A second example is antigenic variation or drift in certain viruses. The best example is influenza A, an RNA virus surrounded by a lipid envelope into which two important proteins (hemaglutinin and neurominidase) are inserted. The virus evades the neutralizing antibodies to this virus by constantly modifying the structure of these two proteins by either antigenic drift or antigenic shift. In the former, the change is minor and accounts for minor epidemics of flu during each winter. Antigenic shift is a major change in the structure of those proteins, which can cause a major pandemic of devastating proportions. Viewed in this light, the influenza vaccine currently in use is generally protective against common flu epidemics in the world but would not be protective against a major antigenic shift.