Epidemics are most often caused by a single serotype, mainly influenza A viruses, in a given season and are confined to a limited geographical region. If different influenza viruses appear in one location, the dominating virus type will be responsible for the epidemic. The WHO Collaborating Influenza Centers, along with local public health services, aim at early isolation and identification of the type and antigenic character of influenza viruses, which will enables them to recommend continued vaccination efforts.
During an epidemic, the number of cases peaks in about three weeks and subsides after another three or four weeks. Half the population of a community may be infected. Epidemics usually start at schools and spread to families.
Flu epidemics may have a tremendous impact on the economic condition of a country by causing surges of absenteeism, high morbidity and mortality. The prevention of flu epidemics remains one of the biggest challenges for public health services in providing adequate information about the disease and encouraging the population to receive vaccinations.