Influenza A and influenza B are the two types of flu virus responsible for seasonal epidemics, and while they cause nearly identical symptoms, they differ in important ways: how they mutate, which species they infect, how severe they tend to be, and their potential to cause pandemics. Both carry eight segments of RNA and use the same two surface proteins to enter your cells, but influenza A is the more unpredictable and dangerous of the two.
They Infect Different Species
The biggest biological distinction is host range. Influenza A circulates naturally in wild birds and can jump to a wide variety of animals, including ducks, chickens, pigs, horses, seals, dogs, cats, cows, and even whales. Nearly all known combinations of its surface proteins have been found in birds, with a couple found only in bats. When one of these animal-adapted viruses gains the ability to infect humans, it can trigger a pandemic because most people have no existing immunity to it.
Influenza B, by contrast, circulates almost exclusively in humans. It doesn’t maintain large animal reservoirs, which means it can’t pull the same species-jumping trick that makes influenza A so unpredictable. This single difference is the main reason every flu pandemic on record has been caused by an influenza A virus, not B.
How They Mutate
Both types change over time through a process called antigenic drift: small, gradual mutations that accumulate season after season. This is why the flu vaccine needs updating every year. Your immune system recognizes last year’s version of the virus, but this year’s version looks slightly different.
Influenza A, however, can also undergo antigenic shift, an abrupt, major reshuffling of its surface proteins. This happens when two different influenza A viruses (say, one from birds and one from humans) infect the same host and swap genetic material. The result can be a virus with surface proteins most humans have never encountered. Antigenic shift is the mechanism behind pandemics. Influenza B does not undergo antigenic shift because it lacks the broad animal reservoir that makes these genetic exchanges possible.
Subtypes vs. Lineages
Influenza A is classified into subtypes based on the specific versions of its two surface proteins. The subtypes currently circulating in people are H1N1 and H3N2. Many other subtypes exist in birds and other animals, creating a vast pool of genetic diversity.
Influenza B doesn’t have subtypes. Instead, it historically split into two main lineages: Victoria and Yamagata. For years, vaccines included both. But no Yamagata lineage virus has been detected anywhere in the world since March 2020. The lineage appears to have gone extinct, likely driven out by the public health measures during the COVID-19 pandemic. As a result, flu vaccines for the 2024-25 and 2025-26 seasons are trivalent: two influenza A components (one H1N1, one H3N2) and a single influenza B component from the Victoria lineage.
Symptoms Are Nearly Identical
If you’re lying in bed with the flu, you almost certainly can’t tell whether it’s type A or B based on how you feel. Both cause the same set of symptoms: fever (typically 100 to 102°F), chills, body aches, headache, sore throat, runny or stuffy nose, fatigue, and sneezing. The onset is similarly abrupt for both types.
That said, influenza A tends to cause more severe illness overall. Influenza B generally produces milder symptoms in healthy adults, though children under 5 and older adults can still develop serious cases. Because the symptoms overlap so completely, the same antiviral treatments are used for both types.
Severity and Hospitalization
Both types of flu can land people in the hospital, but influenza A carries a higher risk. In a large clinical comparison, 76.5% of influenza A patients required hospital admission compared to 70.8% of influenza B patients. Death rates within 90 days were 13.4% for influenza A and 8.7% for influenza B, though these figures came from a population with an average age of 69, meaning they skew toward people already at higher risk. Age 65 and older was the strongest risk factor for hospitalization regardless of type.
For the general population, the gap in severity is real but more modest. Most healthy adults recover from either type within one to two weeks. The greater danger of influenza A comes not just from individual infections being slightly more severe, but from its ability to produce entirely new strains through antigenic shift, potentially overwhelming population-level immunity.
Seasonal Timing
Influenza A typically dominates the early part of flu season, peaking in the winter months in temperate countries. Influenza B often circulates later in the season, sometimes peaking in late winter or early spring. In some years the two overlap significantly, and seasons with both types co-circulating tend to last longer. In tropical countries, the timing is less predictable, with influenza A and B peaks coinciding less frequently and overall flu activity stretching over a longer period.
How Testing Tells Them Apart
Rapid flu tests (the kind you might get at an urgent care visit or use at home) can distinguish between influenza A and B. These tests must meet a minimum 80% sensitivity threshold for both types, meaning they correctly identify about 4 out of 5 true infections. Specificity is higher, at 95%, so a positive result is quite reliable. Molecular tests like PCR are more accurate and can identify the specific subtype or lineage, but rapid tests are sufficient for most clinical decisions since treatment is the same regardless of type.
Knowing which type you have matters less for your personal treatment plan and more for public health surveillance. Tracking the balance of influenza A and B in a given season helps officials predict severity, update vaccine formulations, and watch for the emergence of new influenza A subtypes that could signal a pandemic threat.