Influenza A and influenza B are the two types of flu virus responsible for seasonal epidemics, but they differ in important ways: how they mutate, what animals they infect, how severe they tend to be, and whether they can cause pandemics. Influenza A is more common, more genetically diverse, and the only type that has ever triggered a global pandemic. Influenza B circulates alongside it every winter and can still cause serious illness, but it evolves more slowly and infects a narrower range of hosts.
Surface Proteins and Subtypes
Both viruses use proteins on their surface to latch onto your cells and spread infection. The two key proteins are hemagglutinin (H) and neuraminidase (N). Influenza A has enormous variety here: 18 different hemagglutinin subtypes and 11 neuraminidase subtypes. That’s why you see names like H1N1 or H3N2, each label describing a specific combination of surface proteins. This diversity is central to why influenza A is so unpredictable from year to year.
Influenza B doesn’t have subtypes at all. Instead, it’s divided into two lineages: B/Victoria and B/Yamagata. These lineages are far more similar to each other than, say, H1N1 is to H5N1. The practical result is that influenza B presents fewer moving targets for your immune system and for vaccine designers.
Animal Reservoirs and Host Range
One of the biggest differences between the two types is what species they infect. Influenza A viruses have a natural reservoir in wild aquatic birds, and they also circulate in pigs, horses, dogs, and other animals. This broad host range is not just a wildlife concern. It’s the reason entirely new flu strains can jump into human populations. Pigs, for example, can be infected by bird, human, and swine flu viruses simultaneously, creating a mixing vessel where gene segments shuffle into novel combinations.
Influenza B, by contrast, circulates almost exclusively among humans. Without that large animal reservoir, it has far fewer opportunities to pick up dramatically new genetic material. This single difference explains much of the gap in pandemic risk between the two types.
How Each Virus Evolves
Both influenza A and B undergo a process called antigenic drift, where small mutations accumulate in their surface proteins over time. These gradual changes are the reason you need a new flu shot every year. Even minor shifts in the virus’s outer coat can help it slip past antibodies your body made during a previous infection.
Influenza A, however, is also capable of something called antigenic shift, a sudden, dramatic reshuffling of its genetic material. This happens when two different influenza A subtypes infect the same host at the same time and swap entire gene segments. The result can be a virus with surface proteins that most humans have never encountered before. When that virus spreads efficiently, the outcome is a pandemic.
The 2009 H1N1 pandemic is a clear example. The virus that emerged contained genes from North American swine flu, Eurasian swine flu, human flu, and avian flu viruses. Because it was so genetically distinct from circulating strains, most people had little or no pre-existing immunity. Influenza B cannot undergo antigenic shift in this way, which is why no influenza B pandemic has ever been recorded.
Pandemic History
Every known flu pandemic has been caused by influenza A. The major ones include the 1957 H2N2 pandemic, the 1968 H3N2 pandemic (whose descendant viruses still circulate today), and the 2009 H1N1 pandemic. Influenza A was first isolated in 1933, and influenza B followed a few years later in 1940. In the decades since, influenza B has contributed to seasonal outbreaks but has never been the cause of a global pandemic.
Severity and Who Gets Hit Hardest
In most flu seasons, influenza A causes more illness and more deaths than influenza B. CDC surveillance of pediatric flu deaths found that influenza A viruses accounted for 86% of deaths in children across recent seasons. In all but two seasons since tracking began, influenza A has been associated with more pediatric deaths than influenza B.
That said, influenza B is not mild. It can cause severe illness, hospitalization, and death, particularly in children and older adults. In some years, influenza B arrives later in the season and drives a second wave of illness after influenza A activity has peaked. The two seasons where influenza B was linked to more pediatric deaths than influenza A (2012–13 and 2019–20) show it’s capable of being the dominant threat in certain years.
How the Flu Vaccine Covers Both
Current flu vaccines in the United States are trivalent, meaning they protect against three strains: two influenza A subtypes and one influenza B lineage. For the 2024–2025 season, vaccines include an H1N1 strain, an H3N2 strain, and a B/Victoria lineage strain. The specific virus strains are updated each year based on global surveillance of which variants are circulating and evolving.
Because influenza A has more subtypes and mutates more unpredictably, getting the A components right is often the bigger challenge for vaccine designers. In years when the vaccine is a poor match for the dominant circulating strain, effectiveness drops, and that mismatch more commonly involves influenza A.
Testing and Diagnosis
Rapid flu tests available at clinics and pharmacies can distinguish between influenza A and influenza B, though they’re not equally accurate for both. These rapid tests are required to detect influenza A with at least 80% sensitivity when compared against the gold-standard lab method (RT-PCR), and they must meet the same 80% threshold for influenza B. Specificity is higher for both: at least 95%, meaning false positives are uncommon.
A negative rapid test doesn’t rule out the flu, especially early in the illness when viral levels may be low. If your doctor suspects flu despite a negative rapid result, they may order a more sensitive molecular test. From a treatment standpoint, the distinction between A and B matters less for individual patients. The same antiviral medications work against both types, and the decision to prescribe them depends more on how sick you are and how long you’ve had symptoms than on which type you have.
Quick Comparison
- Subtypes: Influenza A has 18 H and 11 N subtypes. Influenza B has no subtypes, only two lineages (Victoria and Yamagata).
- Animal hosts: Influenza A circulates in birds, pigs, horses, and other animals. Influenza B is essentially limited to humans.
- Mutation style: Both undergo gradual drift. Only influenza A undergoes sudden antigenic shift.
- Pandemic potential: Every recorded flu pandemic has been caused by influenza A. Influenza B has never caused one.
- Seasonal impact: Influenza A typically causes more hospitalizations and deaths, but influenza B can dominate in certain seasons.
- Vaccine strains: Current U.S. vaccines include two influenza A strains and one influenza B strain.