Although vaccination is above all an act of individual prevention, in the case of directly transmissible diseases the vaccination of individuals may contribute indirect protection through the group immunity effect known as collective or herd immunity. The effect is due to the reduced or absent contagiousness of immunized subjects and the reduced likelihood of encounters between contagious and receptive subjects when immunized subjects are numerous. For many diseases, a vaccination coverage of 80% is sufficient to prevent epidemics. Vaccines may be classified as strictly egoistic, strictly altruistic, or simultaneously egoistic and altruistic. Rabies vaccine, which offers 100% protection if administered in time, is an example of a strictly egoistic vaccine that offers no collective benefit. German measles vaccine is strictly altruistic, since it prevents a condition that is dangerous only during fetal development. Vaccines that are both altruistic and egoistic are numerous. Measles, diphtheria, and hepatitis vaccines are examples.