Flu B (influenza B) is one of two main types of influenza virus that cause seasonal flu outbreaks each year. Unlike influenza A, which infects multiple animal species and can cause pandemics, influenza B circulates almost exclusively in humans. It produces the same core symptoms as flu A, including fever, body aches, cough, and fatigue, but it mutates more slowly and tends to hit children particularly hard.
How Flu B Differs From Flu A
The influenza B virus generates new variants at least two times slower than influenza A. Its internal copying machinery makes fewer errors each time it replicates, which means the virus changes less from season to season. In practical terms, this slower mutation rate is why your immune system may retain some protection against flu B longer than it does against flu A. About 71% of flu B samples show no significant genetic variation within a single infected person, compared to only 20% of flu A samples.
Because flu B mutates less, it also puts less pressure on vaccine developers to constantly update the B component of the seasonal flu shot. The now-extinct Yamagata lineage of flu B, for example, stayed so genetically stable that its vaccine strain wasn’t updated after 2015. That said, flu B still evolves enough to require monitoring, and new variants do emerge over time.
Flu B was first isolated in the 1940s and split into two distinct lineages, Victoria and Yamagata, in the 1970s. The Yamagata lineage appears to have gone extinct in recent years, likely a casualty of the public health measures during the COVID-19 pandemic. As of the 2024-2025 flu season, the FDA recommended removing Yamagata from U.S. vaccines entirely, shifting from four-strain (quadrivalent) vaccines to three-strain (trivalent) ones that include only the Victoria lineage for the flu B component.
Symptoms and How Long They Last
Flu B causes the same set of symptoms as flu A: sudden onset of fever, chills, muscle aches, headache, fatigue, dry cough, sore throat, and a runny or stuffy nose. Not everyone develops a fever, especially older adults and people with weakened immune systems. Young children with flu B are more likely to experience nausea, vomiting, or diarrhea alongside respiratory symptoms. In older adults living in care facilities, flu B can show up as confusion, behavioral changes, or loss of appetite rather than classic flu signs.
Most people recover within three to seven days, though a lingering cough and general tiredness can stick around for two weeks or longer. This is especially common in older adults and people with chronic lung conditions.
How Flu B Spreads
Flu B spreads the same way as flu A: through respiratory droplets when an infected person coughs, sneezes, or talks. You can also pick it up by touching a surface with the virus on it and then touching your mouth, nose, or eyes.
Symptoms typically appear about two days after exposure, though the window ranges from one to four days. You become contagious roughly one day before symptoms start and remain contagious for five to seven days after getting sick. The first three days of illness are when you’re most likely to spread it. Children and immunocompromised individuals can stay contagious even longer.
Complications in Adults vs. Children
Flu B can lead to serious complications in both age groups, but the pattern looks different. During a well-studied hospital outbreak in Germany, 68% of hospitalized adults developed at least one complication: pneumonia was the most common, followed by sepsis and heart-related events. The mortality rate among hospitalized adults was 6%.
Children fared better overall. About 17% of hospitalized children developed complications, and none died during that outbreak. The complications children experienced were distinct from adults: irritation of the brain’s protective lining, febrile seizures, ear infections, and muscle inflammation. Interestingly, among children in that study, flu A caused complications more often than flu B (82% vs. 35% of cases).
One complication more closely linked to flu B than flu A is Reye syndrome, a rare but serious condition affecting the brain and liver. It’s associated with aspirin use during a viral illness, which is why aspirin is no longer recommended for children with flu or chickenpox. Since that guidance was adopted in the 1980s, Reye syndrome has become extremely rare.
Treatment Options
Four antiviral medications work against flu B. The most widely prescribed is oseltamivir (Tamiflu), taken as a pill twice daily for five days. It’s approved for all ages, including infants. A newer option, baloxavir (Xofluza), requires only a single dose and has shown particular promise against flu B. In clinical trials, baloxavir reduced symptom duration in flu B patients by more than 24 hours compared to oseltamivir.
Two other antivirals, zanamivir (an inhaled medication) and peramivir (given by IV), are also effective but used less commonly. Zanamivir is approved for children seven and older for treatment, while peramivir is an option for those six months and older who need intravenous care.
All of these medications work best when started within 48 hours of symptom onset. For most healthy people, flu B resolves on its own with rest and fluids. Antivirals are most important for people at higher risk of complications: young children, adults over 65, pregnant women, and those with chronic health conditions.
Flu B and the Seasonal Vaccine
The annual flu shot remains the primary way to protect yourself against flu B. For the 2024-2025 season, U.S. vaccines contain a Victoria lineage strain (B/Austria/1359417/2021) alongside two influenza A strains. Because the Yamagata lineage is no longer circulating, there’s no need for a second B strain in the vaccine.
Flu B’s slower evolution is a modest advantage here. The B component of the vaccine doesn’t need to be updated as frequently as the A components, which means the match between vaccine and circulating virus tends to be more reliable for flu B. Still, the vaccine’s effectiveness varies year to year, and getting vaccinated annually remains important because your immune protection wanes over time.