Influenza A has dominated the current flu season, accounting for the vast majority of confirmed cases. The season started with A(H1N1) strains leading the way in late 2025, then shifted toward A(H3N2) as the primary circulating subtype. This has been a notably severe season, with hospitalization rates among the highest recorded since 2010.
The Strains Circulating Right Now
Two influenza A subtypes have driven this season in waves. The early months were dominated by A(H1N1), specifically a subclade called D.3.1.1. As the season progressed, A(H3N2) took over. Among nearly 10,000 patients tested from September through December 2025, about 20% tested positive for influenza, and almost all of those positive results were H3N2. Genetic sequencing of over 300 samples found that roughly 85% of H3N2 viruses belonged to a single genetic group known as subclade K.
Influenza B has played a much smaller role. One of the two historical B lineages, B/Yamagata, appears to have gone extinct. It hasn’t been reliably detected anywhere in the world since early in the COVID-19 pandemic. The World Health Organization and the FDA both recognized this shift by recommending that flu vaccines drop B/Yamagata protection entirely, moving from four-strain (quadrivalent) to three-strain (trivalent) formulations. The only B lineage still circulating is B/Victoria, though it has made up a small fraction of cases this season compared to influenza A.
How Severe Is This Season?
By multiple measures, this has been one of the more intense flu seasons in recent memory. The cumulative hospitalization rate for the 2024-25 season reached 127.1 per 100,000 people, surpassing every end-of-season rate going back to 2010-11. The current season is tracking at similarly high levels, with a cumulative rate of 67.0 per 100,000 by the fifth week of 2026, the second highest at that point in the season since surveillance began tracking this metric.
Adults 75 and older have been hit hardest, with a hospitalization rate of nearly 599 per 100,000 during the 2024-25 season. That’s roughly 15 times higher than the rate for children aged 5 to 17. Among younger children, infants under 1 year old are the most vulnerable, with a hospitalization rate of 108.6 per 100,000, nearly double the rate for toddlers aged 1 to 4.
Influenza A vs. B Symptoms
Both types cause the classic flu package: fever, body aches, cough, fatigue, and sore throat. But there are meaningful differences. Influenza A generally causes more severe illness overall and accounts for far more cases. In late 2024, flu A made up 96% of reported cases.
The differences are more noticeable in children. Influenza A is linked to higher rates of ear infections, while influenza B tends to cause more gastrointestinal symptoms like vomiting and diarrhea, along with a higher risk of seizures. Since this season has been overwhelmingly driven by influenza A, most people getting sick are experiencing the typical respiratory-heavy presentation rather than stomach symptoms.
How Well the Vaccine Is Working
This season’s flu vaccine is providing moderate protection. Among patients tested from November 2025 through January 2026, vaccine effectiveness against confirmed influenza was 36.4% overall. Against H3N2 specifically, effectiveness was slightly higher at 39.9%. That means vaccinated people were roughly 36% to 40% less likely to get sick enough to seek medical care compared to unvaccinated people.
Those numbers might sound modest, but even partial protection reduces the odds of severe illness, hospitalization, and complications. Vaccination remains more effective at preventing the worst outcomes than at preventing infection altogether, which is consistent with how flu vaccines have performed historically. Nearly all circulating strains remain susceptible to standard antiviral treatments, so if you do get sick, medications prescribed within the first day or two of symptoms can shorten the illness and reduce severity.
What About Bird Flu?
Avian influenza A(H5N1) has generated headlines, but it remains a separate concern from seasonal flu. As of late January 2026, 58 human cases tied to animal exposure have been confirmed across several states, with the largest numbers in California (38 cases) and Washington (12 cases). These cases have been linked to contact with infected poultry or dairy cattle, not person-to-person spread.
CDC surveillance systems show no indicators of unusual influenza activity in people from H5 viruses. The risk to the general public remains low. Seasonal flu, not bird flu, is what’s making people sick in communities, offices, and schools right now.
Testing for Flu This Season
Because flu, COVID-19, and RSV all circulate during the same winter months and cause overlapping symptoms, combination tests that check for all three at once have become widely available. Rapid antigen tests that screen for all three viruses from a single nasal swab show acceptable accuracy compared to the gold-standard PCR lab tests. If you’re feeling sick with fever, cough, and body aches, knowing which virus you have matters because flu responds to antivirals while COVID and RSV require different management approaches. Your doctor or local pharmacy can help you figure out which test makes sense.