Influenza A and influenza B are both responsible for seasonal flu, but they differ in important ways: how they’re classified, which species they infect, how quickly they mutate, and their potential to cause pandemics. Influenza A is the more common and more dangerous of the two, accounting for the large majority of flu hospitalizations and all known flu pandemics. Influenza B still causes significant illness each year but circulates in a narrower, more predictable pattern.
How the Two Viruses Are Classified
Influenza A viruses are organized into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). There are 18 known H subtypes and 11 N subtypes, which is where names like H1N1 and H3N2 come from. Only two of these combinations, A(H1N1) and A(H3N2), currently circulate in humans, but the sheer number of possible subtypes gives influenza A enormous genetic flexibility.
Influenza B doesn’t have subtypes at all. Instead, it’s split into two lineages: B/Victoria and B/Yamagata. These lineages evolve more slowly and don’t recombine the way influenza A subtypes can. In fact, the B/Yamagata lineage appears to be extinct or very nearly so. It hasn’t been reliably detected in global surveillance since the COVID-19 pandemic, and the World Health Organization has recommended removing it from flu vaccines. Current trivalent vaccines include only a single B/Victoria lineage strain alongside two influenza A strains.
Animal Hosts and Pandemic Risk
This is one of the most consequential differences between the two types. Influenza A infects a wide range of animals: wild waterfowl (its primary natural reservoir), domestic poultry, pigs, horses, dogs, cats, bats, seals, and even dairy cattle. Because the virus circulates across so many species, different subtypes can mix inside a single animal and swap entire gene segments, a process called reassortment. When this produces a version of the virus that humans have never encountered before, it can trigger a pandemic.
All four flu pandemics in the past century were caused by influenza A. The mechanism behind them, called antigenic shift, is an abrupt, major change in the virus’s surface proteins that leaves most of the human population without immunity. Influenza B doesn’t undergo antigenic shift. It circulates almost exclusively in humans (with rare detections in seals), so it lacks the animal mixing grounds that make dramatic genetic leaps possible. This makes influenza B a seasonal concern but not a pandemic threat.
How They Mutate Over Time
Both influenza A and B undergo antigenic drift, the slow accumulation of small mutations that gradually change the virus’s surface proteins. This is why flu vaccines need to be updated every year. Your immune system may recognize last year’s strain, but after enough small changes, the virus can partially or fully evade your existing antibodies.
Influenza A drifts faster and less predictably than influenza B, which is part of why it causes more severe seasons and why vaccine mismatches tend to be more common with A strains. Influenza B’s slower drift means immunity from past infections or vaccinations often holds up a bit longer, though not indefinitely.
Severity and Hospitalization Rates
Influenza A causes far more hospitalizations. During the 2024-25 season, the CDC estimated a cumulative hospitalization rate of 122.0 per 100,000 for influenza A infections, compared to just 4.8 per 100,000 for influenza B. Within influenza A, the H1N1 subtype drove more hospitalizations than H3N2 that season (72.2 vs. 49.5 per 100,000), though this balance shifts from year to year. Adults 75 and older had the highest hospitalization rates for both subtypes.
Overall severity markers were consistent with recent seasons: about 17% of hospitalized flu patients were admitted to intensive care, 6% needed mechanical ventilation, and 3% died in the hospital. These figures reflect flu patients broadly, not one type versus the other, but the overwhelming majority of those hospitalizations were caused by influenza A.
Risk to Children
Influenza A also accounts for most pediatric flu deaths. In all but two flu seasons since surveillance began, influenza A has been associated with more pediatric deaths than influenza B. During the 2024-25 season, influenza A was linked to 86% of the 280 pediatric flu deaths reported. That said, influenza B is not harmless in children. It consistently causes a meaningful share of pediatric illness each year, and some seasons see proportionally higher B activity in school-age kids.
Symptoms: More Similar Than Different
If you’re sick with the flu, you generally can’t tell whether it’s type A or B based on symptoms alone. Both cause fever, cough, body aches, fatigue, sore throat, and sometimes vomiting or diarrhea (more common in children). A rapid flu test or PCR test can identify the type, and some tests can also identify the specific influenza A subtype.
One pattern that clinicians have noted is that influenza B can sometimes cause more pronounced leg pain or muscle soreness in children, but this isn’t reliable enough to serve as a distinguishing feature. The practical takeaway is that testing is the only way to know which type you have.
Treatment Options
The same antiviral medications work against both influenza A and B. The most commonly prescribed is oseltamivir (Tamiflu), a pill taken twice daily for five days. Three other antivirals are also approved, including baloxavir (Xofluza), which requires only a single dose.
For influenza B specifically, baloxavir may have an edge. In a randomized controlled trial, baloxavir reduced symptom duration by more than 24 hours compared to oseltamivir in adolescents and adults with influenza B infections. For influenza A, the two medications perform more comparably. Regardless of the type, antivirals work best when started within 48 hours of symptom onset.
Vaccination Coverage
Flu vaccines are designed to protect against both types. For the 2024-25 northern hemisphere season, the WHO recommended trivalent vaccines containing an A(H1N1) strain, an A(H3N2) strain, and a B/Victoria lineage strain. Some quadrivalent vaccines still in circulation include a B/Yamagata component, but the WHO has said this is no longer warranted given that lineage’s likely extinction.
The shift from four-strain to three-strain vaccines is a direct result of B/Yamagata’s disappearance. It simplifies manufacturing without reducing protection, since there’s effectively only one B lineage left to defend against. Annual vaccination remains the primary way to reduce your risk from both influenza A and B, though effectiveness varies each season depending on how well the vaccine strains match what’s actually circulating.