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BULLETIN OF THE WORLD HEALTH ORGANIZATION. 1993; 71(3-4):421-8.Worldwide coverage of measles vaccine is about 80%, but many communities and countries have considerably lower coverage rates. WHO is concerned about measles occurring in infants between 6 and 12 months old, especially in densely populated African cities. Measles rarely occurs in infants under 6 months old, but the measles case fatality rate is greatest in the 1st year of life. WHO aims for an effective measles vaccine to be administered at 6 months old. A high titer vaccine appears to reduce survival among children receiving it. Some countries have reduced measles incidence by as much as 90% by achieving coverage levels greater than 90% with a single dose measles vaccine. Another method to prevent early measles cases and later vaccine failures is administration of the 1st dose around 6 months and a 2nd dose no earlier than 12 months. Measles vaccine policy in the US and some countries in Europe is routine 2-dose measles schedules: 1st dose between 12-19 months and 2nd dose at school entry. This schedule is appropriate in developed countries with good immunization coverage. Other countries schedule the 1st dose anywhere between 6-9 months and the 2nd dose between 12 months and 7 years. All mathematical models of the effects of 2-dose schedules indicate that 2-dose schedule are a great benefit. The literature shows that developing countries with high immunization coverage and well-managed immunization programs can effectively execute and sustain 2-dose measles schedules. Measles vaccination early in life sometimes results in a blunted antibody response. The 2-dose schedules are probably more expensive than 1-dose schedules and require more cold storage space. No field trials have looked at clinical efficacy of 2-dose measles schedules in developing countries. Ideal field trials would be randomized controlled trials. Demonstration projects can evaluate operational issues, e.g., dropout rates, cost, and vaccine usage. Case control studies can address technical and epidemiological issues.