Influenza A and influenza B are the two types of flu virus responsible for seasonal epidemics, but they differ in important ways. Influenza A is more common, causes the majority of hospitalizations, and has the potential to trigger pandemics. Influenza B circulates alongside it, often peaking a bit later in the season, and can be just as severe on an individual level, particularly in children.
How the Two Viruses Differ
Influenza A is the more diverse and unpredictable of the two. It infects humans, birds, pigs, and other animals, and its surface proteins come in a huge number of combinations: 18 types of one protein and 11 types of another, yielding more than 130 subtypes identified in nature so far. Only two of those subtypes routinely circulate in people: H1N1 and H3N2.
Influenza B, by contrast, infects only humans. It doesn’t have subtypes at all. Instead, it’s classified into two lineages: B/Victoria and B/Yamagata. That narrower range means influenza B lacks the animal reservoirs that make influenza A so volatile and harder to predict year to year.
Why Influenza A Drives Pandemics
Both types gradually accumulate small genetic mutations over time, a process called antigenic drift. These mutations are why you can catch the flu more than once and why the vaccine needs updating every year. But influenza A has an additional trick: it can undergo a sudden, dramatic genetic overhaul called antigenic shift, where entirely new surface proteins appear on the virus. This typically happens when a flu virus from an animal species (like birds or pigs) mixes with a human flu virus.
When antigenic shift occurs, most people have little or no preexisting immunity against the resulting virus. That’s the recipe for a pandemic. Influenza B, because it circulates only in humans, cannot undergo this kind of wholesale reassortment and has never caused a pandemic.
Symptoms Are Similar, With Some Differences
If you’re sick with the flu, the experience feels largely the same regardless of type. Fever occurs in roughly 88 to 90 percent of cases for both A and B. Cough is similarly common, showing up about 80 to 84 percent of the time. A runny nose is actually slightly more frequent with influenza B (74 percent versus 70 percent for A).
The clearest differences show up in fatigue and muscle aches. A large meta-analysis found fatigue was reported in 60 percent of influenza A cases but only 21 percent of influenza B cases. Muscle aches followed the same pattern: 32 percent for A compared to 22.5 percent for B. Sore throat was also somewhat more common with influenza A (49 percent versus 38 percent). These are population-level averages, though. Any individual case of either type can range from mild to severe.
Influenza A Causes Far More Hospitalizations
CDC surveillance data from 2010 through 2023 consistently shows that influenza A is responsible for the vast majority of flu-related hospitalizations. During the 2022-23 season, for example, the hospitalization rate for influenza A was 59.5 per 100,000 people, compared to just 2.2 per 100,000 for influenza B. The 2017-18 season saw rates of 74.4 for A and 27.8 for B.
The gap varies from year to year. In seasons where the B/Victoria or B/Yamagata lineage circulates more heavily, influenza B’s share rises. The 2019-20 season was one such year, with influenza B hospitalizations reaching 17.7 per 100,000, nearly a third of A’s rate. Across all seasons, intensive care admission ranged from 14 to 22 percent of hospitalized flu patients, and in-hospital deaths ranged from 2.2 to 3.5 percent, though these figures weren’t broken down by type.
Children Face Similar Risks From Both Types
One important nuance: influenza B is not necessarily milder in children. A study of pediatric patients during the 2017-18 season found that hospitalization rates, fever duration, and recovery outcomes were nearly identical between the two types. About 34 percent of children with influenza A and 32 percent with influenza B required hospitalization, and stays averaged around five and a half days for both groups. Over 96 percent of children with either type recovered without lasting effects.
Where influenza B stood out was in rare but serious neurological complications. Two children with influenza B in the study developed permanent complications, including one case of encephalitis (brain inflammation) with a poor prognosis. The overall takeaway from the researchers was that influenza B should not be treated as a lesser threat in pediatric care.
When Each Type Circulates
In temperate climates like most of the United States, flu season runs roughly from November through March. Influenza A typically dominates the early and peak months of the season, while influenza B often picks up later, sometimes peaking in February or March after influenza A has already begun to decline. In tropical regions closer to the equator, influenza B tends to circulate at low levels year-round rather than following a sharp seasonal pattern.
This staggered timing is why the flu season can feel like it drags on. You might dodge the initial wave of influenza A in December only to encounter influenza B in March.
How Testing Tells Them Apart
Rapid flu tests available in clinics and urgent care centers can distinguish between influenza A and B in about 15 to 30 minutes. These tests are highly specific, meaning a positive result is very reliable (at least 95 percent accuracy for both types). Sensitivity is lower, though. The FDA requires rapid tests to correctly identify at least 80 percent of true influenza A and B infections when compared to the gold-standard PCR test. That means false negatives are possible, especially with influenza B, which sits right at the minimum sensitivity threshold. If your rapid test comes back negative but your symptoms strongly suggest the flu, a PCR test can provide a more definitive answer.
What This Means for the Vaccine
For the 2024-25 season, all flu vaccines in the United States are trivalent, meaning they protect against three strains: two influenza A viruses (an H1N1 and an H3N2) and one influenza B virus from the Victoria lineage. Previous years used quadrivalent vaccines that included both B lineages, but the B/Yamagata lineage has not been detected in circulation since early in the COVID-19 pandemic, so it was dropped.
Because both influenza A and B mutate continuously, the specific strains in the vaccine are reviewed and updated each year by the World Health Organization. The goal is to match the vaccine as closely as possible to whichever versions of the virus are expected to circulate. Getting vaccinated each fall remains the most effective way to reduce your risk of both types.