There is no single fixed number of flu strains because the virus mutates constantly, generating new strains every year. But the flu family has a clear structure: four types of influenza virus (A, B, C, and D), with influenza A alone capable of producing 198 possible subtype combinations based on its surface proteins. Within those subtypes, individual strains number in the thousands and keep climbing as the virus evolves.
Four Types of Influenza Virus
Flu viruses are sorted into four broad types, each labeled with a letter.
- Influenza A is the most diverse and the most dangerous. It infects humans, birds, pigs, and other animals, and it is the only type that causes pandemics. Two subtypes currently circulate in people: A(H1N1) and A(H3N2).
- Influenza B infects only humans and is responsible for seasonal outbreaks alongside influenza A. It has two main lineages, Victoria and Yamagata, though the Yamagata lineage has not been detected in circulation since 2020 and is being dropped from vaccine formulations.
- Influenza C causes mild respiratory illness in humans and is not tracked the way A and B are. It does not cause epidemics.
- Influenza D primarily affects cattle and has also been found in pigs, sheep, goats, and horses. It is not known to cause illness in people.
When people talk about “the flu,” they almost always mean influenza A or B. These are the two types covered by the seasonal flu vaccine, and they drive the illness and hospitalizations that peak each winter.
Why Influenza A Has So Many Subtypes
Influenza A viruses are classified by two proteins on their surface: hemagglutinin (H) and neuraminidase (N). There are 18 known versions of H and 11 known versions of N. In theory, any H can pair with any N, creating up to 198 possible subtype combinations. Each combination gets a shorthand name like H1N1, H3N2, or H5N1.
Most of those 198 combinations circulate in wild birds, which serve as the natural reservoir for influenza A. Only a handful have ever adapted well enough to spread easily among humans. Right now, just two subtypes circulate in people year after year: H1N1 and H3N2. Others, like H5N1 (bird flu) and H7N9, occasionally jump from animals to humans but have not gained the ability to spread efficiently from person to person.
Subtypes vs. Strains
A subtype like H3N2 is a broad category. Within it, there are many individual strains, each slightly different at the genetic level. A specific strain is identified by its type, host (if not human), geographic origin, a lab reference number, and the year it was collected. So a strain name like A/Thailand/8/2022 (H3N2) tells you it is an influenza A virus, subtype H3N2, isolated in Thailand in 2022. When a strain originates in an animal like a pig and then infects a person, a lowercase “v” for “variant” is added to distinguish it from the seasonal human version.
Because the virus mutates continuously, new strains emerge every year. There is no master count of every flu strain ever identified, but global surveillance networks catalog thousands of them. The practical number that matters to most people is much smaller: each flu season, the World Health Organization selects a handful of strains to include in the vaccine, representing the versions most likely to circulate in the coming months.
How New Strains Are Created
Flu viruses change through two main processes, and understanding them explains why the strain count never stops growing.
The first is called antigenic drift. Every time the virus copies itself inside a host, small mutations accumulate in its surface proteins. Over months and years, these tiny changes add up until your immune system no longer recognizes the virus as well as it once did. Drift is the reason flu comes back every season and why the vaccine formula is reviewed and updated each year for both the Northern and Southern Hemispheres.
The second process, antigenic shift, is rarer and more dramatic. It happens only with influenza A. If two different flu subtypes infect the same animal cell at the same time, they can swap major genetic segments and produce a virus with a completely new combination of surface proteins. Because most people have no prior immunity to the new virus, shift can trigger a pandemic. Every major flu pandemic on record has been the result of antigenic shift introducing a subtype that was new to the human population.
Pandemic Strains in History
Four influenza pandemics have occurred since 1900, each caused by a different subtype or a dramatically changed version of an existing one:
- 1918 (H1N1) killed an estimated 50 million people worldwide and remains the deadliest flu pandemic in modern history.
- 1957 (H2N2) emerged when an avian H2N2 virus acquired the ability to spread among humans.
- 1968 (H3N2) arose from a shift that introduced a new hemagglutinin into the circulating virus. H3N2 is still circulating today, though the strains have drifted far from the 1968 original.
- 2009 (H1N1) was a novel reassortment of swine, avian, and human flu genes. It replaced the previous seasonal H1N1 lineage and is now referred to as H1N1pdm09.
What the Annual Vaccine Covers
For the 2024–2025 Northern Hemisphere season, the WHO recommended a trivalent (three-strain) vaccine containing one A(H1N1)pdm09 strain, one A(H3N2) strain, and one B/Victoria lineage strain. Some quadrivalent vaccines still in production include a B/Yamagata component, but because B/Yamagata appears to have stopped circulating, the WHO has advised that it is no longer needed.
The specific strains chosen for the vaccine change from year to year as surveillance labs track which versions of the virus are gaining ground. This is why getting vaccinated annually matters: last year’s shot may target strains that have since drifted enough to reduce your protection. Each season’s vaccine is essentially a best prediction of which three or four strains, out of the many circulating worldwide, will dominate in the months ahead.