Influenza viruses are from the family Orthomyxoviridiae, and are classified by their antigenic differences into influenza virus A, B and C. Inﬂuenza A viruses include a number of subtypes, classiﬁed on the basis of two surface antigens:
Examples of influenza A viruses include H1N1, H2N2 and H3N2, which have caused previous epidemics and pandemics. Influenza B is associated with widespread outbreaks and epidemics. Influenza C virus is associated with sporadic cases of mild upper respiratory infection.
Influenza A and B viruses undergo frequent small changes (mutations) in the DNA coding regions responsible for H and N surface antigens. Over time, these mutations accumulate so that a new virus variant emerges. This is known as antigenic drift and is responsible for annual influenza outbreaks and the need to reformulate influenza vaccines. New variants are described by their type, geographic site of isolation, culture number and year of isolation; for example, the H3N2 virus A/Wellington/1/2004.
Influenza A viruses can also significantly change the DNA coding regions responsible for H and N surface antigens, causing a completely new virus subtype to emerge. This is known as antigenic shift and is largely responsible for pandemics. These new subtypes typically result from the adaption of an avian inﬂuenza virus to human virus DNA coding regions, or the reassortment of human and avian inﬂuenza virus genes.