The 2025-26 flu season has started unusually early, driven by the H3N2 subclade K strain that emerged in summer 2025 after the season's vaccine composition was already finalized. This mutated strain has caused significant outbreaks in Japan, the UK, and Canada, with health officials warning it could produce the worst flu season in over a decade.

The CDC classifies flu season severity using its Influenza Risk Assessment Tool (IRAT) and tracks cumulative hospitalization rates through the FluSurv-NET surveillance system. A "high severity" classification is typically assigned when cumulative hospitalization rates significantly exceed historical baselines and meet specific threshold criteria.

Resolution criteria: This market resolves YES if the CDC officially classifies the 2025-26 influenza season as "high severity" in any official report, press release, or surveillance summary published by June 1, 2026.

Resolves NO if the season is classified as "low" or "moderate" severity, or if no official severity classification is issued by the resolution date.

Resolves N/A if the CDC is unable to issue surveillance reports due to prolonged government shutdown or other operational disruptions.