The 2025-26 flu season presents challenges, notably a mismatch between the predominant flu strains and the seasonal vaccine. Canadian researchers recently reported that the H3N2 subclade K has become the dominant strain, diverging from the vaccine formulation based on earlier variants.
Key Findings on Seasonal Flu Strain and Vaccine Mismatch
Data published by a group of Canadian researchers indicate a growing concern regarding the effectiveness of this season’s flu vaccine. The report emphasizes the need for intensified surveillance in North America, especially in the U.S., where the Centers for Disease Control and Prevention (CDC) has not released regular respiratory illness surveillance data since September 26.
Dominant Flu Strains: H3N2 and H1N1
- The H3N2 subclade K strain has shown early dominance in the U.K. and Japan, representing 90% of flu samples.
- H1N1 was the predominant strain in the Southern Hemisphere before H3N2 emerged towards the season’s end.
Dr. Danuta Skowronski, an epidemiology expert at the British Columbia Centre for Disease Control, highlighted the importance of monitoring U.S. flu trends due to their impact on the North American context. She noted that significant mutations in influenza A strains could affect vaccine protection levels.
Surveillance and Expectations for the Season
Experts are currently uncertain about the overall flu activity in the United States. Dr. Joshua Petrie from Wisconsin’s Marshfield Clinic expects H3N2 will dominate but emphasizes that regular surveillance is essential for tracking the situation. He expressed concern that the influenza season could escalate quickly if surveillance is not maintained.
- Advanced genetic characterization performed by the CDC is crucial for future vaccine preparations.
- H3N2 seasons typically result in lower vaccine effectiveness and more severe outcomes for older adults compared to H1N1 seasons.
Vaccine Effectiveness Insights
Early data from the U.K. Health Security Agency showed that the 2025-26 vaccine might be 70% to 75% effective in preventing hospital attendance among children aged 2 to 17 years. For adults, the effectiveness ranges from 30% to 40%.
Despite concerns over strain mismatches, experts underscore that receiving the flu vaccine remains essential. It still affords some protection against severe illness, even in seasons with lower effectiveness against the circulating strains.
In summary, as researchers and healthcare professionals brace for an uncertain flu season, vigilance and widespread surveillance are imperative to navigate the challenges posed by mismatched strains.
