10 Influenza

10.1 Virology

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.

10.1.1 Antigenic drift

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.

10.1.2 Antigenic shiftTop

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.