Influenza Cases Rising in Japan After Early Start to Season

An Early Start to Flu Season

The number of patient cases of influenza reported by Japanese medical institutions exceeded 1.00 per institution during August 17–23, or week 34 of 2026, indicating the start of an outbreak. Case counts were much higher than the same period in 2025 and have continued to rise through September.

This marked the second earliest start to the influenza season since the current method of keeping records was introduced in 1999; only 2009 saw it kick off earlier. It was also the first time the outbreak threshold was reached in August since 2023, when an ongoing epidemic carried over from the previous year.

Suggested reasons for the summer outbreak include the increase of foreign tourists from the southern hemisphere, where it is winter during Japan’s summer, and dry indoor air caused by the widespread use of air conditioning. The Ministry of Health, Labor, and Welfare is urging the public to take preventive measures, such as avoiding crowded places when feeling unwell, keeping rooms well ventilated, and washing hands with soap after spending time in public places.

Data Sources

• Updates on influenza during the 2026–27 season and data on week 34 (Japanese) from the Ministry of Health, Labor, and Welfare.

(Translated from Japanese. Banner photo © Photo AC.)