Seasonal influenza is caused by influenza viruses which circulate worldwide, resulting in acute respiratory disease, predominantly during the winter months. Influenza virus is an enveloped virus with a segmented RNA genome. Laboratory and surveillance reports state the type and often the subtype of the influenza virus.
What do the types and subtypes of influenza viruses mean?
There are 4 types of influenza viruses, types A, B, C and D, of which influenza A and B are the most important in human infection and disease (Table 1).
Influenza A viruses are further classified into subtypes according to the combinations of the hemagglutinin (HA) and neuraminidase (NA) proteins on the surface of the virus. At least 18 subtypes of HA, (H1 – H18) and 11 subtypes of NA, (N1 – N11) have been identified. However, only 3 haemagglutinin subtypes (H1, H2 and H3) and 2 neuraminidase subtypes (N1 and N2) have circulated consistently in the human population.
Understanding seasonal influenza virus types