What are the differences between influenza A and influenza B?

Influenza A and influenza B are two distinct types of seasonal influenza viruses that cause similar clinical illness but differ significantly in epidemiology, virology, and public health impact. Influenza A viruses are classified into subtypes based on two surface glycoproteins—hemagglutinin (HA) and neuraminidase (NA)—with 18 known HA and 11 NA subtypes identified to date. Common human subtypes include H1N1 and H3N2. Influenza A has a broad host range—including humans, birds, swine, and other mammals—and is capable of undergoing antigenic shift (major genetic reassortment), which can lead to pandemic outbreaks. It typically causes more severe disease, higher hospitalization rates, and greater mortality, especially among older adults, young children, and immunocompromised individuals.

In contrast, influenza B viruses are not divided into subtypes but are categorized into two main lineages: B/Victoria and B/Yamagata (though the latter has not been detected globally since March 2020 and is now considered likely extinct). Influenza B infects almost exclusively humans and undergoes only antigenic drift (gradual point mutations), making pandemics extremely unlikely. While generally associated with milder illness than influenza A, influenza B can still cause significant morbidity—particularly in children—and may lead to severe complications such as pneumonia, myocarditis, or encephalitis. Notably, influenza B often peaks later in the flu season and may circulate concurrently with or after influenza A.

Both viruses are transmitted via respiratory droplets and aerosols, share overlapping symptoms—including fever, cough, myalgia, fatigue, headache, and sore throat—and are diagnosed using rapid molecular assays (e.g., RT-PCR) or rapid antigen tests. Antiviral treatment with neuraminidase inhibitors (e.g., oseltamivir, zanamivir) or cap-dependent endonuclease inhibitors (e.g., baloxavir marboxil) is effective for both when initiated within 48 hours of symptom onset. Annual quadrivalent influenza vaccines contain two influenza A strains (H1N1 and H3N2) and two influenza B strains (one from each lineage) to provide broad protection against circulating variants.