Immunizations must be repeated yearly because antigenic shifts in the virus necessitate production of new vaccines to protect against new epidemic strains. Antibodies to these antigens decrease both the susceptibility to infection and the severity of disease when infection occurs. Unfortunately, because of antigenic variations, vaccination or infection with one strain may not provide protection against other strains. Although influenza B is much more stable than influenza A, antigenic variation does occur.
Influenza is a worldwide problem.
When major antigenic shifts in the virus occur, widespread epidemics ensue. In the winter pandemics of 1957 and 1968, for example, at least 25 percent of the United States population were infected. More than 80 percent of influenza-related deaths occur in patients older than 65 years. A new influenza vaccine is prepared each year. A and B strains that are expected to predominate.