There are four types of influenza: A, B, C, and D. Only two of them, A and B, cause the seasonal flu outbreaks that send millions of people to bed each winter. Types C and D are far less relevant to human health, with C causing only mild illness and D primarily infecting cattle.
Influenza A: The Most Dangerous Type
Influenza A is the type behind every flu pandemic in recorded history. It infects humans, birds, pigs, and other animals, and its ability to jump between species is what makes it uniquely dangerous. The virus is classified by two proteins on its surface: hemagglutinin (H) and neuraminidase (N). There are 18 known H subtypes and 11 known N subtypes, which combine to create labels like H1N1 or H3N2.
Despite that enormous variety, only two subtypes currently circulate in humans: H1N1 and H3N2. Both are included in every seasonal flu vaccine. The other subtypes circulate in birds and other animals, where they occasionally mutate in ways that let them infect people. When that happens and the new subtype spreads easily between humans, the result is a pandemic. The 1957 pandemic was caused by H2N2, the 1968 pandemic by H3N2, and the 2009 pandemic by a new version of H1N1 that contained genes from swine, bird, and human flu viruses.
Influenza A changes in two distinct ways. Small, continuous mutations in its surface proteins, called antigenic drift, are the reason last year’s flu shot doesn’t fully protect you this year. These gradual changes accumulate until your immune system no longer recognizes the virus well enough to fight it off. This is why vaccine formulations are reviewed and updated every year for both the Northern and Southern Hemispheres. The second, rarer process is antigenic shift: a sudden, major change that produces an entirely new subtype. Shift is what creates pandemic viruses, because virtually no one has pre-existing immunity to the new combination.
Influenza B: Slower to Change, Still Serious
Influenza B circulates almost exclusively in humans. It causes seasonal outbreaks that can be just as miserable as influenza A for the individual person, though it mutates more slowly and lacks the animal reservoirs that give type A its pandemic potential. For these reasons, influenza B has never caused a pandemic.
Historically, influenza B split into two distinct lineages: Victoria and Yamagata. For years, flu vaccines included both lineages in a four-component (quadrivalent) formula. That changed recently. The Yamagata lineage has not been reliably detected in global surveillance since around 2020, and a systematic review published in The Lancet Microbe concluded that B/Yamagata is likely on the verge of extinction, if not already extinct. As a result, the 2025–2026 U.S. flu vaccine has shifted back to a three-component (trivalent) formula that includes only the B/Victoria lineage alongside the two influenza A subtypes.
Influenza C and D: Minor Players
Influenza C infects humans but typically causes only mild respiratory symptoms, similar to a common cold. It does not produce epidemics or seasonal waves the way types A and B do, and most people are exposed to it during childhood without ever knowing it was influenza.
Influenza D primarily affects cattle, with occasional spillover to pigs and other livestock. It is not known to cause illness in people. Researchers monitor it mainly because any influenza virus circulating in animals has at least a theoretical chance of eventually adapting to human hosts, but so far type D shows no signs of doing so.
What the Seasonal Vaccine Covers
For the 2025–2026 flu season, the FDA recommended a trivalent vaccine targeting three viruses: an H1N1 strain, an H3N2 strain, and a B/Victoria lineage strain. This covers the influenza types and subtypes currently circulating in humans. Types C and D are not included because C rarely causes significant illness and D does not infect people.
The specific strains chosen for each season’s vaccine change based on global surveillance data tracking which versions of these viruses are spreading. Even within a single subtype like H3N2, antigenic drift means the circulating virus can look different enough from year to year that a fresh vaccine match is needed. This is why annual flu vaccination offers better protection than relying on last season’s shot.
Why Type A Gets the Most Attention
Of the four types, influenza A receives the most scientific and public health scrutiny for a straightforward reason: it is the only type capable of antigenic shift. Because it circulates in birds, pigs, and humans simultaneously, it has opportunities to swap genetic material between species that the other types simply do not have. Every pandemic in modern history traces back to an influenza A virus that made that jump successfully. Influenza B can still cause severe seasonal illness and contributes to tens of thousands of hospitalizations each year, but its inability to undergo antigenic shift means it will not be the source of the next pandemic. Types C and D, for now, remain biological footnotes.