Loss of maternal measles antibody during infancy in an African city.

Measles epidemics with 20% of the cases under nine months of age continue to occur in Brazzaville, Congo, even though measles vaccination coverage was 77% in the 12- to 23-month age group in 1986. In order to estimate the duration of passive immunity against measles, we conducted a serologic survey of infants aged 2 to 9 months. Measles antibody was measured from capillary blood with the plaque inhibition test. An antibody titre of 40 milli-International Reference Units per ml of serum (mIRU/ml) or less was defined as seronegative. Among the 252 infants studied, the proportion with detectable antibody dropped from 95.8% at 2 months of age to 48.5% at 4 months of age, and to 8.2% in the 7-8 month age group. A simple logistic model with age as the only variable provided an excellent fit to the observed values. Between the ages of 8 and 28 weeks, there was an almost steady decline of approximately 4.7% per week in the proportion of infants who were seropositive. These findings suggest that loss of maternal measles antibody during infancy might be faster than reported in other African populations. Giving measles vaccine to infants before the age of nine months currently recommended by the Expanded Programme on Immunization may be useful in some populations. Further studies of seroconversion and impact on measles are needed.