High-titer measles virus vaccines: protection evaluation
The immunogenicity and clinical efficacy of high-titer measles vaccines were evaluated in a randomized controlled trial in Niakhar, a rural area of Senegal that is under demographic surveillance. Two high-titer vaccines were studied: a high-titer Edmonston-Zagreb vaccine (EZ-HT) and a high-titer Schwarz vaccine (SW-HT). Both HT vaccines were administered at five months of age, and they were compared to a Schwarz standard vaccine, administered at 10 months of age. At age five months, 56.9% of the children had medium or high levels of maternal antibodies (at least 250 milli-international units), whereas at 10 months of age only 25.5% of children had residual maternal antibodies.
Both high-titer vaccines were immunogenic when given to children with low levels of maternal antibodies; seroconversion rates were 98.6% (95% confidence interval, CI = 95.9 – 99.9) for the EZ-HT and 85.5% (CI = 78.5 – 93.2) for the SW-HT. There was no definitive evidence of seroconversion among children with high levels of maternal antibodies. However, levels of antibodies at age 10 months were significantly higher in the vaccinated groups than in the placebo group. The EZ-HT vaccine produced a higher response than the SW-HT vaccine (P<0.01). Both the titer at time of vaccination and the age at vaccination had a significant impact on the rate of seroconversion and the percentage of seropositive children at 10 months.
Clinical efficacy (EF) of HT vaccines was highest and significant among children with low levels of maternal antibodies [EF = 100.0%, CI = 50.0–100.0, P = 0.0001 for EZ-HT; and EF = 86.3%, CI = 43.9–96.7, P = 0.0060 for SW-HT], and lowest and not significant among children with high levels of maternal antibodies. Vaccine failures after the HT vaccines were 8.75 times more frequent than after the standard vaccine (CI = 1.02–75.1, P = 0.048]. These results indicate that vaccination of children with Schwarz measles vaccine at nine months of age remains the safest and most effective strategy for controlling measles.
KeywordsHemagglutination Inhibition Maternal Antibody Seroconversion Rate Measle Vaccine Measle Vaccination
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