Photo/Illutration An additional volume about post-infection symptoms of the health ministry’s treatment guidelines for COVID-19 (Asahi Shimbun file photo)

From 11.7 percent to 23.4 percent of adults infected with the novel coronavirus experienced “post-infection” symptoms, two to four times the rate for children, a health ministry research team said.

The post-infection rate was around 6.3 percent among children, the team’s survey found.

Both adults and children who had been vaccinated before infection had a lower rate of post-infection symptoms than those who were unvaccinated, according to the survey.

Most infected people see their symptoms disappear within days or weeks. But some suffer from certain post-infection symptoms over an extended period, including fatigue, malaise, joint pain, shortness of breath, decreased concentration and disturbances in smell and taste.

These symptoms have been referred to as “long COVID” or “post-COVID,” but the definition has not been firmly established internationally, and many things about the symptoms remain unclear.

In Japan, the health ministry categorizes them as “post-infection symptoms.”

Hiroyasu Iso, a member of the research team and head of the Institute for National Center for Global Health and Medicine’s Institute for Global Health Policy Research, said his team defined the symptoms as those persisting for at least two of the three months following infection.

The team conducted online and paper-based surveys on COVID-19 patients and uninfected people in Tokyo’s Shinagawa Ward, Yao in Osaka Prefecture, and Sapporo.

The researchers analyzed 53,642 cases (25,736 adults and 27,906 children) after sorting them by city or ward.

The research team also compared the post-infection rates based on peak infection times in the pandemic.

Among adults who contracted the virus during the outbreak of the Alpha and Delta variants in spring and summer 2021, 25 percent to 28.5 percent reported having post-infection symptoms.

The rate decreased to 11.7 percent to 17 percent during the peak of the Omicron variant from early 2022 to summer that year. Omicron continues to be the dominant strain in Japan.

The health ministry plans to incorporate the findings into treatment guidelines for post-infection symptoms.

It also intends to conduct further research to clarify how the symptoms trend over the coming years and what is causing the symptoms.

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