The influenza virus is divided into three main types, Influenza A, Influenza B, and Influenza C, which are distinguished by differences in two major virus surface proteins (Hemagglutinin (HA) and Neuraminidase (NA)). Hemagglutinin (HA) and Neuraminidase (NA) both serve as antigenic determinants found on the surface of the Influenza virus.
Influenza A virus is the most common flu virus infecting humans, animals, and birds. Influenza B infection mostly occurred in humans and it does not branch into multiple subtypes. Infection of influenza C virus does not cause any severe symptom in human or mammals and hence it is not well studied.
The influenza A virus can be subdivided into different serotypes based on the type of hemagglutinin and neuraminidase. The "H" in "H1N1" for example refers to hemagglutinin (HA) and "N" in "H1N1" refers to neuraminidase (NA). There are 16 different types of HA and 9 different types of NA, therefore, there are potentially 144 different subtypes of influenza A viruses. Among them, two subtypes of influenza A, H1N1 and H3N2, most commonly infect humans. For each subtype virus, the hemagglutinin gene mutates all the time and hence there are many variants of the same subtype viruses, and hence the need to change the virus strain for seasonal flu vaccines on an annual bases. A list of human influenza A virus is exhibited below.
This type of influenza mutates at a rate 2-3 times lower than type A and consequently is less genetically diverse, with only one influenza B serotype. Reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.