Background: Measles kills more than 1 million infants per year and is particularly lethal in infants < 1 year old in developing countries. Recent reports have suggested that measles vaccines of different strains and titre differ in their immunogenicity in young infants. We sought to identify strains and titres of measles vaccines which would be effective in 6 and 9 month old infants.
Methods: We conducted a randomized trial of AIK-C, Edmonston-Zagreb (EZ), Leningrad-16 and Schwarz strains of measles vaccine at different titres in 1202 6 month old and 1250 9 month old infants. Antibody levels were measured by haemagglutination inhibition assay. Seroconversion was defined as a change from seronegative to seropositive or a fourfold rise in titre above the expected level after antibody decay (assumed antibody half-life = 6 weeks). Chi-square tests were used to compare seroconversion rates and rates of adverse reactions among the groups. Comparison of geometric mean titres (GMT) was done by the Student's t-test.
Results: No severe or unusual adverse reactions occurred during the 6 weeks after vaccination. All strains induced high seroconversion rates in 9 month old infants. In 6 month olds, medium- and standard-titre AIK-C induced the highest rates of seroconversion. Antibody titres at 6 weeks after vaccination were highest in recipients of Schwartz vaccine and lowest for EZ vaccine recipients.
Conclusions: Standard-titre AIK-C may be more effective than other measles vaccine strains for early measles immunization and should be evaluated further for efficacy, long-term immunogenicity, and long-term safety.