Seasonal influenza activity has reached a record-breaking level, making the 2025-2026 influenza season one of the most intense ones of the past few seasons. While the full data for this season is yet to be made available, flu activity remains strong throughout the country.
Statistics from the Centers for Disease Control and Prevention show flu activity reports from week one to week four, being the latest report for the year 2026. Data shows a persistent increase in influenza activity across the United States. This includes rising hospitalization rates, positive rates, an increase in illnesses from flu, and deaths. More reports, however, are yet to be made accessible on the website.
According to public health officials, the intensity and prolonged flu activity make this season a bit different from the past ones. But the question is, why was this season’s flu activity so intense? The answer lies in the influenza type.
CDC reports that public health laboratories recorded Influenza A-H3N2 as the dominant virus in this season so far. In the latest week’s report, genetic analysis revealed that 91.5% of the A-H3N2 virus belonged to subclade K of H3N2.
Now, what is subclade K? Subclade K is the very rumored “new flu strain” that has been the talk of the town. Researchers in Australia and New-Zealand were studying flu trends and published in Euro surveillance, an open-access medical journal. Their study mentioned that subclade J of A-H3N2 viruses spread from early-mid 2025 and some of those viruses had mutations leading to their formation of subclade K.
This causes changes in the strain which might lead to the immune system and prior flu vaccines being less effective. This was one reason why flu activity peaked in Dec. 2025 and again in Jan. 2026. Unvaccinated people are also at high risk of getting illnesses and infections from the flu. As per the CDC, 90% of pediatric deaths observed this season were due to not being fully vaccinated.
Influenza types A and B are both in season, increasing flu cases one way or another. Influenza A-H3N2 seems to be at a stability now, but type B activity seems to be increasing slowly. Clinical laboratory data from the CDC shows that activity in week four has a slight decrease in some regions. However, cases are increasing in HHS Region 10, which has four states: Idaho, Oregon, Washington, and Alaska.
Despite the surge in flu activity, health officials and organizations emphasize the importance of vaccination against not only influenza but other pathogens too. Having flu vaccination reduces the risk of getting infected to a certain extent.
Influenza viruses constantly mutate. Therefore, vaccines are updated every season for protection and to boost the immune system. It is generally recommended by the CDC to get flu vaccinations every season.
