2015 Measles Outbreaks Fueled By Low Vaccination Rates

First Posted: Mar 16, 2015 04:17 PM EDT

New findings published in JAMA Pediatrics show that adequate vaccine coverage will help to be a driving force behind ongoing issues with the measles outbreak in Disneyland to keep the health problem there and other places at bay.

For the study, researchers examined case numbers reported by the California Department of Public Health and current and historical case data captured by the HealthMap disease surveillance system, that was led by Maimuna Majumder, MPH, and John Brownstein, PhD, of Boston Children's Informatics Program--estimating that the measles vaccination rate was among the case clusters in California, Illinois and Arizona at between 50 and 86 percent, and below 96 to 99 percent necessary to create a large-scale immunity effect.

As measles is highly contagious, researchers estimate than an infected individual can be susceptible to the spread of the virus between 11 and 18 additional people. The number is also called the virus's basic reproduction rate, or R0, where at least some individuals can be immune to measles as the virus spreads from person to person more slowly and the rate of the spread in an immune population that can be called the virus's effective reproduction rate or RE.

Researchers used data, RO and measles' serial interval, Majumder and Brownstein to calculate that the virus's RE in the Disneyland outbreak that was between 3.2 and 5.8. Then they calculated the vaccination estimate where researchers were quick to note that the estimate did not reflect the vaccination across the United States of California or even among the population of visitors at the park during their first outbreak. Rather, researchers noted how it reflected the vaccination rate among the exposed populations in each cluster of cases that are linked to the outbreak thus 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," added Majumder, in a news release. 

The HealthMap team then seperately released an interactive model to illustrate how different rates of vaccine coverage could affect the growth of measles outbreak over time,

"Our data tell us a very straightforward story--that the way to stop this and future measles outbreaks is through vaccination," said Brownstein, a digital epidemiologist and co-founder of HealthMap and VaccineFinder, an online service that allows users to search for locations offering a variety of vaccinations, including the MMR vaccine that protects against measles, in a news release. "The fundamental reason why we're seeing the number of cases we are is inadequate vaccine coverage among the exposed.

"We hope these data encourage families to ensure they and their loved ones are vaccinated," he continued, "and help local public health officials in their efforts to control this outbreak."

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