The flu season in Japan has continued into May, with experts attributing the prolonged spread of infections to low herd immunity and government relaxation of coronavirus measures.

Group infections have forced some schools to close entirely or close some classes even after infections peaked in February.

An electron microscopic image shows the influenza A virus. (Courtesy of U.S. Centers for Disease Control and Prevention)(Kyodo)

"This is the first time we've had so many infections," said an official at a private high school in Oita, southwestern Japan, where around 500 people, including staff and a quarter of all students, were infected from May 11 to 22 following a sports festival.

Also this month, around 500 students at a high school in Miyazaki, southwestern Japan, were infected around the time of a sports festival, while about 100 students got sick at an elementary school in Chofu, western Tokyo.

Experts also noted that measures against COVID-19 had prevented mass influenza infections from late 2020, meaning people have decreased immunity against the illness. They warned of a possible surge in infections this winter.

According to a tally by the National Institute of Infectious Diseases, the weekly number of influenza cases reported at designated medical institutions exceeded one patient in December, marking the start of the flu season for the first time in three years.

But while the number of cases peaked at 12.91 per institution in the first half of February, the season has yet to end. The number was 1.36 cases from May 8 to 14, compared to 0.74 for the same period in the pre-pandemic year of 2019.

Norio Sugaya, a visiting professor at Keio University specializing in infectious diseases, said that influenza infections plummeted during the COVID-19 outbreak as people adopted mask-wearing en masse and avoided going out to eat.

Sugaya said this year's protracted spread was caused by weakened herd immunity as well as the government's decision to downgrade the legal status of COVID-19 earlier this month and broadly ease anti-virus guidelines, while also warning group infections could occur even in summer if the virus is brought by people arriving from overseas.

"The elderly, young children and people with underlying conditions are at risk of becoming seriously ill," Sugaya warned of the coming winter, urging them to "get vaccinated and be prepared."