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.
Mass infections have forced some schools to close entirely or close some classes even after infections peaked in February.
"This is the first time we've had so many infections," said an official at a private high school in Oita Prefecture where around 500 people, including staff and a quarter of all students, were infected between May 11 and 22 following a sports festival.