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Antigenic drift occurs more regularly than antigenic shift. Antigenic variation
within the serotype is thought to occur as a result of antibody pressure existing
in the population. As herd immunity to a specific variant of influenza virus in
the population builds up, the virus is "forced" to mutate in order to survive.
Even though the mutated virus belongs to the same serotype, it is sufficiently
different to cause epidemics. The severity of the epidemic is dependent on the
degree of antigenic variation.
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