There are four types of influenza virus (A, B, C, and D), but the number of variants within those types is enormous and constantly growing. Influenza A alone has 18 known subtypes of one surface protein and 11 of another, creating the potential for 198 combinations. More than 130 of those combinations have already been identified in nature. When you factor in the smaller genetic changes that happen every flu season, the total number of distinct flu variants that have circulated over the decades is effectively uncountable.
The Four Types of Influenza
Flu viruses are grouped into four broad types: A, B, C, and D. Only A and B cause the seasonal epidemics that send millions of people to bed each winter. Influenza C causes mild respiratory illness and is rarely tested for or reported. Influenza D primarily affects cattle and is not known to infect people.
Of the two types that matter most for human health, influenza A is by far the more diverse and dangerous. It’s the only type capable of triggering pandemics, and it circulates in birds, pigs, horses, and other animals in addition to humans. Influenza B circulates almost exclusively in people and changes more slowly.
Influenza A: The Most Diverse Type
Influenza A viruses are classified by two proteins that sit on their outer surface. One helps the virus enter your cells, and the other helps newly made copies break free to infect more cells. Scientists have identified 18 versions of the first protein (labeled H1 through H18) and 11 versions of the second (N1 through N11). That’s where the familiar shorthand like H1N1 or H3N2 comes from.
Theoretically, those 18 and 11 versions could combine into 198 distinct subtypes. In practice, more than 130 combinations have been found in nature, mostly in wild birds, which serve as the virus’s natural reservoir. The reason not all 198 exist (yet) is partly chance, but flu viruses readily swap genetic material when two different subtypes infect the same animal at the same time. That mixing means new combinations can appear without warning.
Only a handful of influenza A subtypes regularly infect humans. The two that cause seasonal flu right now are H1N1 and H3N2. But within each subtype, the virus drifts genetically from year to year, producing slightly different strains each season. That’s why a flu shot from two years ago offers limited protection today, and why the vaccine is reformulated annually.
How New Variants Keep Appearing
Flu generates new variants through two distinct processes. The first, called antigenic drift, is a slow, constant accumulation of small mutations as the virus copies itself. These tiny changes gradually alter the virus’s surface proteins enough that your immune system may no longer recognize it. Sometimes even a single mutation in the right spot is enough to make last season’s antibodies ineffective. Drift is the reason flu comes back every year in a slightly different form.
The second process, antigenic shift, is far more dramatic. It happens when a flu virus from an animal population picks up entirely new surface proteins and gains the ability to infect humans. Because most people have no pre-existing immunity to the new combination, shift events can spark pandemics. The 2009 H1N1 pandemic is a recent example: a novel virus with gene segments from bird, pig, and human flu strains emerged and spread worldwide within months. Antigenic shift only occurs with influenza A, which is one reason it’s watched more closely than influenza B.
Influenza B: Fewer Variants, Still Significant
Influenza B doesn’t have the same subtype system as influenza A. Instead, it’s divided into two lineages named after the places where they were first characterized: Victoria and Yamagata. For decades, both lineages circulated simultaneously, which is why flu vaccines contained four components (two A strains and two B strains) from 2012 onward.
That changed recently. The Yamagata lineage has not been detected in global surveillance since March 2020. Its disappearance, likely accelerated by the public health measures used during the COVID-19 pandemic, prompted a shift back to three-component vaccines starting with the 2024-2025 flu season. Today’s vaccines include only the Victoria lineage of influenza B alongside the two influenza A strains.
Bird Flu Variants on the Radar
While seasonal flu involves just a few subtypes, dozens of influenza A subtypes circulate in wild birds. Five of those avian subtypes (H5, H6, H7, H9, and H10) have caused confirmed human infections. The two responsible for the most cases are H5N1 and H7N9, both of which can cause severe illness in the rare instances when they jump to people.
Other combinations like H5N6, H9N2, H6N1, H10N3, H10N7, and H10N8 have also infected small numbers of humans. None of these avian subtypes currently spread easily from person to person, but they represent the pool from which a future pandemic virus could emerge through antigenic shift. That’s why global surveillance networks track bird flu variants closely, even when human cases are rare.
What the Vaccine Targets Each Year
Because the flu landscape shifts constantly, the World Health Organization convenes expert panels twice a year (once for each hemisphere) to select the specific strains the next season’s vaccine should target. For the 2024-2025 U.S. flu season, the vaccine includes three components: an H1N1 strain, an H3N2 strain, and a B/Victoria lineage strain. The exact strains chosen are based on which variants are spreading most widely and which are expected to dominate in the coming months.
This annual selection process highlights a practical truth about flu variants: the sheer number matters less to you than which specific ones are circulating right now. Hundreds of subtypes exist in birds and other animals, and countless drifted strains have come and gone over the years. But in any given flu season, your risk comes down to the two or three variants making the rounds in the human population, and the vaccine is designed to match those as closely as possible.