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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. |
