The Impact of Population Immunity on Antigenic Drift During Large Epidemics and Small Outbreaks

Population immunity is described as the number of individuals in a given population that is sufficient to prevent the transmission of a disease from the infectious to the susceptible. Although high population immunity usually prevents epidemics, we have seen many epidemics that were caused by infectious agents such as the Severe Acute Respiratory Syndrome (SARS) and Influenza type A H1N1 viruses.