Is flu going around in Japan?

Japan's influenza season typically peaks between January and February, following a climb through December. The winter window is narrower and often more sharply defined than in Europe or North America. Post-2020 seasons have shown some irregularity, including unusual summer activity in 2024–2025, but the dominant pattern remains a winter peak in the first two months of the calendar year.

Influenza surveillance in Japan is led by the National Institute of Infectious Diseases (NIID), which coordinates a national sentinel network of approximately 5,000 physicians reporting weekly ILI cases, supplemented by prefectural public-health institutes and designated reference laboratories. NIID publishes weekly Infectious Agents Surveillance Reports (IASR) and transmits aggregated data to WHO's FluNet platform.

Japan's NIID shares weekly virological and epidemiological indicators for influenza and RSV with the WHO Western Pacific Regional Office, which consolidates them into WHO FluNet. FluNet aggregates sentinel data from around the world into harmonised weekly datasets, letting Japan's flu trajectory be compared directly with other Western Pacific countries such as South Korea, Australia, and China.

Japan's peak typically arrives earlier than in continental Europe and ends more abruptly. Activity rises sharply through late December and retreats by early March. Because Japan reports into WHO FluNet, its curve can be overlaid directly with other Northern-Hemisphere countries. Unusual years, such as the 2024 summer wave, stand out clearly against that historical baseline.

NIID classifies influenza intensity using qualitative tiers (baseline, caution, warning, epidemic) derived from the ILI per-sentinel threshold. Warning and epidemic levels trigger heightened public-health communications and vaccine-uptake reminders. These tiers are anchored to historical reference ranges, which makes them more stable indicators than raw case counts, particularly given how testing behaviour shifts across years.