Influenza A and influenza B are both responsible for seasonal flu, and they cause very similar symptoms. The real differences between them lie in how they spread, who they tend to hit hardest, how quickly they mutate, and whether they can trigger pandemics. In most flu seasons, influenza A causes the majority of infections and hospitalizations, but influenza B can be just as severe on an individual level.
How the Two Viruses Are Classified
Influenza A is divided into subtypes based on two proteins on its surface. The two subtypes that currently circulate in humans are A(H1N1) and A(H3N2). These surface proteins are what your immune system recognizes, and they’re also what make influenza A so unpredictable from year to year.
Influenza B works differently. It isn’t divided into subtypes at all. Instead, it’s split into two lineages: B/Victoria and B/Yamagata. These lineages change more slowly than influenza A subtypes, which is one reason flu B tends to be more stable and predictable over time.
Why Only Flu A Causes Pandemics
This is the single biggest distinction between the two types. Influenza A infects birds, pigs, horses, and other animals in addition to humans. Influenza B circulates almost exclusively in people. That animal reservoir matters enormously, because when a flu virus jumping between species picks up new surface proteins, it can change so dramatically that human immune systems don’t recognize it at all. This kind of abrupt, major genetic change is called antigenic shift, and it only happens with influenza A.
Every flu pandemic on record, from the 1918 Spanish flu to the 2009 H1N1 pandemic, was caused by an influenza A virus. The shift happens when a virus from an animal population gains the ability to infect humans with surface proteins different enough that most people have no existing immunity. Influenza B mutates too, but only through slower, smaller changes that your immune system can partially recognize from previous infections.
Symptoms Are Nearly Identical
If you’re lying in bed with the flu, you almost certainly can’t tell whether you have type A or type B based on how you feel. Both cause fever, body aches, cough, sore throat, fatigue, and headache. A CDC study comparing hospitalized adults with flu A and flu B found that the two caused equally severe disease outcomes. Length of hospital stay and ICU admission rates were comparable between the types.
There are some general tendencies. Flu A is sometimes associated with higher fevers, particularly during widespread outbreaks. Flu B is more commonly linked to gastrointestinal symptoms like nausea and vomiting, especially in children. But these are tendencies, not reliable rules. Either type can cause mild illness or land someone in the hospital. Both usually last about a week, though fatigue and cough can linger. How sick you get depends more on your age, overall health, and immune history than on which type of virus you caught.
Flu A Drives More Hospitalizations
While individual severity is similar, influenza A is responsible for far more total hospitalizations simply because it infects more people in most seasons. CDC hospitalization surveillance data from 2010 through 2023 makes this clear. In the 2017-18 season, for example, the hospitalization rate for influenza A was 74.4 per 100,000 people compared to 27.8 for influenza B. In 2022-23, it was 59.5 versus just 2.2. Across every season tracked, influenza A consistently produced higher hospitalization rates, often by a factor of five to ten.
The A(H3N2) subtype contributed the largest share of hospitalizations in every age group, followed by A(H1N1) and then influenza B. This pattern holds year after year, which is why public health planning tends to focus heavily on influenza A.
Flu B Hits Children Harder
One important exception to the “flu A is worse” narrative involves kids. Influenza B can be particularly severe in children, and certain lineages skew heavily toward younger patients. Research from CIDRAP found that patients infected with B/Victoria may be up to 20 years younger on average than patients infected by other strains. B/Victoria tends to drive more flu activity in schools, while B/Yamagata historically affected older adults in long-term care facilities.
The reason likely involves immune memory. Children may have never been exposed to the B lineage circulating in a given season, so they have no built-in defense. Richard Webby, a researcher at St. Jude Children’s Research Hospital, has pointed to immunologic imprinting as a factor. B/Victoria, for instance, hasn’t dominated a U.S. flu season since 1992-93, meaning children born after that have had little opportunity to build immunity against it. When it does circulate, they’re especially vulnerable.
Timing During Flu Season
Both types circulate during the same general flu season (roughly October through May in the Northern Hemisphere), but they don’t always peak at the same time. Influenza A typically dominates the early and middle portions of flu season, while influenza B often picks up later, sometimes peaking in the spring. In some unusual seasons, influenza B dominates early, which can catch health systems off guard since it tends to hit schools and pediatric populations harder.
Testing Can Miss Both Types
Rapid influenza diagnostic tests can distinguish between type A and type B, but they aren’t perfect. Their sensitivity runs around 50 to 70 percent, meaning they miss a significant number of true infections. False negatives are common, particularly when flu is circulating widely in the community. The FDA now requires that all rapid tests achieve at least 80 percent sensitivity compared to more advanced lab methods, but older tests still fall short. A negative rapid test during flu season doesn’t reliably rule out infection with either type.
More precise tests like RT-PCR are highly accurate and can identify both the type and subtype, but results take longer. In practice, treatment decisions are often made based on symptoms and timing rather than waiting for definitive test results.
How Vaccines Cover Both Types
Flu vaccines are designed to protect against both influenza A and influenza B. For the 2025-26 season, all flu vaccines in the U.S. are trivalent, meaning they cover three viruses: an A(H1N1) strain, an A(H3N2) strain, and a B/Victoria lineage virus. Previous seasons used quadrivalent vaccines that also included B/Yamagata, but that lineage has not been detected in circulation recently, so it was dropped.
Because influenza A mutates faster and more dramatically, the A components of the vaccine are updated more frequently. The vaccine composition is reassessed twice a year by the World Health Organization to match the strains expected to circulate. Getting vaccinated each season remains the most effective way to reduce your risk of both types, even though vaccine effectiveness varies from year to year depending on how well the selected strains match what actually circulates.