Three types of flu are circulating in the United States during the 2025-2026 season: influenza A H3N2, influenza A H1N1, and influenza B Victoria. As of early April 2026, influenza B has edged ahead to account for about 53% of positive tests at public health labs, while influenza A makes up the other 47%. Among the influenza A cases, H3N2 is dominant at 76% of subtyped samples, with H1N1 making up the remaining 24%.
The season’s overall positivity rate has been trending downward, sitting at 11.5% of clinical lab tests in late March. That means the worst of this flu season is likely behind us, but all three strains are still actively spreading.
H3N2: The Season’s Hardest-Hitting Strain
The influenza A H3N2 strain has defined much of this flu season. Specifically, a variant known as the “subclade K” strain emerged as the dominant circulating virus earlier in the season and has been linked to more severe illness and higher hospitalization rates than a typical flu year. H3N2 seasons tend to be rougher in general, and this one has followed that pattern.
H3N2 has historically hit older adults especially hard. The virus mutates frequently, which makes it harder for your immune system to recognize from year to year and more challenging for vaccine manufacturers to match precisely.
Influenza B Is Now the Majority
In an interesting late-season shift, influenza B Victoria lineage viruses now make up the slight majority of circulating flu. This is a common pattern: influenza A tends to dominate early in the season, while influenza B picks up later and can extend the tail end of flu activity into spring.
Influenza B generally causes somewhat milder illness in adults compared to H3N2, but it can still lead to serious complications, particularly in children. One lineage of influenza B, called Yamagata, has not been detected in global surveillance since the COVID-19 pandemic. Only the Victoria lineage remains in circulation, which is why this season’s vaccines were reformulated as trivalent (three-strain) rather than the four-strain vaccines used in prior years.
What Symptoms to Expect
Flu symptoms tend to be consistent regardless of which strain you catch. The hallmark is sudden onset: you may feel fine in the morning and miserable by the afternoon. Common symptoms include fever or chills, cough, sore throat, a runny or stuffy nose, muscle and body aches, headaches, and significant fatigue. Not everyone with flu develops a fever, though, so its absence doesn’t rule out infection.
Some people experience vomiting and diarrhea, though this is more common in children than adults. Because H3N2 subclade K has been associated with more severe symptoms this season, people infected with that strain may notice more intense body aches, higher fevers, or longer recovery times compared to a milder flu year.
How Well the Vaccine Matches This Year’s Strains
All flu vaccines for the 2025-2026 season are trivalent, covering one H1N1 strain, one H3N2 strain, and one B/Victoria strain. That lineup matches the three types currently circulating, which is good news on paper. The real question with any flu vaccine is how closely the specific vaccine strains resemble the viruses actually making people sick, and H3N2’s tendency to mutate quickly can sometimes reduce that match.
If you were vaccinated this season, your protection against H1N1 and B/Victoria is generally more reliable than against H3N2. Even when the vaccine match isn’t perfect, vaccination tends to reduce the severity of illness if you do get infected, making hospitalization and complications less likely.
Antivirals Still Work Against Nearly All Strains
The CDC tests thousands of flu virus samples each season to check whether they’re resistant to antiviral medications. The results this season are reassuring. Out of nearly 4,000 viruses tested, the vast majority remain fully susceptible to the most commonly prescribed antiviral treatments.
A small number of H1N1 viruses, roughly 1-2% of those tested, showed reduced responsiveness to one widely used antiviral. But H3N2 and B/Victoria strains have shown almost no resistance. One older class of antiviral medication (the adamantanes) remains completely ineffective against both types of influenza A and is not recommended.
If you’re prescribed an antiviral for flu, the key factor is timing. These medications work best when started within the first 48 hours of symptoms. They can shorten illness by about a day and, more importantly, reduce the risk of serious complications in people who are at higher risk, including adults over 65, young children, pregnant women, and people with chronic health conditions.
How to Tell Flu From Other Circulating Viruses
COVID-19 and RSV are also circulating alongside flu this spring, and the symptoms overlap significantly. The biggest clue that you’re dealing with flu rather than a cold is the speed and intensity of onset. Colds tend to build gradually over a couple of days, while flu hits fast and hard, often with prominent body aches and fatigue that feel out of proportion to your other symptoms.
The only way to know for certain which virus you have is a test. Many clinics and pharmacies offer combination tests that check for flu, COVID-19, and RSV from a single nasal swab. Getting tested matters most if you’re in a higher-risk group or if you’re within that 48-hour window where antivirals can make a difference.