Type B influenza is one of two flu viruses responsible for seasonal epidemics each winter, alongside the more familiar type A. It infects only humans, causes the same core symptoms as other flu strains, and tends to peak later in the season. While type A gets more attention because of its pandemic potential, type B is a significant cause of illness, hospitalization, and death, particularly in children.
How Type B Differs From Type A
Influenza viruses come in four types: A, B, C, and D. Only A and B cause the seasonal flu outbreaks that sweep through communities each winter. Types C and D are either mild or don’t infect humans at all.
The biggest distinction between A and B is their host range. Type A circulates in birds, pigs, horses, and other animals, which gives the virus more opportunities to swap genes between species and produce dramatically new strains. That’s why all flu pandemics have been caused by type A. Type B, by contrast, infects humans almost exclusively. Seals can catch it, but they don’t appear to play a meaningful role in spreading or reshaping the virus. Because type B lacks a wild animal reservoir, it mutates more slowly than type A and doesn’t produce the sudden, radical shifts that spark pandemics.
Type A is classified into subtypes based on two surface proteins (H1N1 and H3N2 are the ones currently circulating in people). Type B doesn’t have subtypes. Instead, it splits into two lineages: B/Victoria and B/Yamagata, named after the locations where each was first identified. As of 2024, B/Yamagata appears to have stopped circulating entirely, leaving B/Victoria as the only active lineage. This is why flu vaccines have shifted from four-strain (quadrivalent) formulas to three-strain (trivalent) ones starting with the 2025-26 season.
When Type B Hits During Flu Season
Type B typically arrives later than type A. During the 2024-25 season, type A specimens peaked in late January at a positivity rate of about 30%, while type B didn’t reach its peak until late March, topping out at just 4.3%. This pattern is fairly consistent from year to year: type A dominates the early and middle parts of the season, and type B circulates at lower levels, often extending the tail end of flu season into spring.
That later timing matters because people sometimes let their guard down after the initial winter surge passes. If you catch the flu in March or April, there’s a good chance it’s type B.
Symptoms and Severity
From the patient’s perspective, type B flu feels essentially the same as type A: fever, body aches, cough, sore throat, fatigue, and sometimes vomiting or diarrhea (especially in children). There’s no reliable way to tell the two apart based on symptoms alone.
Overall, type A causes more total deaths and hospitalizations simply because it infects far more people each season. But type B is not mild. It hits children particularly hard. CDC surveillance of pediatric flu deaths over multiple seasons shows that type B is consistently responsible for a significant share. In the 2024-25 season, for example, 38 of 280 pediatric flu deaths (about 14%) were linked to type B. In two past seasons (2012-13 and 2019-20), type B actually caused more pediatric deaths than type A. For an individual child who catches it, type B can be just as dangerous as type A.
How It’s Diagnosed
Rapid influenza diagnostic tests, the kind you might get at an urgent care clinic or your doctor’s office, can detect type B, but they’re somewhat less reliable for it than for type A. The FDA requires these rapid tests to catch at least 80% of type B infections when compared against the gold-standard lab method (RT-PCR). For type A, the bar is the same 80% against RT-PCR but 90% against viral culture. In practical terms, this means a negative rapid test doesn’t completely rule out type B, especially if you have classic flu symptoms during an active season. Molecular tests like PCR are more accurate and can confirm the diagnosis when it matters clinically.
Treatment Options
The same antiviral medications used for type A also work against type B. The most commonly prescribed is oseltamivir (Tamiflu), taken as a pill twice daily for five days. It works by blocking a protein the virus needs to spread from cell to cell.
A newer option, baloxavir (Xofluza), works through a different mechanism, interfering with how the virus copies its genetic material. It’s taken as a single dose, which is a practical advantage. In a clinical trial comparing the two, baloxavir reduced symptom duration in adolescents and adults with type B infections by more than 24 hours compared to oseltamivir. That’s a notable difference, and it suggests baloxavir may be the stronger choice specifically for type B, though both remain effective.
Antivirals work best when started within the first 48 hours of symptoms, so early testing and treatment matter.
Vaccination and Prevention
The annual flu vaccine is the primary defense against type B. For years, vaccines included strains from both the B/Victoria and B/Yamagata lineages to cover the possibility that either could dominate a given season. Now that B/Yamagata has disappeared from circulation, all flu vaccines for the 2025-26 season and beyond are trivalent: one type A H1N1 strain, one type A H3N2 strain, and one B/Victoria strain.
Because type B mutates more slowly than type A, vaccine matches for the B component tend to be more reliable from year to year. That slower evolution is one small advantage in the effort to prevent infections. Standard prevention measures also apply: hand washing, avoiding close contact with sick individuals, and staying home when you’re symptomatic. Since type B often peaks later in the season, getting vaccinated even in January or February can still provide meaningful protection during the spring wave.