Biology of Influenza

Influenza A and B are the two types of influenza viruses that cause epidemic human disease (14). Influenza A viruses are categorized into subtypes on the basis of two surface antigens: hemagglutinin and neuraminidase. Currently circulating influenza B viruses are separated into two distinct genetic lineages but are not categorized into subtypes. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have circulated globally. In certain recent years, influenza A (H1N2) viruses that probably emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses also have circulated. Both influenza A subtypes and B viruses are further separated into groups on the basis of antigenic similarities. New influenza virus variants result from frequent antigenic change (i.e., antigenic drift) resulting from point mutations that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses.

Immunity to the surface antigens, particularly the hemagglutinin, reduces the likelihood of infection (15). Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype of influenza virus. Furthermore, antibody to one antigenic type or subtype of influenza virus might not protect against infection with a new antigenic variant of the same type or subtype (16). Frequent emergence of antigenic variants through antigenic drift is the virologic basis for seasonal epidemics as well as the reason for annually reassessing the need to change one or more of the recommended strains for influenza vaccines.

More dramatic changes, or antigenic shifts, occur less frequently and can result in the emergence of a novel influenza A virus with the potential to cause a pandemic. Antigenic shift occurs when a new subtype of influenza A virus emerges (14). New influenza A subtypes have the potential to cause a pandemic when they are demonstrated to be able to cause human illness and demonstrate efficient human-to-human transmission, in the setting of little or no previously existing immunity among humans.

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