[Measles control in developing countries]

The EPI has made significant progress in reducing global morbidity and mortality from measles. Many countries are considering the need for additional strategies in response to the changes in measles epidemiology which occur when vaccination coverage increases. Measles vaccination changes the relative age distribution of cases towards older groups, because measles transmission becomes less intense so that children reach older ages before being exposed to wild virus. The proportion of cases occurring below the target age group for vaccination may also increase, though the number of cases does not always increase. Outbreaks are likely to occur after some years of low incidence in countries which have moderate or high coverage, for several reasons. Firstly, susceptibles gradually accumulate over a number of years until a "post-honeymoon" outbreak occurs. Second, there may be pockets of low coverage which occur for example in urban slums, in nomadic and other inaccessible populations, or among groups with religious or philosophical objections to vaccination. Third, outbreaks can occur among vaccinated children who did not respond to the vaccine. Even in industrialized countries, where measles vaccine is administered in the second year of life and is over 90% effective in protecting against disease, small outbreaks have occurred among vaccine failures under conditions of close contact such as school settings. In developing countries, where measles vaccine is usually administered at age 9 months, approximately 15% of children are not protected, and vaccine failures can be expected to play an increasing role in outbreaks in the future. Furthermore, if vaccine efficacy is reduced because of poor vaccine storage or handling, large outbreaks may occur.(ABSTRACT TRUNCATED AT 250 WORDS)