Influenza A and influenza B are the two types of flu virus responsible for seasonal epidemics each year. Both cause the same familiar illness (fever, body aches, cough, fatigue), but they differ in important ways: the animals they infect, how quickly they mutate, how common they are in any given season, and who they hit hardest. Understanding those differences helps explain why flu vaccines change every year and why some seasons are worse than others.
How Influenza A and B Differ Biologically
Influenza A viruses circulate naturally in wild birds and can jump to a wide range of animals, including pigs, horses, dogs, cats, seals, whales, and cows. This large animal reservoir is what makes influenza A especially unpredictable. When the virus replicates in different species, it has more opportunities to mix genetic material and produce new strains that human immune systems have never encountered.
Influenza B, by contrast, spreads almost exclusively among humans. Without that broad animal reservoir, it evolves more slowly and produces fewer dramatically new variants. That distinction is the single biggest biological difference between the two types, and it drives most of the other differences in severity, seasonality, and pandemic potential.
Subtypes, Lineages, and Why They Matter
Influenza A is classified into subtypes based on two surface proteins the virus uses to enter and exit your cells. These subtypes are labeled with H and N numbers. The two subtypes currently circulating in people are H1N1 and H3N2. During the 2024–25 U.S. flu season, public health labs found these two subtypes in roughly equal proportions: 53% H1N1 and 47% H3N2.
Influenza B doesn’t have subtypes but is divided into two lineages: Victoria and Yamagata. The Yamagata lineage has not been detected in circulation anywhere in the world since March 2020. That effectively leaves Victoria as the only influenza B lineage people need protection against, which is why flu vaccines in the United States shifted from four components to three starting with the 2024–25 season.
Which Type Is More Common?
Influenza A dominates most flu seasons. In the 2024–25 season, clinical laboratories in the U.S. found that nearly 89% of all positive flu tests were influenza A, while about 11% were influenza B. The two types also peak at different times. Influenza A cases climbed through late fall and peaked in late January, when about 30% of respiratory specimens tested positive. Influenza B circulated at much lower levels and peaked later, in late March, topping out at just 4.3% positivity.
This pattern is typical. Influenza A tends to drive the main wave of illness each winter, while influenza B often causes a smaller, later surge. In some years, though, influenza B accounts for a larger share of cases, particularly among school-age children.
Severity in Adults and Children
A common assumption is that influenza A causes more severe illness because it dominates hospitalizations by sheer volume. The reality is more nuanced. A study of 391 children hospitalized with flu found no significant differences in intensive care admissions, length of hospital stay, or clinical features between influenza A and B when analyzed across age groups.
Some evidence actually points in the opposite direction for kids. A large Canadian surveillance study covering eight flu seasons found that, even after adjusting for age and underlying health conditions, children with influenza B had significantly more headache, abdominal pain, and muscle inflammation, and higher mortality, than those with influenza A. For adults, both types can lead to serious complications like pneumonia, especially in older adults and people with chronic health conditions.
The bottom line: neither type is categorically “milder.” Both can cause severe illness, and influenza B deserves as much respect as influenza A, particularly in children.
Why Influenza A Causes Pandemics
Flu viruses change in two ways. The first, called antigenic drift, is a slow, continuous process where small mutations accumulate over time. Both influenza A and B undergo drift, which is why the flu vaccine needs updating most years.
The second, called antigenic shift, is sudden and dramatic. It happens when an influenza A virus from an animal population acquires entirely new surface proteins and gains the ability to infect humans. Because most people have no pre-existing immunity to the new virus, it can spread rapidly around the world. This is how pandemics start. The 2009 H1N1 pandemic, for example, emerged from a reassortment involving swine flu viruses.
Influenza B cannot undergo antigenic shift because it lacks the broad animal reservoir needed for that kind of genetic mixing. This is why every flu pandemic on record has been caused by influenza A.
Symptoms and How Flu Spreads
The symptoms of influenza A and B are essentially identical: sudden fever, chills, muscle aches, sore throat, cough, runny nose, headache, and fatigue. You cannot tell which type you have based on how you feel. The only way to know is through a diagnostic test.
Both types spread the same way, primarily through respiratory droplets when an infected person coughs, sneezes, or talks. Symptoms typically appear about two days after exposure, though the window ranges from one to four days. You can start spreading the virus to others a full day before your symptoms appear and remain contagious for five to seven days after getting sick. The first three days of illness are the most contagious period. Young children and people with weakened immune systems may shed the virus for longer.
Testing for Flu A and B
Rapid influenza diagnostic tests, the kind you might get at an urgent care clinic with results in 15 minutes, can distinguish between influenza A and B. However, their sensitivity is only moderate, roughly 50–70% in older versions, meaning they miss a significant number of true infections. The FDA now requires newer rapid tests to achieve at least 80% sensitivity. Specificity is high (95–99%), so a positive result is reliable, but a negative result during peak flu season doesn’t necessarily rule out infection.
Rapid molecular assays, available in some clinics and emergency departments, deliver results within about 30 minutes with much higher accuracy. These are increasingly replacing the older antigen-based rapid tests in clinical settings.
How the Vaccine Covers Both Types
Current U.S. flu vaccines are trivalent, meaning they protect against three virus strains: two influenza A strains (one H1N1 and one H3N2) and one influenza B strain (Victoria lineage). Until recently, most vaccines were quadrivalent and included the Yamagata lineage as well, but since that lineage has disappeared from circulation, the extra component was dropped.
Each year, the World Health Organization analyzes global surveillance data and selects the specific strains most likely to match what will circulate in the upcoming season. For 2024–25, the influenza A components were based on viruses identified in Victoria, Australia, and Thailand, while the B component was based on a virus from Austria. Different manufacturing methods (egg-based, cell-based, and recombinant) use slightly different reference strains, but all target the same three virus types.
Because influenza A mutates faster and has more genetic diversity, the A components of the vaccine are updated more frequently than the B component. Getting vaccinated each year remains the most effective way to reduce your risk of serious illness from both types.