There is no single fixed number of influenza strains. The flu virus exists as four broad types (A, B, C, and D), but within those types, constant genetic mutation produces new strains every year. Influenza A alone has 18 known versions of one surface protein and 11 of another, creating dozens of possible subtypes, each of which spawns countless individual strains over time. The total number of distinct strains ever cataloged runs into the thousands, and it grows every flu season.
Understanding why the count keeps changing matters more than pinning down a single number. Here’s how the system works, from the broadest categories down to the specific strains that show up in your annual flu shot.
The Four Types of Influenza
Influenza viruses fall into four types, labeled A through D. Types A and B are responsible for the seasonal flu epidemics that hit every winter. Type C causes mild respiratory illness and doesn’t drive epidemics. Type D primarily affects cattle and is not known to infect people.
Of the four, influenza A is by far the most diverse and the most dangerous. It’s the only type that has caused pandemics, and it circulates in birds, pigs, horses, and other animals in addition to humans. That animal reservoir is a big part of why it keeps generating new strains. Influenza B, by contrast, circulates almost exclusively in humans and changes more slowly.
Influenza A: Subtypes and Their Surface Proteins
Influenza A viruses are classified into subtypes based on two proteins that stud the virus’s outer surface. One protein, hemagglutinin (H), helps the virus latch onto your cells. The other, neuraminidase (N), helps newly made copies of the virus break free to infect more cells. Scientists have identified 18 distinct versions of H and 11 of N, which is where shorthand names like H1N1 and H3N2 come from.
Not all of those combinations circulate in people. Only two subtypes currently cause seasonal flu in humans: H1N1 and H3N2. But many other subtypes thrive in birds and occasionally jump to humans through direct contact with infected animals or contaminated environments. Since 1997, avian H5N1 has caused hundreds of human cases with a high fatality rate. H7N9 emerged in China in 2013 and resulted in over 1,500 reported human infections before being brought under control. Sporadic human cases have also been linked to H9N2, H5N6, H3N8, H5N2, H7N7, and several others.
Each of these subtypes contains many individual strains. A single subtype like H3N2 has produced hundreds of genetically distinct strains since it began circulating in humans in 1968.
Influenza B: Two Lineages, One Fading
Influenza B doesn’t have subtypes like influenza A. Instead, it split into two major lineages in the 1980s: B/Victoria and B/Yamagata. For decades these two lineages co-circulated, often trading dominance from one season to the next. Between roughly 2000 and 2020, B/Yamagata was the predominant lineage in 10 flu seasons, B/Victoria in six, and the two shared roughly equal circulation in four.
That pattern appears to have shifted. B/Yamagata has not been reliably detected in circulation since around 2020, and flu vaccines for the 2025-2026 season include only a B/Victoria component. Each lineage, like each influenza A subtype, contains many individual strains that evolve over time.
Why New Strains Keep Appearing
Two mechanisms drive the creation of new flu strains, and they work on very different timescales.
The first, called antigenic drift, happens constantly. Every time the virus copies itself inside an infected person, small errors creep into its genetic code. Most of these mutations are meaningless, but occasionally one changes the shape of a surface protein just enough that your immune system no longer recognizes it. Over a few years, these small changes accumulate until last season’s antibodies bind poorly or not at all to this season’s virus. Drift is the reason you need a new flu shot every year and the reason the World Health Organization reviews vaccine composition twice annually, once for the Northern Hemisphere and once for the Southern.
The second mechanism, antigenic shift, is rarer and more dramatic. It happens when two different influenza A viruses infect the same animal cell and swap large segments of their genetic material. The result can be a virus with a surface protein combination that most humans have never encountered. Shift events are what trigger pandemics. The 2009 H1N1 pandemic, for example, arose when gene segments from bird, pig, and human flu viruses reassorted in pigs and then jumped to people.
How Strains Are Named
Every identified flu strain gets a standardized name that reads like an address. It includes the virus type, the geographic location where it was collected, a sample number, the year of collection, and (for influenza A) the subtype. A human strain might be labeled A/Perth/16/2019 (H3N2). An animal strain adds the host: A/duck/Alberta/35/76 (H1N1).
This naming system is why you’ll sometimes see very specific labels on vaccine packaging. The FDA-recommended egg-based flu vaccine for the 2025-2026 U.S. season targets A/Victoria/4897/2022 (H1N1), A/Croatia/10136RV/2023 (H3N2), and B/Austria/1359417/2021 (B/Victoria lineage). Each of those names represents a single strain chosen because it’s the closest match to what’s expected to circulate that winter.
What This Means for Your Flu Shot
Because the virus constantly drifts into new strains, vaccine manufacturers can’t build a shot that covers every possible version. Instead, global surveillance networks track which strains are spreading in real time, and an international committee selects the best representative strains months before flu season begins. Current U.S. flu vaccines are trivalent, targeting two influenza A strains (one H1N1 and one H3N2) and one influenza B strain (B/Victoria). Until recently, many vaccines were quadrivalent and included a B/Yamagata strain as well, but that component was dropped after B/Yamagata stopped circulating.
The match between vaccine strains and circulating strains varies from year to year. In seasons when the match is close, the vaccine offers stronger protection. When the virus drifts significantly between the time the vaccine is formulated and the time flu season peaks, effectiveness drops. This ongoing mismatch is the core challenge of flu prevention and the reason researchers continue pursuing broader-acting vaccines.