Yes, the flu is actively circulating across the United States during the 2025–26 season, with influenza A(H3N2) as the dominant strain. Activity has been widespread in most states, and this season carries an added wrinkle: the circulating virus has drifted significantly from the strain used to make this year’s vaccine, reducing its effectiveness compared to typical years.
Which Strain Is Spreading This Season
Influenza A(H3N2) accounts for about 88% of all subtyped flu cases this season. Among the remaining cases, influenza A(H1N1) makes up roughly 12% of influenza A detections, and a smaller number of influenza B/Victoria cases are also circulating. H3N2-dominant seasons tend to hit harder overall, particularly among older adults, because this subtype is associated with more severe illness and higher hospitalization rates than H1N1.
The specific version of H3N2 spreading this year belongs to a genetic subgroup called clade 2a.3a.1 subclade K. What matters about that technical detail is this: it was first identified by the CDC in June 2025, after the vaccine viruses for this season had already been selected. That timing gap means the virus and the vaccine don’t match as well as hoped.
How Well the Vaccine Is Working
Interim estimates from the CDC paint a mixed picture. For children and teens, the vaccine reduces the risk of a flu-related doctor visit by 38% to 41% and cuts hospitalization risk by about 41%. For adults, the numbers are lower: 22% to 34% protection against outpatient visits and around 30% against hospitalization. Those figures are modest, but even partial protection can mean the difference between a miserable week at home and a trip to the emergency room.
Protection against influenza B is noticeably better. Among children and teens, the vaccine prevents 45% to 71% of flu B outpatient visits. Among adults, that figure is about 63%. So while the H3N2 mismatch drags down overall effectiveness, the vaccine still provides meaningful protection against the other strains in circulation.
Getting vaccinated remains worthwhile even with reduced effectiveness. A 30% reduction in hospitalization risk is significant at a population level, and for people with chronic conditions, pregnancy, or weakened immune systems, any added protection matters.
How the Flu Spreads and When You’re Contagious
You can start spreading the flu about one day before your symptoms appear, which is part of what makes it so hard to contain. You’re most contagious during the first three days of illness, though you can continue shedding the virus for five to seven days after getting sick. Children and people with weakened immune systems may be contagious even longer.
The virus travels mainly through respiratory droplets produced when an infected person coughs, sneezes, or talks. It can also survive on surfaces like doorknobs and phones, though person-to-person transmission through the air is the primary route.
Flu vs. COVID vs. RSV vs. a Cold
One of the most common frustrations this time of year is figuring out which virus you actually have. The flu, COVID-19, and RSV all share core symptoms: fever, cough, body aches, and fatigue. There’s no reliable way to tell them apart based on symptoms alone.
That said, the flu does have a signature pattern. It typically hits fast. You might feel fine in the morning and be flat on your back by evening, with a high fever, chills, muscle aches, and exhaustion that feel disproportionate to your other symptoms. COVID tends to come on more gradually and is more likely to cause loss of taste or smell. RSV is most dangerous in very young children and older adults, and it often centers on the lower respiratory tract, producing wheezing and difficulty breathing. A common cold generally stays milder, with a runny nose and sore throat but rarely a significant fever.
The only way to know for certain is testing. Combination tests that check for flu, COVID, and RSV simultaneously are widely available at clinics and pharmacies.
How Accurate Flu Tests Are
If you take a rapid flu test at a pharmacy or urgent care, know that these tests are highly specific but not very sensitive. That means a positive result is almost certainly correct (specificity above 97%), but a negative result doesn’t rule out the flu. Rapid tests catch only about 50% to 70% of true infections, with one large analysis putting the average sensitivity at just 62%. If your rapid test comes back negative but you have classic flu symptoms, a PCR-based test is far more accurate and your provider may recommend one, especially if treatment decisions depend on the result.
Treatment Works Best in the First 48 Hours
Antiviral treatment is most effective when started within two days of your first symptoms. That narrow window is why timing matters so much. If you develop sudden-onset fever, body aches, and chills during flu season, getting tested and seeking treatment quickly can shorten your illness and reduce the risk of complications.
Antivirals are typically taken for five days. They don’t cure the flu instantly, but they can shave a day or two off your symptoms and, more importantly, reduce the chance of the infection progressing to pneumonia or requiring hospitalization. They’re especially valuable for people at higher risk of complications: adults over 65, children under 5, pregnant women, and anyone with chronic health conditions like asthma, diabetes, or heart disease.
For otherwise healthy adults with mild symptoms, rest, fluids, and over-the-counter fever and pain relievers are usually enough. Most people recover within one to two weeks, though fatigue can linger.
How to Protect Yourself Right Now
Even with a vaccine mismatch, basic prevention strategies make a real difference. Wash your hands frequently with soap and water for at least 20 seconds, especially after being in public spaces. Avoid touching your face. If someone in your household is sick, try to limit close contact during their first three days of symptoms, when they’re shedding the most virus.
If you haven’t been vaccinated yet, it’s not too late. Flu seasons can run well into spring, and partial protection is better than none. For those in high-risk groups who develop symptoms, the priority is getting tested and starting antiviral treatment within that 48-hour window. Having a plan before you get sick, knowing where to get tested quickly and how to reach your provider on short notice, can make all the difference in a season where the virus and the vaccine aren’t perfectly aligned.