Protective efficacy of high-titre measles vaccines administered from the age of five months: a community study in rural Senegal

Abstract

Using data on incidence and secondary attack rates, we examined the protective efficacy of high-titre Edmonston-Zagreb (EZ) and Schwarz (SW-HT) measles vaccines administered at 5 months. Control children were assigned to placebo at age 5 months and standard Schwarz (SW-std) measles vaccine at 9–10 months of age. A large proportion of measles cases was verified serologically. Though high-titre vaccines seemed to be protective before 10 months of age, a significant reduction in disease could not be demonstrated due to low incidence of measles. After 10 months of age, SW-std given at 10 months gave a vaccine efficacy of 100% and induced better protection than SW-HT ( P = 0·030) and EZ-HT ( P = 0·128) administered at 5 months. In studies of secondary attack rates in the compound, vaccine efficacy was 91% (75%–97%) for EZ-HT, 85% (40%–96%) for SW-HT, and 100% for SW-std. Attack rates were correlated with intensity of exposure ( P = 0·0006), being much higher for children exposed in the same hut than for those living in the same compound but in a different household (relative risk = 3·36 [1·32–8·57]). The attack rate was significantly lower among vaccinated than unvaccinated children with no detectable measles antibody (relative risk = 0·41 [0·18–0·93]). In rural areas with a high coverage in the surrounding community, a single dose at 9–10 months may provide sufficient protection. Since high-titre vaccines have been associated with higher mortality than SW-std, further improvements in measles control before 9 months may require two-dose strategies with standard vaccines.