Progeny virions are formed and released by budding . Avian and human strains recombining in pigs in the Far East may permit virulent human strains to evolve. Undergoes relatively slow change in HA with time. Known only in man. Uncommon strain, known only in man.
Birds have the greatest number and range of influenza strains.
Avian haemagglutinins sometimes appear in pig human and horse influenza strains. See chart below indicating main pandemic strains in previous years. This constant antigenic change down the years means that new vaccines have to be made on a regular basis. Influenza epidemics may cause economically significant absenteeism. The drug Amantadine may prevent influenza if taken continuously by high-risk persons at the time of an epidemic, but is not used widely.