22% of women show adverse reaction to Moderna vaccine, 4 times more than men

Yomiuri Shimbun file photo
A vaccine manufactured by U.S. pharmaceutical firm Moderna Inc.

Women are about four times likelier than men to experience adverse reactions, such as arm swelling, after receiving a vaccine against the novel coronavirus made by U.S. pharmaceutical company Moderna Inc., a team led by researchers from Self-Defense Force Central Hospital and others has found.

More than 20% of women showed the reactions, according to the findings, published in a medical journal.

The reactions, called Moderna Arm, appear about one week after the inoculation. However, what causes the reactions is not yet fully known.

The team analyzed responses to a questionnaire administered at the time of the second vaccination to 5,893 people who had received the first vaccination at a large-scale SDF-run vaccination center in Tokyo. The results showed that 22.4% of the women had Moderna Arm, while 5.1% of men had the adverse reactions. Women tended to have a later onset date and more persistent symptoms than men. It is possible that differences in immune response between men and women may have influenced the results.

By age group for both men and women, those under 30 had the lowest rate at 9.0%, while those in their 30s to 60s had relatively high rates. The rate for those aged 70 and older was 10.5%.

Tetsuo Nakayama, a specially appointed professor of clinical virology at Kitasato University, said, “Analysis of the components of the vaccine may reveal the cause of adverse reactions.”