Flu B refers to influenza B, one of two main types of influenza virus that cause seasonal flu in humans. If you received a test result showing “influenza B” or “flu B positive,” it means the specific virus making you sick is type B rather than type A. Both types cause the same general illness, but they differ in important ways.
Influenza B vs. Influenza A
Seasonal flu is caused by two virus types: influenza A and influenza B. Both circulate every winter and produce similar symptoms, but they behave differently behind the scenes.
Influenza A is the more diverse and volatile type. It has dozens of subtypes based on proteins on its surface, and it infects birds, pigs, and other animals in addition to humans. That animal reservoir is why influenza A is the only type capable of causing pandemics. The subtypes currently circulating in people are H1N1 and H3N2.
Influenza B, by contrast, circulates almost exclusively in humans. It doesn’t have subtypes but instead splits into two lineages, named after the places where reference strains were first identified: B/Victoria (from a 1987 strain) and B/Yamagata (from a 1988 strain). These two lineages diverged genetically in the 1970s and are different enough that immunity against one doesn’t protect well against the other. Influenza B also mutates more slowly than influenza A, which makes it somewhat more predictable from year to year.
What Happened to the Yamagata Lineage
For decades, both B/Victoria and B/Yamagata circulated together, which is why flu vaccines used to include four strains (two A subtypes and both B lineages). That changed after the COVID-19 pandemic. B/Yamagata essentially vanished from global surveillance after March 2020. The few cases reported since then appear to be vaccine-derived, misidentified, or unconfirmed, and researchers now believe the lineage is likely extinct.
As a result, flu vaccines for the 2025-2026 season in the U.S. are trivalent, covering three strains instead of four: an H1N1 virus, an H3N2 virus, and a B/Victoria lineage virus. If you’re diagnosed with flu B today, it’s almost certainly a Victoria lineage strain.
Symptoms of Flu B
Flu B causes the same core illness as flu A: sudden onset of fever, chills, muscle aches, headache, fatigue, cough, sore throat, and a runny or stuffy nose. “Sudden” is the key word. Unlike a cold that builds gradually, flu symptoms typically hit within a day or two of infection, with an incubation period ranging from one to four days.
Most people recover in three to seven days, though a lingering cough and general tiredness can drag on for more than two weeks, especially in older adults or anyone with chronic lung conditions. Young children with flu B may also experience nausea, vomiting, or diarrhea alongside respiratory symptoms, which can make it easy to mistake for a stomach bug.
One complication linked more strongly to flu B than flu A is Reye syndrome, a rare but serious neurological condition associated with taking aspirin-type medications during a viral illness. This is why aspirin is avoided in children and teenagers with the flu.
Who Flu B Hits Hardest
Influenza B is sometimes perceived as the “milder” flu, partly because it can’t cause pandemics. That perception is misleading. Flu B causes significant illness and death every season, and it disproportionately affects children. Kids are more likely to be hospitalized from influenza B than many people realize, and awareness of that risk tends to lag behind the attention given to influenza A.
Older adults in long-term care facilities can also present unusually. Instead of the classic fever-and-cough picture, they may show confusion, behavioral changes, or loss of appetite as their primary signs of infection.
How It Spreads and How Long You’re Contagious
Flu B spreads the same way as flu A: through respiratory droplets when an infected person coughs, sneezes, or talks, and occasionally by touching contaminated surfaces. You can start spreading the virus a full day before your symptoms appear, which is part of what makes flu so hard to contain. After symptoms begin, you’re most contagious during the first three days, though you can continue shedding the virus for five to seven days.
Testing for Flu B
When your doctor tests you for the flu, the most common option is a rapid diagnostic test, which returns results in about 15 to 20 minutes. These rapid tests are required to detect influenza B with at least 80% sensitivity and 95% specificity compared to PCR, the gold-standard lab test. That means rapid tests catch most flu B infections, but they can miss roughly one in five cases. A negative rapid test doesn’t always rule out the flu, especially if your symptoms strongly point to it. PCR testing is more accurate but takes longer to process.
Some rapid tests simply tell you “influenza positive” without specifying the type. Others distinguish between A and B, which is what produces the “flu B” result people search for.
Treatment Options
Antiviral medications work against flu B, but timing matters. Treatment is most effective when started within 48 hours of your first symptoms. After that window, antivirals still offer some benefit for people who are severely ill or hospitalized, but the advantage shrinks the longer you wait.
The most commonly prescribed antiviral is an oral medication you take twice daily for five days. An inhaled option and a single-dose oral alternative are also available. All of these work against both influenza A and B. Older antiviral drugs from a different class no longer work against circulating flu viruses and are not recommended.
For most otherwise healthy people, flu B resolves on its own with rest, fluids, and over-the-counter fever and pain relievers. Antivirals are typically reserved for people at higher risk of complications: young children, adults over 65, pregnant women, and anyone with chronic health conditions like asthma, diabetes, or heart disease.