Antigenic shift (reassortment)

Antigenic shift refers to an abrupt, major change to produce a novel influenza A virus subtype in humans that was not currently circulating among people (see more information below under Influenza Type A and its subtypes). This can occur when viruses from two different host species (man and bird) infect the same cell and swap genetic information. Vaccines against either of the two original viruses will no longer be effective.