Childhood MMR vaccination and the incidence rate of measles infection: a ten year longitudinal cohort study of American children born in the 1990s

BACKGROUND: Measles (rubeola) is a highly contagious disease with significant morbidity/mortality. Measles-Mumps-Rubella (MMR) is a live-attenuated vaccine used in the United States (US) since the early 1970s to prevent measles infection. This retrospective longitudinal cohort study examined childhood MMR vaccination effectiveness (VE) on preventing diagnosed measles cases. METHODS: The Independent Healthcare Research Database (IHRD) is composed of non-identifiable linked eligibility and claim healthcare records prospectively generated from the Florida Medicaid system. The SAS system was utilized to examine a cohort of 101,736 persons eligible for Florida Medicaid from 1990 to 2009 and continuously eligible with ≥10 outpatient office visits during the 120-month period following birth. There were 32,870 persons (224,492 person-years) in the cohort receiving a single dose of childhood MMR vaccine (vaccinated) and 43,538 persons (434,637 person-years) in an unvaccinated cohort (no exposures to measles-containing vaccine). The frequency of diagnosed measles (ICD-9 code: 055xxx) was examined. Cox proportional hazards models evaluated MMR vaccination and diagnosed measles over time. RESULTS: MMR vaccinated cohort members were at significantly reduced risk of measles in the unadjusted (VE = 83.6, 95% CI = 67.2–91.8%) and adjusted (VE = 80.7, 95% CI = 61.5–83.9%) models as compared to the unvaccinated cohort. VE = 80% among younger MMR recipients (12–15 months), whereas VE = 90% among older MMR recipients (16–20 months) as compared to the unvaccinated cohort. CONCLUSION: Routine childhood MMR vaccination significantly reduced the incidence rate of childhood measles infections, and the VE was greater in the older recipients (16–20 months) than in the younger recipients (12–15 months).