Session II: Antigenic Shift--What is it?

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Antigenic shift is contrasted with antigenic drift, which is the natural mutation over time of known strains of influenza to evade the immune system. Antigenic drift occurs in all types of influenza including influenza A, B and C. Antigenic shift, however, occurs only in influenza A because it infects more than just humans. Affected species include other mammals and birds, giving influenza A the opportunity for a major reorganization of surface antigens. Influenza B and C principally infect humans, minimizing the chance to mutate drastically.

Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and/or new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans. Shift results in a new influenza A subtype. When shift happens, most people have little or no protection against the new virus. While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.

Antigenic shift can also refer to a change in species-specificity of an influenza virus. The 1918 “Spanish” influenza pandemic is an example of species shift. Genetic analysis of the 1918 flu genome suggests that the H1N1 virus was an avian influenza virus that jumped directly into the human population. The virus had to mutate to replicate and spread in the human population, but there was not a genomic reassortment.

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