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.