moderna’s intramuscular COVID-19 vaccine in the defence forces, who were prioritised for administration, are currently being studied. However, adverse events associated with the intramuscular injection of Moderna’s COVID-19 vaccine in young people have not yet been well studied in Japan. 1,2. In addition, racial differences in adverse events following coronavirus vaccines have not been sufficiently studied.

We have already reported adverse events associated with the first dose of Moderna’s intramuscular injection of its COVID-19 vaccine. In Japan, the intramuscular injection of Moderna’s COVID-19 vaccine will continue to be used in large-scale inoculation venues such as universities and workplaces.

Therefore, to clarify the adverse events following coronavirus vaccination in the young Japanese population, we conducted a questionnaire survey after the second dose for students, faculty, and staff who belong to an educational foundation, Kyushu Bunka Gakuen.