Influenza A is one of two types of flu virus responsible for seasonal epidemics, and it’s the only type that causes pandemics. It circulates in humans, birds, pigs, and other animals, which gives it a unique ability to mutate in dramatic ways. The subtypes you’ve likely heard of, H1N1 and H3N2, are both influenza A viruses.
How Influenza A Works
The surface of an influenza A virus is covered in tiny protein spikes. Two of these proteins define the virus: hemagglutinin (H) and neuraminidase (N). Hemagglutinin acts like a key, latching onto cells in your respiratory tract and allowing the virus to slip inside. Neuraminidase does the opposite job: once the virus has replicated inside a cell, it cuts the newly made copies free so they can spread to neighboring cells. These two proteins are also what your immune system learns to recognize, which is why they matter so much for vaccines and immunity.
There are 18 known types of hemagglutinin and 11 types of neuraminidase, creating a huge number of possible combinations. That’s where subtype names like H1N1 and H3N2 come from. Only a few of these combinations circulate in humans at any given time, but the sheer variety in the animal world is what makes influenza A unpredictable.
H1N1 vs. H3N2: The Two Circulating Subtypes
The two influenza A subtypes that regularly infect people are H1N1 and H3N2. Both are included in every seasonal flu vaccine. In head-to-head comparisons, infection rates in school-age children are nearly identical between the two. H1N1 tends to infect adults and very young children (under two) at somewhat lower rates. H3N2, on the other hand, is more likely to cause febrile illness and lower respiratory symptoms in young children, though the overall severity in infected people is similar across subtypes.
The real difference between the two often comes down to epidemiology rather than the virus itself. How widely a subtype spreads through a community, and how well that season’s vaccine matches it, typically matters more than any inherent difference in how dangerous one subtype is compared to the other.
Why Influenza A Keeps Coming Back
Influenza A evolves roughly three times faster than influenza B. It changes through two distinct mechanisms, and understanding them explains why you can catch the flu year after year.
The first is called antigenic drift. Small, gradual mutations accumulate in the virus’s surface proteins, slightly altering how they look to your immune system. This is why getting the flu one year doesn’t fully protect you the next, and it’s the reason health authorities review and update the flu vaccine composition every year for both the Northern and Southern Hemispheres.
The second mechanism, antigenic shift, is far more dramatic and only happens with influenza A. This occurs when two different influenza A viruses (say, one from a bird and one from a human) infect the same host and swap genetic material. The result can be an entirely new subtype with surface proteins most people have never encountered. When this happens, widespread immunity is essentially nonexistent, and a pandemic can follow. There have been four flu pandemics in the past 100 years, all caused by influenza A.
Animal Reservoirs and Spillover
Wild aquatic birds are the primary natural reservoir for most influenza A subtypes. This is a key distinction from influenza B, which circulates almost exclusively in humans. Because influenza A thrives in such a wide range of animal hosts, eradication is considered impossible.
H5N1 avian influenza, for example, has caused hundreds of millions of poultry infections and has spread to marine mammals, dairy cattle, and other wild and domestic animals. Humans can become infected through direct contact with sick or dead animals, through contaminated environments like live bird markets, or even by handling infected poultry during food preparation. Workers on animal farms face elevated risk during activities like culling. Swine influenza viruses similarly spread to humans through close proximity to infected pigs, including at agricultural fairs where pigs are exhibited.
Symptoms and How Long You’re Contagious
Influenza A typically hits fast. The hallmark is an abrupt onset of fever, cough, chills or sweats, muscle aches, and general malaise. Symptoms usually begin about two days after exposure, though the incubation period can range from one to four days. In children, vomiting and diarrhea sometimes occur alongside the respiratory symptoms.
Most people feel sick for two to eight days. You can start spreading the virus to others about one day before your own symptoms appear, and you generally remain contagious for five to seven days after getting sick. The first three days of illness are the most contagious period. Young children and people with weakened immune systems may shed the virus for longer.
Treatment With Antivirals
Four antiviral medications are currently approved for treating influenza A. Three of them work by blocking the neuraminidase protein on the virus’s surface, preventing new viral copies from escaping infected cells. The fourth works by disrupting the virus’s ability to copy its genetic material.
The critical detail with all of them is timing. Clinical benefit is greatest when treatment starts within 48 hours of symptom onset. That said, antivirals can still help people with severe or progressive illness even when started later, particularly those who are hospitalized. Two older antiviral drugs (amantadine and rimantadine) are no longer recommended because circulating influenza A viruses have developed high levels of resistance to them.
How Vaccines Are Matched Each Year
Seasonal flu vaccines include two influenza A components (one H1N1 strain, one H3N2 strain) alongside an influenza B component. For the 2025-2026 U.S. flu season, the FDA moved to a trivalent (three-strain) formulation. The specific influenza A strains selected are chosen to match whatever is circulating most closely, and the egg-based and cell-based versions of the vaccine sometimes use slightly different reference strains to account for how viruses behave in different manufacturing systems.
Because antigenic drift constantly reshapes the virus’s surface proteins, the strains in the vaccine are reevaluated every year. Some seasons the match is close and the vaccine works well. Other seasons the virus drifts enough between strain selection and flu season that effectiveness drops, though vaccination still tends to reduce the severity of illness even when the match is imperfect.