Flu B means you’ve been diagnosed with influenza type B, one of the four types of influenza virus. It causes the same core illness as flu A (the other common type), with fever, body aches, cough, and fatigue, but it differs in important ways: it mutates more slowly, only infects humans, and cannot cause pandemics. For most people, a flu B infection feels and resolves much like any other bout of the flu, typically within one to two weeks.
The Four Types of Influenza
There are four types of influenza virus: A, B, C, and D. Types A and B are responsible for the seasonal flu outbreaks that hit every winter. Type C causes only mild illness and doesn’t drive epidemics. Type D primarily affects cattle and isn’t known to infect people.
When a rapid flu test or PCR test comes back positive, it usually specifies whether you have type A or type B. That distinction matters for tracking how flu spreads through communities, though the treatment approach is largely the same for both.
How Flu B Differs From Flu A
The biggest difference is scope. Flu A infects humans, birds, pigs, and other animals, and that cross-species mixing lets it undergo dramatic genetic shifts. Those shifts are what cause pandemics. Flu B infects only humans, which limits its ability to change rapidly and means it will never trigger a pandemic.
Flu B also mutates more slowly than flu A, particularly compared to the H3N2 subtype of flu A, which is notorious for changing fast enough to dodge immune defenses year after year. Because flu B is more genetically stable, your immune system has a somewhat easier time recognizing it from one season to the next.
That said, “slower mutation” doesn’t mean harmless. Flu B still causes severe illness and hospitalizations every year, and it tends to circulate later in the flu season, often peaking in February or March. It also hits children and adolescents particularly hard compared to flu A.
Lineages: Victoria and Yamagata
Flu B is divided into two lineages rather than subtypes: B/Victoria and B/Yamagata. For decades, both lineages circulated simultaneously, which made it difficult to predict which one would dominate in a given season. That unpredictability is why flu vaccines switched from covering three strains to four (adding a second B lineage) starting around 2012.
The Yamagata lineage, however, has not been confirmed in circulation since March 2020. Surveillance data shows that no verified Yamagata sequences have been uploaded to global databases since then, and the handful of sporadic detections reported in 2021 and 2022 were likely linked to the nasal spray vaccine rather than actual community spread. As a result, all flu vaccines for the 2025-26 season have dropped the Yamagata component and gone back to three strains: two flu A strains and one flu B strain from the Victoria lineage.
Symptoms of Flu B
Flu B symptoms are virtually identical to flu A. They come on abruptly, usually about two days after exposure (though the incubation period ranges from one to four days), and typically include:
- Fever and chills
- Muscle aches
- Headache
- Dry cough
- Sore throat
- Runny or stuffy nose
- Fatigue and general malaise
Young children with flu B may also experience nausea, vomiting, or diarrhea alongside respiratory symptoms. In older adults living in care facilities, the flu can show up in less obvious ways, such as confusion, behavioral changes, or loss of appetite, without the classic fever-and-cough presentation.
You’re contagious starting about one day before your symptoms appear and for five to seven days after getting sick, which means you can spread the virus before you even know you have it.
Complications to Watch For
Most people recover from flu B without incident, but complications do happen. The most common serious one is bacterial pneumonia. More than 20% of flu patients develop a secondary bacterial lung infection, and being co-infected with both the flu virus and bacteria raises the risk of death by roughly 2.6 to 3.4 times compared to the flu alone. Nearly a quarter of seasonal flu deaths are attributed to these secondary bacterial infections.
One complication more closely tied to flu B than flu A is Reye syndrome, a rare but serious condition that affects the brain and liver. It’s associated with taking aspirin or aspirin-containing products during a flu infection, which is why aspirin should never be given to children or teenagers who have the flu.
Signs that flu B is becoming more serious include difficulty breathing, chest pain, persistent vomiting, sudden dizziness, or symptoms that improve and then return with a worse fever and cough (a pattern that can signal a secondary bacterial infection setting in).
Treatment and Recovery
Antiviral medications work against both flu A and flu B. They’re most effective when started within the first 48 hours of symptoms, and they can shorten the illness by about a day while reducing the risk of complications. Your doctor is most likely to prescribe antivirals if you’re in a higher-risk group: young children, adults over 65, pregnant women, or people with chronic health conditions.
For otherwise healthy people, flu B is managed the same way you’d handle any flu. Rest, fluids, and over-the-counter fever reducers (avoiding aspirin in anyone under 18) are the standard approach. Most people feel significantly better within a week, though fatigue and a lingering cough can stick around for another week or two after the worst is over.
Does the Flu Vaccine Cover Flu B?
Yes. Every seasonal flu vaccine includes at least one flu B component. Current vaccines for the 2025-26 season contain a B/Victoria lineage strain alongside two flu A strains. Because flu B mutates more slowly than flu A, the vaccine match tends to be more reliable from year to year. That slower rate of change is one reason why getting vaccinated each season offers relatively consistent protection against flu B, even in years when the flu A component is a less-than-perfect match.