All the following are true regarding Type A influenza virus, EXCEPT:
There are only 14 subtypes of haemagglutinin
There are only 9 subtypes of neuraminidase antigen
Humans are generally infected by virus of subtype H1, H2 or H3 and N1 or N2.
It shows both antigenic shift and drift
There are 16 subtypes of hemagglutinins. Rest of the statements given are true. Antigenic changes occurs only to a lesser degree in group B influenza virus. Influenza C virus appears to be antigenically stable. Note: Almost all of the pandemic which has ever occurred is due to Type A influenza virus. Ref: Park, Edition 21, Page - 143,144