Japan should prepare for the possible arrival of a “twindemic” — a mixture of influenza and COVID-19 infections — experts say, after an unusually early flu outbreak at a school in Tokyo this week.

A public elementary school in Tachikawa, western Tokyo, canceled both of its third grade classes on Tuesday and Wednesday after 14 of the 45 students tested positive for influenza, according to the Tokyo Metropolitan Government.

This was the first cancellation of a school class due to a flu outbreak in the 2021-2022 season, which started last September. School closures or class cancellations due to flu viruses are common in the winter but rare in the summer.

The risk of flu infection is rising, because of reduced immunity in society stemming from a much lower number of influenza cases during the coronavirus pandemic, experts say. The past two years have seen very few reported infections, partly as a result of people taking anti-infection measures against COVID-19, such as mask-wearing, hand-washing and restricting movement.

Another possible explanation for the decrease in flu cases during the past two winters is viral interference, whereby people’s infection with the coronavirus could have made them less susceptible to infection with a flu virus.

Mitsuo Kaku, chairman of the expert board at the Tokyo Center for Infectious Disease Control and Prevention, cited a recent spike in flu cases in Australia as a cause for concern.

“In Australia, in the two weeks between May 23 and June 5, a total of 48,000 flu cases were confirmed,” Kaku told the board meeting Thursday. “That’s significantly higher than the country’s five-year average and is comparable to its peak marked in 2017.

“Japan could have a similar spike in flu infections this year. It’s extremely important for us to prepare for the simultaneous spread of the flu virus and the coronavirus.”

Takaji Wakita, chair of the health ministry panel on COVID-19 measures, said Thursday that the situation requires close monitoring, though there will still be some time before flu viruses become widespread in Japan.

“Influenza viruses and the coronavirus have very similar infection routes, which means measures to prevent them are very similar too,” said Wakita, who is concurrently director of the National Institute of Infectious Diseases. “Hand-washing and mask-wearing will remain important.”