IMMUNOGENICITY OF HIGH-TITER AIK-C OR EDMONSTON-ZAGREB VACCINES IN 3.5-MONTH-OLD INFANTS, AND OF MEDIUM-TITER OR HIGH-TITER EDMONSTON-ZAGREB VACCINE IN 6-MONTH-OLD INFANTS, IN KINSHASA, ZAIRE

The effect of measles vaccine potency was evaluated among 485 children aged 6 months, and the effect of vaccine strain was evaluated among 538 children aged 3.5 months, in Kinshasa, Zaire. Children aged 6 months were randomly assigned to receive either high-titre Edmonston-Zagreb (EZ-H), potency 5.7 log(10)/dose, or medium-titre EZ (EZ-M), potency 4.7 log(10)/dose; those aged 3.5 months were randomly assigned to receive either AIK-C, potency 5.5 log(10)/dose, or EZ-H, and were revaccinated with EZ-M vaccine at age 9.5 months. Measles antibodies were measured using the plaque reduction neutralization assay. Among children vaccinated at age 6 months, the seroresponse was significantly higher after EZ-H than EZ-M vaccine, with 92 and 83% seroconverting by 6 months postvaccination and 59 and 40% respectively having antibody titres > 200 mIU. Among children vaccinated at age 3.5 months, only 24% (AIK-C) and 22% (EZ-H) attained antibody titres greater than or equal to 200 mIU 6 months postvaccination. After revaccination at age 9.5 months, 81% of children in the AIK-C group and 73% in the EZ-H group had antibody levels > 200 mIU (p = 0.056). A retrospective survey was conducted in January 1993 to determine the mortality experience of vaccine groups, and information was obtained for 94% of the children. A total of 44 deaths (4%) were identified, with no significant differences between groups when stratified by age at vaccination. Although high-titre EZ vaccine is immunogenic among 6-month-old children, this vaccine is no longer recommended for use because of the finding of increased mortality among recipients of this vaccine in other studies. The efficacy of two doses of standard titre vaccine requires further study.