Women were 5.3 times more likely than men to develop a skin rash, part of what is commonly called ¡°Moderna arm,¡± after getting the COVID-19 vaccine produced by the U.S. pharmaceutical giant, a large-scale study in Japan by the Self-Defense Forces Central Hospital has found.

The swelling, pain and/or itchiness of the arm that received the vaccine, also called ¡°COVID arm¡± in the U.S. and other countries, was also more common among people in their 30s through 60s and less likely to occur in younger people, according to the study, published in JAMA Dermatology on Thursday.

Such skin reactions typically appear around seven days after a Moderna shot and last about four days, but the reason for their emergence is not well understood. While skin reactions do occur with vaccines for other conditions, they are much rarer.

Researchers led by Toshihide Higashino analyzed the answers of 5,893 people who showed up for a second shot of Moderna¡¯s vaccine at a mass vaccination center in Tokyo¡¯s Otemachi district between May and November 2021. The participants were asked about the first dose, also from the same vaccine-maker, that they received four to six weeks earlier.

The results found that 12.7% of the participants said they had developed a skin rash, known in medicine as a ¡°delayed large local reaction¡± (DLLR). But there was a stark difference by sex, with the condition affecting 22.4% of women compared with 5.1% of men.

Such skin rashes seem to be more common among people in Japan than those in other countries. In previous studies, 1.1% of female Moderna recipients in the U.S. reported DLLR, while in Germany it was 0.16% of the general population.

The SDF doctors said this might be because Japanese vaccine recipients were more aware of the skin symptoms, leading to more reporting than in other countries.

Japan¡¯s Moderna rollout started in May 2021, about six months after it began in the U.S., where the occurrence of COVID arm was widely reported both by medical experts and the media.

Women also had a slightly later onset and longer duration of the rash, the researchers said. Body weight as well as hormonal and environmental factors may partially explain the difference between the sexes, they said.

¡°We hope our research will help with the discovery of DLLR¡¯s mechanism, which is clinically different to other allergic reactions to vaccines seen before, and lead to the development of vaccines with fewer side effects,¡± the researchers said.