The role of herd immunity in control of measles.

In densely populated developing countries it is impossible to protect the majority of the measles-susceptible population regardless of the immunization schedule used. In such countries a large proportion of measles cases occur early in life and by the time children have lost maternal antibodies and thus become responsive to vaccine many will already have become infected. Revaccination at a later age is not a satisfactory solution to the problem of vaccine effectiveness in developing countries because early vaccination inhibits the effects of a second dose. If effective control of measles is to be achieved in developing countries a program must be designed to build upon and expand herd immunity. Herd immunity is difficult to establish against measles because the virus is extremely contagious and a high level of population immunity is required. Population immunity affects the frequency of virus transmission and thus the age at infection. Active immunization makes the most significant contribution to herd protection but the duration of passive protection from maternal antibodies is variable and important in providing protection during infancy when measles is often fatal. When given early in life measles vaccine can reduce the rate of virus circulation and raise the age at which children are infected. Once this is achieved higher seroconversion rates can be obtained by raising the age at vaccination. During the period that vaccine is given at an early age the titers in responding children will be low leaving the next generation with little protection. Thus if measles vaccine is administered early the measles control program should be intensively and consistently applied to control virus circulation before the next generation is born.