(HealthDay News) — Measles, mumps, and rubella (MMR) vaccination rates are estimated at between 50–86% among the population exposed to the recent measles outbreak, according to a research letter published online March 16 in JAMA Pediatrics.

Maimuna S. Majumder, MPH, from the Massachusetts Institute of Technology in Cambridge, and colleagues used cumulative incidence data from the California Department of Public Health and Health-Map media alerts to estimate the vaccination rate in the context of the 2015 measles outbreak.

The researchers found that with a serial interval associated with measles of 10, 12, and 14 days, the effective reproductive numbers (RE) were 3.2, 4.1, and 5.8, respectively. Over the range of estimates of the basic reproduction number from prevaccination era (11 to 18), the estimated vaccination rates varied from 75–86%, 66–81%, and 50–71% assuming the RE values were 3.2, 4.1, and 5.8, respectively.

“Our study estimates that MMR vaccination rates among the exposed population in which secondary cases have occurred might be as low as 50% and likely no higher than 86%,” the authors write. “Given the highly contagious nature of measles, vaccination rates of 96–99% are necessary to preserve herd immunity and prevent future outbreaks. Even the highest estimated vaccination rates from our model fall well below this threshold.”

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