Measles in Children Vaccinated With 2 Doses of MMR

OBJECTIVE: A previous measles outbreak investigation in a high school in Quebec, Canada identified 2-dose vaccine effectiveness of 94%. The risk of measles in 2-dose recipients was significantly higher (2-4 times) when measles vaccine was first administered at 12 versus >= 15 months of age, with no significant effect of the age at second dose. Generalizability of this association was also assessed in the expanded provincial data set of notified cases. METHODS: This matched case-control study included only 2-dose recipients. All confirmed (laboratory or epidemiologically linked) cases in patients aged 5 to 17 years were included. Each case was matched to 5 controls. RESULTS: A total of 102 cases and 510 controls were included; 89% of cases were in patients 13 to 17 years old. When the first dose was administered at 12 to 13 months compared with >= 15 months of age, the risk of measles in participants outside the outbreak school was 6 times higher (95% confidence interval, 1.33-29.3) and was 5.2 times higher (95% confidence interval, 1.91-14.3) in the pooled estimate (participants from the outbreak school + outside that school). CONCLUSIONS: A significantly greater risk of measles among 2-dose recipients whose first dose was given at 12 to 13 months rather than >= 15 months of age is confirmed in the larger Quebec data set. The mechanism remains unknown, but vaccine failures in 2-dose recipients could have substantial implications for measles elimination efforts through 2-dose vaccination. The optimal age at first dose may warrant additional evaluation.