What's going around in Japan?

NIID aggregates weekly reports from its national sentinel networks for influenza and RSV, together with virological testing at prefectural laboratories. Published in the Infectious Agents Surveillance Report (IASR), these indicators describe the burden carried by primary care and paediatric services. The same data is transmitted to WHO FluNet for international comparison.

WHO FluNet, the international platform that aggregates Japan's sentinel data, covers influenza and RSV, not SARS-CoV-2. COVID-19 has its own surveillance streams, including genomic and wastewater systems. That is why data pages built on FluNet for Japan, India, and China focus on flu and RSV only. COVID-19 monitoring in Japan continues via other NIID and Ministry of Health channels.

Japan's winter respiratory peak reflects the usual mix of cold, dry indoor air, reduced ventilation, and dense indoor contact in workplaces and schools. Influenza drives the sharp January–February spike, while RSV may contribute either earlier or during winter depending on the year. The combined curve is one of the most sharply defined in the Northern Hemisphere.

Japan's flu peak typically arrives earlier and is more tightly concentrated in January–February than European or US peaks. RSV timing has been less stable since 2021. Because NIID feeds WHO FluNet, Japan's trajectory can be overlaid directly with other Western Pacific countries (South Korea, Australia, China) and with Northern-Hemisphere countries elsewhere.

NIID uses qualitative tiers (baseline, caution, warning, epidemic) for influenza, with separate activity classifications for RSV anchored to historical reference ranges. These tiers describe whether current sentinel reporting rates are ordinary, elevated, or unusually intense. They intentionally avoid implying precision that sentinel-based surveillance cannot support.