Effectiveness of vaccination at 6 to 11 months of age during an outbreak of measles

Hypothesis. Monovalent measles vaccine can be administered to children 6 to 11 months of age during an outbreak. Efficacy and effectiveness of this control measure still have to be assessed. Methods. During an outbreak of measles, monovalent measles vaccine was administered as part of outbreak control to children aged 6 to 11 months. Active surveillance was used to detect cases of measles occurring during the following month. Children who did not develop measles were tested for measles antibody before their revaccination at 15 months of age. Results. Of 81 children 6 to 11 months of age, 56 were vaccinated and two received immunoglobulins; the latter were excluded from the analysis. Measles occurred in 15 of the 79 children during and after the vaccination campaign, for an overall attack rate of 19%. The attack rate among unvaccinated children was 39% (9 of 23), compared with 11% (6 of 56) among those vaccinated (relative risk = 3.6, 95% confidence interval [CI] = 1.5 to 9.1). All of those who sustained measles in the vaccinated group developed the disease within 10 days after vaccination. The overall vaccine effectiveness was 73% (95% CI = 32% to 89%) when children were classified as vaccinated as soon as they were given measles vaccine. It rose to 96% (95% CI = 72% to 99%) when children were considered vaccinated 1 week postimmunization. Nineteen infants who were vaccinated and who did not develop measles during the outbreak were tested for measles antibody status at 15 months of age before revaccination. All had plaque reduction neutralizing antibody titers greater than 120. Conclusion. This study confirms that measles vaccination of infants aged 6 to 11 months is an effective intervention measure during measles outbreaks.