bird flu

Flu vaccine efficacy has been estimated by various studies to be between 30 – 70%. The reason for the less than 100% efficacy is a phenomenon called “antigenic drift”, namely that the surface proteins on the virus change in nature from the time the strains are selected in the Spring and the vaccine is manufactured in the late summer. Your immune system is being primed with inactivated viral strains whose surface features have changed in the months since the vaccine was manufactured.