Influenza B is one of two types of flu virus responsible for seasonal epidemics in humans. Unlike influenza A, which circulates in birds, pigs, and other animals, influenza B infects almost exclusively humans. It causes the same core illness as influenza A, with fever, body aches, and respiratory symptoms, but it behaves differently in terms of who it hits hardest, how quickly it evolves, and when it tends to peak during flu season.
How Influenza B Differs From Influenza A
Influenza A and B both cause seasonal flu, but they differ in important ways. Influenza A has a much larger genetic toolkit. It circulates in animal populations and can reshuffle its genes dramatically, which is why influenza A is behind all known flu pandemics. Influenza B lacks that animal reservoir and changes more slowly over time. Its seasonal antigenic drift, the gradual mutation that lets it dodge your immune system, happens at a lower rate than influenza A. This slower evolution is one reason influenza B doesn’t cause pandemics, but it still causes significant illness every year.
Influenza B viruses split into two distinct lineages back in the 1970s: B/Victoria and B/Yamagata. These two lineages are different enough that immunity to one doesn’t reliably protect you against the other. B/Victoria tends to infect children at higher rates, while B/Yamagata historically affected older adults more often. B/Victoria also evolves faster, with frequent genetic changes in the part of the virus your immune system recognizes, making it harder for vaccines to keep up with.
The Yamagata Lineage Has Likely Gone Extinct
One of the biggest recent developments in flu biology is that B/Yamagata appears to have disappeared. No confirmed detections of Yamagata have occurred globally since the COVID-19 pandemic, likely because the combination of masking, social distancing, and reduced travel broke its chain of transmission. As a result, the World Health Organization recommended removing Yamagata from flu vaccines. For the 2024-2025 flu season, all U.S. flu vaccines are trivalent (three-component), covering two influenza A strains and a single influenza B strain from the Victoria lineage.
Symptoms of Influenza B
Influenza B produces the same general illness as influenza A. It typically comes on suddenly, with fever, chills, muscle aches, headache, fatigue, a dry cough, sore throat, and a runny or stuffy nose. Most healthy people recover within a week or two without treatment.
In children, influenza B is more likely to cause gastrointestinal symptoms like abdominal pain, vomiting, and diarrhea compared to what adults typically experience. Muscle inflammation (myositis), which causes severe leg pain and difficulty walking, occurs in roughly 5 to 15 percent of children with influenza B. Febrile seizures happen in about 10 percent of affected children. One rare but notable association: Reye syndrome, a serious neurological condition linked to aspirin use during viral illness, is more common with influenza B than influenza A. This is why aspirin is avoided in children with flu symptoms.
Who Faces the Highest Risk
Young children bear a disproportionate burden from influenza B. Globally, children exposed to influenza B tend to have more severe disease than adults. While most cases cause mild to moderate respiratory illness in otherwise healthy kids, complications can include pneumonia, ear infections, and, rarely, encephalitis. Children with chronic medical conditions or weakened immune systems face the greatest risk of severe outcomes, including respiratory failure and secondary bacterial infections that develop after the initial viral illness.
Adults can certainly get sick from influenza B, but healthy adults generally experience it as a standard flu illness. Older adults, pregnant individuals, and people with underlying health conditions remain at elevated risk for complications from any type of flu, including influenza B.
When Influenza B Peaks
Influenza B tends to peak later in the flu season than influenza A. In temperate climates, influenza A often dominates in the fall and early winter, while influenza B activity typically rises in winter and spring. This means you can catch the flu well into March or April during a season when influenza B is circulating heavily. The two types don’t follow a strict schedule, though, and some years influenza B circulates earlier or co-circulates with influenza A throughout the season.
Testing and Diagnosis
Rapid flu tests, the kind you might get at a clinic or urgent care visit, can detect influenza B but aren’t perfect. These rapid antigen tests are required to have at least 80 percent sensitivity and 95 percent specificity compared to the gold-standard PCR test. In practical terms, that means a positive rapid test result is very reliable, but a negative result doesn’t rule out the flu. If your doctor suspects influenza despite a negative rapid test, a PCR-based test can confirm or rule it out with much higher accuracy. Most rapid tests can distinguish between influenza A and B, which can matter for treatment decisions.
Antiviral Treatment
The same antiviral medications used for influenza A also work against influenza B. Treatment is most effective when started within 48 hours of symptom onset, though it can still help in severe cases even when started later. Two main options exist for outpatient treatment: a five-day course of an oral antiviral taken twice daily, or a newer single-dose oral medication that works through a different mechanism. Both can shorten illness duration and reduce the risk of complications, particularly in people at high risk.
For most healthy adults and older children with mild symptoms, antiviral treatment isn’t always necessary. It’s typically recommended for people at higher risk of complications: young children, older adults, pregnant individuals, and those with chronic health conditions.
Vaccine Protection Against Influenza B
Annual flu vaccination remains the primary way to protect yourself against influenza B. Because the Victoria lineage continues to evolve with frequent genetic changes, the specific vaccine strain is updated regularly. The 2024-2025 vaccines use a B/Austria/1359417/2021-like virus from the Victoria lineage. Since the Yamagata lineage is no longer circulating, the shift from four-component to three-component vaccines doesn’t leave a gap in protection. It simply reflects the current reality of which viruses are actually out there.
Because influenza B mutates more slowly than influenza A, vaccine matches for the B component tend to be somewhat more stable from year to year. That said, B/Victoria’s pattern of frequent insertions and deletions in key parts of the virus means mismatches can still occur, reducing vaccine effectiveness in some seasons.