Flu season in Japan
Current flu activity in Japan — based on WHO FluNet positivity-rate surveillance, set against the parallel COVID-19 and RSV trajectories.
Current situation: Influenza
In week 14 of 2026, activity of influenza (seasonal flu) in Japan is low. The trend — derived from clinical surveillance — is falling. Over a four-week comparison, a clear decline is visible.
The classification is based on the WHO FluNet aggregate of sentinel laboratory data from Japan's National Institute of Infectious Diseases (NIID). Seasonally, infection waves in Japan typically peak between January and February; activity is usually markedly lower during the summer months. How severe a given season becomes depends on the circulating virus variant and the population's immune status, among other factors.
Data sources and methodology
The current picture for Japan is built on WHO FluNet, the World Health Organization's global platform for influenza surveillance. the National Institute of Infectious Diseases (NIID) reports weekly sentinel laboratory results to FluNet, which we translate into a consultation-equivalent signal.
WHO FluNet
FluNet is the WHO's weekly global influenza surveillance database. National influenza centres and reference laboratories — for Japan this is the National Institute of Infectious Diseases (NIID) — submit the number of respiratory specimens tested and the number positive for influenza and, where available, RSV. Using FluNet ensures a consistent, internationally comparable data source for countries without a European-style sentinel consultation system.
Positivity-based signal
Unlike ECDC ERVISS, FluNet does not carry outpatient consultation rates. Our headline signal is therefore derived from the weekly lab positivity rate (percent of tested specimens that are positive), multiplied by a pathogen-specific scaling factor — 80 for influenza and 50 for RSV — to approximate a consultation-equivalent incidence. COVID-19 is not available via FluNet and is therefore not shown for these countries.
Why this source
FluNet is the only weekly, comparable dataset covering many non-European countries. Positivity is relatively insensitive to changes in overall testing volume, which makes it a reasonable proxy for underlying transmission pressure even when the denominator fluctuates. We apply a SENTINEL-only filter where appropriate to reduce noise from non-sentinel reporting streams.
Qualitative classification
The “low”, “moderate” and “high” categories follow seasonal reference values and epidemiological thresholds calibrated to match our classifications for other countries. The positivity × scaling-factor product is mapped to the same consultation-equivalent scale we use elsewhere so results stay comparable across regions. Data refreshes weekly when WHO publishes the latest FluNet update.
Frequently asked questions
When is flu season 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.
Who runs flu surveillance in Japan?
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.
How does Japan report into WHO FluNet?
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.
How does Japan's flu season compare internationally?
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.
How does NIID classify flu activity levels?
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.
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