MEASLES-VACCINE EFFECTIVENESS IN STANDARD AND EARLY IMMUNIZATION STRATEGIES, NIGER, 1995

Background. An Expanded Programme on Immunization was started in late 1987 in Niger, including vaccination against measles with one dose of standard titer Schwarz vaccine given to infants after 9 months of age, During epidemics an early two dose strategy was implemented (one dose between 6 and 8 months and one dose after 9 months). From January 1, 1995, until May 7, 1995, 13 892 measles cases were reported in Niamey, Niger. Methods. A retrospective cohort study was conducted in a crowd ed area of Niamey at the end of the outbreak to assess the effectivene ss of measles vaccine in standard (after 9 months) and early (before 9 months) immunization strategies under field conditions. Results, High est measles incidence rates were observed among children <1 year of ag e. Vaccine effectiveness estimates increased with age at vaccination f rom 78% with a single dose administered at 6 months of age to 95% at 9 months. Vaccine effectiveness with the early two dose strategy was 93 %. Conclusions. Immunization with a single dose of standard titer Schw arz vaccine before 9 months of age provided higher clinical protection than expected from seropositivity studies. The early two dose strateg y is justified in contexts where measles incidence is high before 9 mo nths of age. Our results raise the issue of lowering the recommended a ge for measles vaccination in developing countries.