There are three types of Influenza virus; A, B, and C. The virus has two major surface protein antigens; the Hemagglutinin (HA) and Neuraminidase (NA) . New subtypes or strains of the virus evolve constantly under natural conditions and produce a constant source of new infections, largely related to evolution of antigenic drift or antigenic shift in the surface protein antigens in the virus.
Antigenic Drift: Infections with Influenza virus have been part of human life for thousands of years. Each infection results in an immune response. New infections with new strains of the virus with small changes in the antigenic properties do often exhibit cross protection from prior infection with earlier strains. However, accumulated genetic changes over time eventually result in virus strains that are antigenically significantly different from earlier strains. Examples include, H1N1, H3N2, and H5N1. Although the new strains may produce more severe infection, they are often easier to treat with available drugs. Antigenic drift occurs in all three influenza virus types (A, B and C).
Antigenic Shift: Such shift occurs as a result of abrupt and major mutations in HA, or NA proteins or combinations of both that have emerged from different animal populations. Such shift results in the formation of new and antigenically distinct subtypes. The recombination involves more than one virus subtype and may result in a jump of the infection from one species to another, and from animals to man. Examples include. 1918 Pandemic with Avian H1N1; 1968 Pandemic with H3(avian) with H2N2 human strain; 2009 infection with new H1N1 virus type. Infections with influenza virus resulting from antigenic shift require a different vaccine for each new virus type and are often difficult to treat. In contrast to antigenic drift, it is important to note that antigenic shift is observed only with Influenza virus Type A.
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