There is no single fixed number of flu strains. Influenza viruses belong to four broad types (A, B, C, and D), but within those types the virus constantly mutates, producing new strains every year. At any given time, a handful of distinct strains circulate in humans, while hundreds more exist in birds, pigs, and other animals.
The confusion is understandable because “strain” gets used loosely. The real picture involves a hierarchy: types, subtypes, lineages, and then individual strains, each level more specific than the last. Here’s how it breaks down.
The Four Types of Influenza
Scientists classify influenza viruses into four types labeled A through D. Only two of them, A and B, drive the seasonal flu epidemics that hit every winter. Influenza C can infect people but generally causes mild illness and doesn’t spark epidemics. Influenza D primarily affects cattle and is not known to cause illness in humans at all.
Of the two types that matter most, influenza A is the bigger concern. It is the only type capable of causing pandemics, those global waves of infection that occur when a dramatically new virus emerges and most people have no immunity to it. Influenza B circulates alongside A every season but tends to cause somewhat narrower outbreaks.
Influenza A Subtypes
Influenza A viruses are categorized 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, that allows for 198 possible subtype combinations, and most of them have been found in wild birds, the natural reservoir for influenza A.
Only a small fraction of those subtypes regularly infect humans. The two circulating in people right now are H1N1 and H3N2. These are the subtypes you see referenced in flu vaccine formulations each year. The current H1N1 descends from the strain that caused the 2009 pandemic, which is why it’s sometimes written as “H1N1pdm09.” H3N2 has circulated in humans since the 1968 pandemic.
Other subtypes, like H5N1 and H7N9, occasionally jump from birds or other animals into humans. These spillover events are closely watched because if one of those subtypes gained the ability to spread easily between people, it could trigger a new pandemic.
Influenza B Lineages
Influenza B doesn’t have numbered subtypes like A. Instead, it split decades ago into two genetic lineages named after the places where they were first characterized: Victoria and Yamagata. Until 2020, both lineages circulated in the population simultaneously, which is why flu vaccines used to include a representative of each one.
The Yamagata lineage has not been reliably detected since the COVID-19 pandemic began. Many researchers believe it may have gone extinct, likely a side effect of the social distancing and masking measures that disrupted transmission of many respiratory viruses. As a result, the 2024–2025 flu vaccines are trivalent, containing only three components: an H1N1 strain, an H3N2 strain, and a single B/Victoria lineage strain.
Why the Number Keeps Changing
Within each subtype or lineage, individual strains emerge constantly through two mechanisms. The first, called antigenic drift, involves small, accumulating mutations that happen every time the virus copies itself inside a host. Over months and years, these tiny changes add up until your immune system no longer fully recognizes the virus. This is the main reason flu vaccines are reviewed and updated every year for both the Northern and Southern Hemispheres.
The second mechanism, antigenic shift, is rarer and more dramatic. It happens when an influenza A virus undergoes a major, abrupt change, typically when a subtype from an animal population acquires the ability to infect humans, or when two different subtypes swap genetic material inside a single host. Antigenic shift is what produces pandemic strains, because the resulting virus can be so different that virtually no one has pre-existing immunity.
Because of drift, the specific strains circulating this winter are not identical to the ones from last winter, even though they belong to the same subtypes. Each named strain, like A/Victoria/4897/2022, represents a snapshot of the virus at a particular time and place. Over the decades, thousands of distinct strains have been identified and catalogued.
How Many Strains Circulate in a Given Season
In any typical flu season, you’re dealing with three main groups circulating in humans: H1N1 strains, H3N2 strains, and B/Victoria strains. Within each group, one or two closely related variants tend to dominate, though multiple slightly different versions co-circulate. The vaccine targets one representative strain from each group to give the broadest practical protection.
For the 2024–2025 U.S. season, for example, egg-based vaccines use an A/Victoria/4897/2022-like virus for H1N1, an A/Thailand/8/2022-like virus for H3N2, and a B/Austria/1359417/2021-like virus for the Victoria lineage. Cell-based and recombinant vaccines use slightly different reference strains (A/Wisconsin/67/2022 for H1N1 and A/Massachusetts/18/2022 for H3N2) but target the same subtypes and lineage.
Strains vs. Subtypes: A Quick Distinction
When people ask “how many flu strains are there,” they often mean subtypes. The answer at that level is manageable: 18 H subtypes and 11 N subtypes for influenza A, two lineages for influenza B (though one may now be extinct), and types C and D rounding out the picture. But if you mean individual strains, the number is effectively uncountable and always growing, because the virus never stops mutating.
For practical purposes, what matters is that only three distinct viral groups typically cause seasonal flu in humans at any one time. The yearly vaccine is built around those three, and the composition shifts as the virus drifts into new territory.