Flu B is one of two main types of influenza virus that cause seasonal flu in humans. While influenza A gets more attention because of its pandemic potential, influenza B is responsible for a significant share of flu hospitalizations each year and can be especially impactful in children and teens. Both types produce similar symptoms, but they differ in important ways when it comes to severity, who they affect most, and how well treatments work against them.
How Flu B Differs From Flu A
Influenza A and influenza B are distinct viruses within the same family. Influenza A circulates in animals and humans, which is why it can spark pandemics when an animal strain jumps to people. Influenza B circulates almost exclusively in humans, so it doesn’t cause pandemics, but it drives substantial waves of seasonal illness.
Influenza B is divided into two lineages, named after the places where they were first identified: B/Victoria and B/Yamagata. For decades, both lineages circulated and traded off dominance from season to season. That appears to have changed. Since March 2020, the Yamagata lineage has virtually disappeared. A systematic review published in The Lancet Microbe concluded that B/Yamagata is likely on the verge of extinction, if not extinct already. The handful of cases reported since 2020 were mostly traced to vaccine-derived virus or data entry errors. As a result, the World Health Organization now recommends removing the Yamagata component from flu vaccines, and the current trivalent vaccines for the 2024-2025 season include only a B/Victoria strain.
Symptoms of Flu B
Flu B produces the classic flu picture: fever, body aches, fatigue, cough, sore throat, and headache. Symptoms typically begin about two days after exposure, though the window ranges from one to four days. In a hospital-based study comparing influenza A and B patients, the two types overlapped so much in their presentation that laboratory testing was the only reliable way to tell them apart.
That said, there are some tendencies. In the same study, flu B patients were more likely to experience vomiting on admission (about 22% compared to 12% for flu A). Flu A patients, on the other hand, tended to have higher fevers and more shortness of breath. These are statistical trends across hundreds of patients, not reliable ways to self-diagnose which type you have.
Who Gets Hit Hardest
Adults over 65 consistently have the highest flu hospitalization rates regardless of virus type. But flu B has a notable pattern with younger age groups. CDC surveillance data from 2010 to 2023 shows that children and teens aged 5 to 17 were hospitalized for flu B at rates similar to those for the H1N1 strain of influenza A. During some seasons, flu B actually dominated in this age group: it accounted for over 50% of hospitalizations among 5- to 17-year-olds in both the 2012-13 and 2019-20 seasons.
Hospitalization rates for flu B swing dramatically from year to year. In the 2017-18 season, the rate reached 27.8 per 100,000 people. Just one season later, it dropped to 2.5 per 100,000. This unpredictability is one reason public health officials track flu B closely even when influenza A dominates the headlines.
One reassuring finding from hospital data: flu A patients tended to have slightly longer hospital stays (8 days vs. 7 days) and a higher in-hospital mortality rate (8.3% vs. 4.7%) during the 2017-18 season. While both types can be serious, flu B generally produced somewhat less severe disease in hospitalized adults during that period.
Complications to Watch For
Most people recover from flu B without complications, but the virus can lead to secondary problems, particularly in children. Ear infections are the most common, occurring in roughly 19% of pediatric flu B cases. Muscle inflammation, called myositis, affects between 4.5% and 15% of children and typically causes intense calf pain that makes walking difficult. It usually resolves on its own within a few days.
More serious but less common complications include pneumonia (from the virus itself or from a secondary bacterial infection), heart-related complications, kidney problems, and encephalitis. Respiratory failure and multi-organ failure can occur in severe cases, particularly when bacterial pneumonia develops on top of the viral infection.
How Long You’re Contagious
You can start spreading flu B to others about one day before your own symptoms appear, which is part of why the virus spreads so efficiently. Most people remain contagious for five to seven days after getting sick, with the first three days of illness being the most infectious period. Young children and people with weakened immune systems may shed the virus for longer.
Treatment Options
Several antiviral medications work against flu B. The most widely prescribed is oseltamivir (Tamiflu), which is taken as a pill twice daily for five days. It works by blocking an enzyme the virus needs to spread from cell to cell. Two other drugs in the same class, zanamivir (inhaled) and peramivir (given by IV), also target flu B, though peramivir has limited clinical trial data specifically for B infections.
A newer option, baloxavir (Xofluza), works through a different mechanism and has shown particular promise against flu B. In a randomized controlled trial, baloxavir reduced the time to symptom improvement by more than 24 hours compared to oseltamivir in adolescents and adults with flu B. It also has the advantage of being a single-dose treatment rather than a five-day course.
Antivirals are most effective when started within the first 48 hours of symptoms. They don’t cure the flu instantly, but they can shorten illness by a day or more and reduce the risk of serious complications, especially in high-risk groups like young children, older adults, and people with chronic health conditions.
Flu Vaccine and B/Victoria
The annual flu vaccine is the primary tool for preventing flu B. With the Yamagata lineage likely gone, current vaccines focus on the B/Victoria lineage. The 2024-2025 northern hemisphere vaccine includes a strain matched to B/Austria/1359417/2021, which is a B/Victoria-lineage virus. Because flu B mutates more slowly than influenza A, vaccine matches for the B component tend to be more stable from year to year, though the strain is still updated when surveillance data warrants it.
The shift from quadrivalent (four-component) to trivalent (three-component) vaccines is underway as manufacturers phase out the now-unnecessary Yamagata component. Some quadrivalent vaccines still in use during this transition period retain it, but the WHO has stated it is no longer warranted.