“Influenza A and influenza B have a lot in common. But there are some overarching differences between the two flu types. For example, influenza A usually accounts for more flu cases and causes more severe illness than type B flu. For example, flu A accounted for 96% of flu cases reported in late 2024.

Another difference is how the flu types affect kids. “In children, influenza A is associated with more ear infections than influenza B, while influenza B is associated with more seizures, vomiting and diarrhea than influenza type A,” says Dr. Mossad.

But the single most important difference is this: Influenza B flu only infects humans. Influenza A, on the other hand, can infect birds and mammals. This explains why antigenic shift can occur with influenza A, but not influenza B.

Anyone can catch these viruses. But babies and older adults are more likely to have more severe symptoms, Dr. Mossad shares.”

Cleveland Clinic (Influenza A vs. Influenza B)