The flu is caused by viruses in the family Orthomyxoviridae. Influenza viruses types A and B cause seasonal flu outbreaks, and the flu vaccine provides protection against both virus types. Although these viruses have many similarities, they also have distinct differences in their structural and clinical characteristics.
Types, Subtypes and Strains
Influenza A and B viruses are genetically similar enough to be included in the same family of viruses, the Orthomyxoviridae family. However, their genetic differences are significant enough to warrant separating the viruses into two different types, A and B. Influenza A viruses are further categorized by subtype and strain. Influenza B viruses are categorized only by strain. This difference is due to the more rapid mutation rate seen in influenza A viruses compared to influenza B viruses.
The Centers for Disease Control and Prevention (CDC) states that wild birds are the natural hosts for all influenza type A viruses. Influenza A viruses also infect a wide variety of mammals including horses, pigs, ferrets and, of course, humans. In contrast, influenza type B viruses infect only mammals—primarily people. Influenza B viruses do not infect birds.
Influenza A and B viruses both cause the illness we recognize as the flu with fever, headache, dry cough, fatigue, muscle aches, sore throat and a runny or stuffy nose. However, the illness caused by the influenza type A virus is usually more severe than that caused by influenza type B.
Influenza A viruses are in a perpetual state of change. Spontaneous changes called mutations occur frequently in their genes. From one flu season to the next, genetic changes in the circulating influenza A viruses are extensive enough to cause the virus to go unrecognized by the immune system—even if you had the flu shot or the flu the previous year. This is why you need a flu shot every year; the shot from the previous year cannot protect you against the newly mutated influenza A virus.
Influenza B viruses mutate more slowly than influenza A viruses. Whereas influenza A viruses change significantly from one flu season to the next, influenza B viruses typically change significantly only every few years.
The high mutation rate of influenza A viruses combined with their broader range of hosts imparts these viruses with pandemic potential that influenza B viruses do not have. All of the influenza pandemics that have occurred in modern times, beginning with the Spanish flu of 1918, have been caused by influenza A viruses.
Response to Antiviral Medications
The antiviral medications zanamivir (Relenza) and oseltamivir (Tamiflu) are active against both influenza A and influenza B viruses. However, rimantadine (Flumadine) and amantadine (Symmetrel) are active only against influenza A viruses.