SEROLOGIC STATUS AND MEASLES ATTACK RATES AMONG VACCINATED AND UNVACCINATED CHILDREN IN RURAL SENEGAL

During a measles vaccine trial in a rural area of Senegal, antibody st atus was examined within 10 days of exposure for 228 previously vaccin ated and 313 unvaccinated children more than 12 months old who were ex posed to measles at home. Thirty-six percent of the children developed clinical measles, the clinical diagnosis being confirmed for 135 of t he 137 children from whom 2 blood samples were collected. Vaccine effi cacy was 90% (95% confidence interval, 83 to 94%). The hemagglutinin i nhibiting antibodies (HI) or plaque neutralizing antibodies (PN) assay s were equally efficient in predicting susceptibility and protection a gainst measles. Vaccinated children who had no detectable HI or PN ant ibodies at exposure had significant protection against measles compare d with seronegative unvaccinated children (HI vaccine efficacy, 49% (9 5% confidence interval, 21 to 68%); PN vaccine efficacy, 43% (95% conf idence interval, 12 to 62%)). The attack rate was high for children wi th a titer of 40 to 125 mIU) 67% (4 of 6) of those with a positive hem agglutinin-inhibiting antibody test and 36% (13 of 36) of those with a positive PN test developed measles. Attack rates among children with HI or PN titers above 125 mIU were 2% (6 of 295) and 3% (7 of 258), re spectively. Because titers of less than or equal to 120 mIU have been found to offer little protection in another study, this antibody level may be the best screening value for assessing susceptibility and prot ection against measles. However, it should be noted that many seronega tive vaccinated children are protected against measles infection.