The most effective way to reduce the impact of influenza is to vaccinate people at high risk for complications shortly before the influenza season each year. Influenza B viruses are not categorized into subtypes. Influenza A and B viruses are further separated into groups on the basis of antigenic characteristics. New influenza virus variants result from frequent antigenic changes that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses.
Children have the highest infection rates for influenza viruses.
Types and subtypes These rapid genetic changes are the reason that one or more new virus strains are usually incorporated into each year's influenza vaccine. A person's immunity to the surface antigens, especially hemagglutinin, reduces the likelihood of infection and the severity of disease if infection occurs. The influenza incubation period is one to four days, with an average of two days. Persons can be infectious starting the day before symptoms begin through about five days after illness onset. Children can be infectious for a longer period.