Influenza A and influenza B are the two types of flu virus responsible for seasonal epidemics, but they differ in important ways. Influenza A is more diverse, infects a wider range of animals, mutates faster, and is the only type that causes pandemics. Influenza B circulates almost exclusively in humans, changes more slowly, and typically produces milder illness in adults, though it can hit children hard.
How the Two Viruses Differ Genetically
Influenza A is classified into subtypes based on two surface proteins that help the virus enter and exit your cells. The two subtypes currently circulating in humans are H1N1 and H3N2, but there are many more combinations found in animals. Wild birds alone carry nearly all known subtypes, and the virus also circulates in pigs, horses, dogs, cats, seals, whales, and cows. Two rare subtypes have even been found exclusively in bats.
Influenza B doesn’t have subtypes. Instead, it splits into two lineages called Victoria and Yamagata, named after the places where they were first identified. Both lineages infect humans, and both can circulate during the same flu season. Because influenza B has far less genetic diversity than influenza A, it changes more slowly over time and poses no pandemic risk.
Why Only Influenza A Causes Pandemics
Both flu types evolve through a process called antigenic drift: small, gradual mutations that accumulate as the virus copies itself. This is why you can catch the flu more than once and why the vaccine is updated every year.
Influenza A, however, can also undergo something called antigenic shift. This is a sudden, major change that produces a version of the virus most people have never been exposed to. It happens when a flu virus from an animal population, often pigs or birds, gains the ability to infect and spread among humans. The 2009 H1N1 pandemic is a textbook example: a virus with gene segments from North American swine, Eurasian swine, human, and bird flu viruses emerged and spread globally because almost no one had existing immunity to it.
Influenza B doesn’t undergo antigenic shift. It circulates almost exclusively in humans, so there’s no animal reservoir mixing and shuffling genes to create a dramatically new virus. This makes influenza B more predictable from year to year.
Who Gets Each Type
Adults are more likely to catch influenza A, while children are more likely to catch influenza B. The reasons aren’t entirely clear, but it may relate to differences in prior immunity. Adults have had decades of exposure to various influenza A strains, yet the virus changes fast enough to keep reinfecting them. Children, with less accumulated immune history, appear more susceptible to influenza B.
Influenza A also tends to peak earlier in the flu season, typically in fall and early winter. Influenza B is more likely to strike later, in late winter and early spring. In some years, a second wave of illness driven by influenza B follows after the initial influenza A surge has subsided.
Symptoms and Severity
The core symptoms of both types are essentially the same: fever, body aches, cough, sore throat, fatigue, and congestion. You can’t reliably tell which type you have based on how you feel. A rapid flu test at a clinic can distinguish between A and B, though it doesn’t always matter for treatment since the same antiviral medications work against both.
That said, influenza A tends to cause more severe illness overall. It’s responsible for the majority of flu-related hospitalizations and deaths in a typical season. Influenza B generally produces milder symptoms in healthy adults, but it’s not harmless. Children under 5 and older adults can develop severe cases of influenza B, and it contributes to a meaningful share of pediatric flu deaths each year.
How the Vaccine Covers Both Types
Seasonal flu vaccines are designed to protect against both influenza A and influenza B. The World Health Organization reviews global surveillance data twice a year and recommends which specific strains to include. For the 2024-2025 Northern Hemisphere season, trivalent vaccines contain one H1N1 strain, one H3N2 strain, and one B/Victoria lineage strain. Quadrivalent vaccines add a B/Yamagata lineage strain as well.
Because influenza A mutates faster and undergoes more dramatic changes, the A components of the vaccine are updated more frequently. The B components tend to be more stable from season to season, though they’re still reviewed and adjusted when needed. Getting vaccinated each year remains the most effective way to reduce your risk of both types, especially since the circulating strains shift unpredictably.
The Bottom Line on A vs. B
Influenza A is the more dangerous and unpredictable of the two. It has a vast animal reservoir, mutates rapidly, and is the only type capable of sparking a pandemic. Influenza B is more stable, limited mostly to humans, and generally milder in adults, but it poses real risks to young children and older adults. Both types circulate every flu season, often peaking at different times, and both are covered by the seasonal vaccine.