The role of herd immunity in control of measles.

Measles vaccine cannot give high sero-conversion rates in developing countries. The high birth rates characteristic of these countries lead to infection at a very early age, thus making it difficult to vaccinate before exposure to the disease. Nevertheless, if given early in life, the vaccine can reduce the rate of virus circulation and thus raise the age at which children are infected. Once that is done, higher sero-conversion rates can be obtained by raising the age at vaccination. During the period when vaccine is given at an early age, the titers in responding children will be low, and this will leave children of the next generation with little protection. It is important, therefore, that if vaccine is used early the program be intensively and consistently applied to control virus circulation before the next generation is born.