text: Influenza A viruses are further classified , based on the viral surface proteins hemagglutinin ( HA or H ) and neuraminidase ( NA or N ) ... Sixteen H subtypes ( or serotypes ) and nine N subtypes of influenza A virus have been identified .
simplified: Influenza A viruses are further classified , based on the viral surface proteins hemagglutinin ( HA or H ) and neuraminidase ( NA or N ) . Sixteen H subtypes ( or serotypes ) and nine N subtypes of influenza A virus have been identified .