Influenza: current concepts.

Changes in surface antigens make preparation of an effective vaccine against the influenza virus a difficult job. When the changes are small (antigenic drift), a vaccine can be expected to be of some value; when larger changes occur (antigenic shift), its effectiveness is greatly curtailed. Improvements in the purification process for the vaccine have greatly lessened the incidence and severity of reactions. Although the current vaccines are still imperfect, they represent the best available control.