Inadequate vaccine coverage is likely a driving force behind the ongoing Disneyland measles outbreak, according to calculations by a research team at Boston Children's Hospital. Their report, based on epidemiological data and published online by JAMA Pediatrics, indicates that vaccine coverage among the exposed populations is far below that necessary to keep the virus in check, and is the first to positively link measles vaccination rates and the ongoing outbreak. By examining case numbers reported by the California Department of Public Health and current and historical case data captured by the HealthMap disease surveillance system, the researchers--led by Maimuna Majumder, MPH, and John Brownstein, PhD, of Boston Children's Informatics Program--estimate that the measles vaccination rate among the case clusters in California, Arizona and Illinois is between 50 and 86 percent, far below the 96 to 99 percent necessary to create a herd immunity effect.

Measles is highly contagious. It's estimated that an infected individual in a population fully susceptible to measles will spread the virus to between 11 and 18 additional people. This number is called the virus's basic reproduction rate, or R0. In a population where at least some individuals are immune to measles, the virus spreads from person to person more slowly. The rate of spread in an immune population is called the virus's effective reproduction rate, or RE.

Using case data, R0 and measles' serial interval (the length of time for each successive wave of transmission to follow the one before), Majumder and Brownstein calculated that the virus's RE in the Disneyland outbreak is between 3.2 and 5.8. From there, the pair calculated their vaccination estimate.

The researchers are quick to note that their estimate does not reflect vaccination across the United States, the state of California or even among the population of Disneyland visitors at the outbreak's start. Rather, it reflects the vaccination rate among the exposed populations in each cluster of cases linked to the outbreak so far.

"It's as though you took everyone exposed to measles in the areas with case clusters, put them in a room and measured the level of vaccine coverage in that aggregate population," says Majumder.