Early signals of the 2025-26 Northern flu season

30 November 2025

Flu seasons in the Northern Hemisphere traditionally intensify in late fall, peaking between December and February as cooler weather and holiday gatherings drive people indoors. Yet, the 2025-2026 season is showing unusually early surges, with health agencies worldwide reporting low but steadily rising influenza positivity rates across the region. This accelerated start echoes patterns seen in the Southern Hemisphere earlier this year, where higher-than-average activity provided a preview of a potential strong season ahead. The dominant player is influenza A(H3N2), a subtype known for causing more severe symptoms in vulnerable groups like young children and older adults. While overall activity remains moderate in many places, the rapid uptick, coupled with a circulating variant that can sometimes slip past immune defenses, has led experts to send warnings to vulnerable groups.

In North America, influenza is low but steadily growing in most states, particularly among children. More than two thirds of influenza A cases are of the H3N2 subtype, signaling its early dominance. Hospitalizations are starting to strain pediatric wards in some states like Texas and Florida. Canada is experiencing similar increased incidence. Provinces like Ontario and Quebec are seeing the sharpest increases, likely driven by school reopenings, with H3N2 making about half of cases.

Europe mirrors this heightened activity, with a rapid increase in cases occurring earlier than usual. Influenza cases surged in October, with 11 countries reporting sporadic to moderate activity one month later. However, these increases are not homogeneous: Northern and Western Europe, including the UK and Germany, are bearing the brunt, with high activity especially in Germany as predicted by our forecasts. In the UK, the National Health Service has issued alerts for one of its worst winters yet, with young adults and schoolchildren hit hardest. In contrast, Southern nations like Spain and Italy still show reduced activity, possibly due to milder weather delaying peaks. Across the European Union, vaccination rates lag below 50% in most countries, exacerbating risks for the elderly.

In Asia, early signs of increased activity appeared as far back as October, outpacing typical timelines and sending an alert for the hemisphere. Japan's National Institute of Infectious Diseases declared a nationwide epidemic on late November, with cases sixfold higher than expected, prompting over 100 school closures in Tokyo and Osaka alone. Officials blame a mix of post-pandemic travel booms and the dominant H3N2 strain for this "unprecedented" surge, with hospitalizations climbing among children and seniors.

China and Taiwan are also seeing higher activity, though not overwhelming. Taiwan faces its worst season in a decade, with several severe cases and over 130 deaths since October. Factors like dense populations and holiday preparations are amplifying spread.

The circulating A(H3N2) subtype is dominated by a specific variant characterized by changes in its surface proteins that help it evade some immune responses more effectively than usual. This variant, which emerged over the summer, carries several key alterations -- around seven in critical spots -- that allow it to spread faster and cause more infections, even in partially immune populations. H3N2 has long been known for hitting vulnerable groups hardest, often leading to higher risks of pneumonia in the elderly and young children, and this version appears to amplify those effects. It's already accounting for 33% of global H3N2 detections since May, rising to nearly 50% in Europe, and driving early outbreaks from Japan to the UK.