Yes. Viruses can change in two ways: antigenic drift and antigenic shift. Antigenic drift refers to changes in the genetic makeup of a virus, which occurs continually and gradually over time. If an individual is infected by a specific strain of virus, antibodies develop in the bloodstream to help fight against that strain. As newer strains evolve, the antibodies to the previous strains are ineffective, and the newer strain is able to become more established. This is why people must receive a new flu shot each winter—last season’s influenza vaccine is no longer effective against this winter’s influenza virus strains due to the drift in genetic make-up.
Antigenic shift is a much more abrupt, dramatic change in the makeup of a virus. It is thought to occur when an individual becomes infected by two different subtypes at the same time. The virus may re-assort its H and N proteins, resulting in a whole new subtype of virus that has not previously affected humans. Scientists studying the current avian influenza outbreak in several countries are concerned about such a shift occurring in the highly pathogenic H5N1 avian influenza virus.
For additional resources and information please visit The Avian Influenza Homepage.