The number of flu cases per regularly monitored medical institution in Japan in the week through Sunday rose 1.08 from the previous week to 11.07, topping the warning level of 10, the health ministry has said.
The ministry collects flu case reports from about 5,000 such medical institutions nationwide. The warning level announced Friday indicates the possibility of a major outbreak within four weeks.
By prefecture, Okinawa had the highest per-institution count, at 25.37, followed by Chiba with 20.86, Saitama with 19.69 and Ehime with 18.45. The per-institution number exceeded 10 in 17 of the country's 47 prefectures.
A total of 1,772 kindergartens, elementary, junior high and high schools nationwide were partially or fully closed due to the flu.
The flu usually spreads in winter, but it has been rampant since the first nationwide epidemic season in three years began at the end of last year.
"Several factors are believed to have played a role in the current spread of the flu, including a decline in immunity as (the flu) had not spread for a long time, and an increase in travel between Japan and overseas," a health ministry official said.
"We want people to continue efforts to prevent infection as they do for COVID-19," the official added.
The ministry also said the per-institution number of COVID-19 infections in the country stood at 3.76 in the week through Sunday, down for the sixth consecutive week.