Influenza A and B are the two types of flu virus responsible for seasonal epidemics every year. Both cause the same familiar illness, with fever, body aches, cough, and fatigue, and both can lead to serious complications including hospitalization and death. The key difference lies in their genetic diversity and long-term threat: influenza A is far more variable, infects multiple animal species, and is the only type capable of causing pandemics. Influenza B circulates almost exclusively in humans and changes more slowly over time.
How Influenza A and B Differ Genetically
Influenza A has enormous genetic diversity. It’s classified into subtypes based on two proteins on its surface: hemagglutinin (H) and neuraminidase (N). There are 18 H subtypes and 11 N subtypes, and more than 130 combinations have been identified in nature, primarily in wild birds. The subtypes you hear about most during flu season, H1N1 and H3N2, are the ones currently adapted to spread among humans.
Influenza B has no subtypes at all. Instead, it’s divided into just two lineages: B/Victoria and B/Yamagata. This narrower genetic range means influenza B evolves more slowly, particularly compared to the fast-changing H3N2 subtype of influenza A. The practical result is that your immune system’s memory of a past influenza B infection tends to hold up a bit longer than it does for influenza A.
Why Only Influenza A Causes Pandemics
Both types of flu virus undergo a process called antigenic drift, where small mutations accumulate over time and gradually change the virus’s surface proteins. This is why you can catch the flu more than once, and why the vaccine is updated each year. Your antibodies from a previous infection or vaccination may no longer recognize the slightly altered virus.
Influenza A, however, can also undergo something far more dramatic: antigenic shift. This is an abrupt, major change that produces an entirely new combination of surface proteins, typically when a flu virus from an animal population (birds or pigs, for example) gains the ability to infect humans. Because most people have little or no immunity against the resulting novel virus, it can spread globally and cause a pandemic. Influenza B doesn’t undergo antigenic shift because it circulates almost exclusively in humans, with no significant animal reservoir to swap genes with.
Symptoms Are Essentially the Same
If you’re lying in bed with the flu, you won’t be able to tell whether it’s type A or type B based on how you feel. Both cause fever, chills, cough, sore throat, runny or stuffy nose, muscle and body aches, headaches, and fatigue. Symptoms typically begin about two days after exposure, though the range is one to four days.
There’s a common misconception that influenza B causes milder illness than influenza A. A CDC study covering eight flu seasons found this isn’t true. Among hospitalized adults, influenza B caused equally severe outcomes as influenza A. There was no significant difference in ICU admissions, length of hospital stay, or the proportion of deaths between the two types. Clinicians are now advised not to assume influenza B is less dangerous when making treatment decisions.
How the Flu Spreads and How Long You’re Contagious
Both types spread the same way: through respiratory droplets when an infected person coughs, sneezes, or talks, and sometimes by touching contaminated surfaces and then touching your mouth, nose, or eyes. You can start spreading the virus to others a full day before your own symptoms appear, and you remain contagious for five to seven days after getting sick. The first three days of illness are when you’re most contagious.
Testing for Influenza A and B
Rapid influenza diagnostic tests, the kind you can get at a doctor’s office or urgent care visit, can return results in about 15 minutes and will typically tell you whether you have influenza A, influenza B, or neither. These tests are reasonably reliable when they come back positive (at least 95% specificity), but they can miss infections. The FDA requires rapid tests to detect at least 80% of true influenza A and B cases when compared to the gold-standard molecular test.
Molecular tests, including PCR, are significantly more accurate and are recommended for hospitalized patients. They can detect lower levels of virus and are better at confirming a diagnosis when clinical decisions depend on it. For most people with typical flu symptoms during peak season, though, a rapid test or even a clinical diagnosis based on symptoms is often sufficient to start treatment.
How Flu Vaccines Cover Both Types
Seasonal flu vaccines are designed to protect against both influenza A and B. For the 2025-2026 season, all flu vaccines in the United States are trivalent, meaning they protect against three viruses: an A(H1N1) strain, an A(H3N2) strain, and a B/Victoria lineage virus. Previous seasons used quadrivalent vaccines that also included a B/Yamagata component, but that lineage has not been detected in global circulation recently, so it was dropped.
The specific strains in the vaccine are updated each year based on surveillance data showing which viruses are circulating and how much they’ve drifted. Because influenza A, especially H3N2, mutates faster than influenza B, the A components are more likely to need updating from one season to the next.
Antiviral Treatment Works for Both
The most commonly prescribed antiviral medications are effective against both influenza A and B. These include neuraminidase inhibitors (the class that includes the well-known oral antiviral often sold as Tamiflu) and a newer single-dose oral antiviral that works through a different mechanism.
Interestingly, the newer single-dose antiviral may have an edge specifically for influenza B. In a clinical trial of higher-risk patients, it reduced the time to symptom improvement by more than 24 hours compared to the older oral antiviral in people with influenza B infections. For influenza A, both medications performed similarly. Antivirals work best when started within the first 48 hours of symptoms, so early testing and treatment matter regardless of which type you have.
Which Type Is More Common in a Given Season
Influenza A typically dominates the early part of flu season and accounts for the majority of cases in most years. Influenza B often circulates later in the season, sometimes peaking in the spring. Some years, though, influenza B makes up a larger share of overall cases, and it tends to disproportionately affect children. The pattern shifts from year to year depending on which strains are circulating, how much immunity exists in the population, and how well the vaccine matches the dominant strains.
Both types deserve the same level of caution. The distinction matters most at the population level, where influenza A’s pandemic potential makes it the bigger long-term public health concern, but for any individual dealing with the flu, the severity, treatment, and recovery timeline are comparable regardless of type.