There are four types of influenza viruses: A, B, C, and D. Only two of them, A and B, cause the seasonal flu outbreaks that sweep through communities every winter. The other two are either too mild to matter much (type C) or don’t infect people at all (type D). Within those broad categories, though, the picture gets more complicated. Influenza A alone has dozens of subtypes, and the viruses are constantly changing in ways that affect how sick you get and how well the vaccine works.
Influenza A: The Most Dangerous Type
Influenza A is the heavyweight of the flu world. It causes the majority of seasonal flu cases, drives the highest hospitalization rates, and is the only type capable of triggering pandemics. During the 2024-25 flu season, the hospitalization rate for influenza A infections was roughly 25 times higher than for influenza B.
What makes influenza A unique is the variety of forms it can take. The virus is classified by two proteins on its surface: one that lets it latch onto your cells, and another that helps newly made copies break free to infect more cells. Scientists have identified 16 versions of the first protein and 9 of the second in birds alone, creating a huge number of possible combinations. Each combination is a different subtype, named with an H and N number.
Only two subtypes currently circulate in people: H1N1 and H3N2. These are the strains responsible for seasonal flu every year. H1N1 drove the 2009 “swine flu” pandemic and continues to circulate in an updated form. H3N2 tends to cause more severe seasons when it dominates. In the 2024-25 season, H1N1 was responsible for more hospitalizations than H3N2, with rates of about 72 per 100,000 people compared to roughly 50 per 100,000.
Animal-Origin Strains
Many other influenza A subtypes circulate in animals, particularly birds and pigs. These don’t normally infect people, but occasionally they jump species. When that happens, the virus is named for the animal it came from: avian (bird) flu, swine flu, or equine (horse) flu.
The most well-known example is H5N1 avian flu, which was first detected in humans in 1997 and has resurfaced repeatedly since. Other bird flu strains that have infected people include H7N9 (first reported in China in 2013), H9N2 (since 1999), and H5N6 (since 2014). At least a dozen different avian subtypes have caused sporadic human infections over the years. Swine flu viruses from the H1 and H3 groups have also occasionally infected people who work closely with pigs or visit places where pigs are exhibited.
These animal-to-human infections are concerning not because they spread easily between people right now, but because of what could happen if the virus changes enough to do so. A pandemic occurs when an influenza A virus emerges that can spread sustainably from person to person and most of the population has no existing immunity against it.
Influenza B: Seasonal but Narrower
Influenza B circulates alongside influenza A during flu season and can make you just as miserable on an individual level. It causes the same range of symptoms: fever, body aches, cough, fatigue. The key difference is scope. Influenza B infects only humans and has far less genetic diversity than influenza A, so it doesn’t produce pandemics or generate new subtypes from animal reservoirs.
Until recently, influenza B had two distinct lineages circulating worldwide: Victoria and Yamagata. That’s no longer the case. The Yamagata lineage hasn’t been detected anywhere in the world since March 2020, making five consecutive flu seasons without a single confirmed case. It appears to have gone extinct, possibly pushed out during the period of widespread masking and social distancing early in the COVID-19 pandemic.
Because of this, flu vaccines have shifted from a four-strain formula (quadrivalent) to a three-strain formula (trivalent). The 2025-26 vaccine targets one H1N1 strain, one H3N2 strain, and one B/Victoria lineage strain.
Influenza C and D: Minor Players
Influenza C was first identified in humans in 1947. It causes mild upper respiratory illness, mostly in infants and young children. It doesn’t trigger epidemics and rarely causes anything more than cold-like symptoms. Most people have been exposed to it by adulthood without ever knowing it.
Influenza D primarily infects cattle, with occasional spillover into pigs and other animals. It is not known to cause illness in people. For practical purposes, you can ignore types C and D when thinking about flu season and vaccination.
How Flu Viruses Change Over Time
One reason flu keeps coming back year after year is that the viruses are constantly evolving. This happens through two distinct processes.
The first is called antigenic drift. Small mutations accumulate gradually in the virus’s surface proteins, changing its appearance just enough that your immune system doesn’t fully recognize it from last year’s infection or vaccination. Drift is why you need a new flu shot every year. It happens in both influenza A and B.
The second process, antigenic shift, is far more dramatic and only happens with influenza A. This is when the virus undergoes a sudden, major change, picking up entirely new surface proteins. This can happen when two different influenza A subtypes infect the same animal (often a pig) and swap genetic material. The result is a virus that looks completely foreign to human immune systems. Antigenic shift is what creates pandemic strains.
What the Vaccine Covers
Each year, the seasonal flu vaccine is reformulated to match the strains experts predict will circulate most widely. For the 2025-26 season, all flu vaccines in the United States are trivalent, targeting three viruses: an A(H1N1) strain, an A(H3N2) strain, and a B/Victoria lineage strain. The specific strains differ slightly depending on how the vaccine is manufactured. Egg-based vaccines use virus strains grown in eggs, while cell-based and recombinant vaccines use strains better matched to what circulates in humans.
The vaccine does not cover influenza C, influenza D, or animal-origin strains like H5N1. It’s designed exclusively for the influenza A and B strains that cause seasonal outbreaks. Protection against those seasonal strains remains the single most effective tool for reducing your risk of hospitalization during flu season.