There are four types of influenza virus: A, B, C, and D. Only two of them, influenza A and influenza B, cause the seasonal flu epidemics that sicken millions of people each year. Influenza C causes only mild illness, and influenza D doesn’t infect humans at all. Understanding what separates these types helps explain why flu season looks different every year and why you need a new vaccine each fall.
Influenza A: The Most Dangerous and Diverse
Influenza A is the only type capable of causing pandemics, and it’s responsible for the most severe flu seasons. What makes it so formidable is its extraordinary diversity. The virus is classified by two proteins on its surface: one that helps it enter your cells (H protein) and one that helps newly made viruses spread to other cells (N protein). There are 18 known versions of the H protein and 11 known versions of the N protein, creating a huge number of possible combinations.
Most of these combinations circulate in wild birds, which are the natural reservoir for influenza A. The virus can also infect and adapt to spread in pigs, horses, dogs, cats, seals, whales, and cows. Only a few subtypes regularly circulate in people. Right now, those are H1N1 and H3N2, the two strains included in every seasonal flu vaccine. H3N2 tends to mutate faster than H1N1 and is often linked to more severe flu seasons, particularly for older adults.
Occasionally, an animal influenza A virus jumps to humans. This is rare but gets significant attention because of the potential consequences. Since February 2024, 71 human cases of H5 bird flu have been reported in the United States, most of them detected through monitoring of people exposed to infected animals rather than through routine flu surveillance. While sporadic animal-to-human infections do happen, sustained person-to-person spread of these viruses has not occurred.
Influenza B: Slower to Change, Still Serious
Influenza B circulates only in humans, which limits its ability to surprise us with entirely new versions of itself. It isn’t divided into subtypes the way influenza A is. Instead, it splits into two lineages: B/Victoria and B/Yamagata. These lineages are genetically distinct enough that immunity to one doesn’t fully protect against the other.
For years, both lineages co-circulated, which is why flu vaccines used to include four strains (two A strains and one from each B lineage). That changed after the COVID-19 pandemic. B/Yamagata viruses have not been detected anywhere in the world since March 2020. Scientists believe the lineage may have gone extinct, likely pushed out by the same social distancing measures that suppressed other respiratory viruses. As a result, current U.S. flu vaccines are trivalent, containing one H1N1 strain, one H3N2 strain, and one B/Victoria strain.
Influenza B generally mutates more slowly than influenza A, which means the immune protection you build from a B infection or vaccination tends to last a bit longer. That said, influenza B still causes substantial illness every season and can be just as dangerous as influenza A for individual patients, especially children.
Influenza C and D: Minor Players
Influenza C infects humans but typically causes only mild respiratory symptoms, similar to a common cold. It’s most often detected in young children. Because it doesn’t cause epidemics or significant public health burden, it isn’t included in seasonal vaccines and rarely makes the news.
Influenza D primarily affects cattle and pigs. It has not been shown to infect humans, so it’s a concern for veterinary health rather than human medicine.
How Flu Viruses Change Over Time
One of the most important things to understand about influenza is that it doesn’t stay the same from year to year. Two distinct processes drive this constant evolution, and they explain why flu keeps coming back even if you’ve had it before.
The first is called antigenic drift. Every time the virus copies itself inside an infected person, small random mutations accumulate in its surface proteins. Over weeks and months, these tiny changes add up until the virus looks different enough that your immune system’s existing antibodies struggle to recognize it. Drift is the reason flu vaccines are reviewed and updated every year for both the Northern and Southern Hemispheres. It’s also why you can catch the flu multiple times throughout your life, even though each infection does build some immunity.
The second process, antigenic shift, is far more dramatic and only happens with influenza A. Shift occurs when a flu virus from an animal population gains the ability to infect humans, introducing surface proteins that are completely unfamiliar to human immune systems. Because most people have little or no pre-existing immunity to the new virus, shift can trigger a pandemic. There have been four flu pandemics in the past century: 1918, 1957, 1968, and 2009. Each was caused by a shifted influenza A virus.
What’s in the Current Flu Vaccine
For the 2024-2025 season, U.S. flu vaccines target three viruses: an H1N1 strain, an H3N2 strain, and a B/Victoria lineage strain. The exact strains differ slightly depending on how the vaccine is made. Egg-based vaccines use virus strains grown in chicken eggs, while cell-based and recombinant vaccines use strains grown in animal cells or manufactured using synthetic techniques. The target viruses are functionally equivalent, but the manufacturing process can affect how closely the vaccine matches what’s actually circulating.
Vaccine composition is decided months before flu season begins, based on global surveillance of which strains are spreading and how they’re evolving. In years when the match is close, the vaccine is more effective. In years when the circulating virus drifts between the time strains are selected and when flu season peaks, protection drops, though vaccination still reduces the risk of severe illness and hospitalization.
How Flu Types Are Identified
When you get a flu test at a clinic, the type of test determines how much information your doctor gets. Rapid antigen tests, the kind that give results in 15 to 30 minutes, can detect influenza but are less reliable. They’re more likely to miss infections, particularly when the amount of virus in your sample is low.
Molecular tests, which detect the virus’s genetic material, are significantly more accurate. Their reported sensitivity ranges from 66% to 100% depending on the specific product, and both sensitivity and specificity are high enough that false results are uncommon. These tests can distinguish between influenza A and B, which matters because it may influence treatment decisions. Public health laboratories use even more detailed molecular methods to identify the exact subtype (H1N1 vs. H3N2, for example) and track how viruses are changing over the course of a season.