March 27, 2017

The math of a mumps outbreak

Daily Briefing

    A new analysis of an ongoing outbreak of mumps in Arkansas provides a striking reminder that low vaccination rates can endanger an entire community—and underscore the importance of herd immunity, Maimuna Majumder writes for NPR's "Shots."

    According to CDC, the signs and symptoms of mumps include a swollen jaw, fever, and headache. While the virus usually isn't fatal, it is extremely contagious.

    Maimuna Majumder—a Ph.D. candidate at the Massachusetts Institute of Technology, and a fellow at HealthMap, a computational epidemiology lab based out of Boston Children's Hospital—writes, "Because of this, the measles-mumps-rubella, or MMR, vaccine is given in two doses"—the first when an infant is between 10 and 15 months old, and the second when the child is between 4 and 6 years old.

    But risks remain: 10 percent of people who receive both doses can still contract the disease, Majumder explains, and the number of vaccinated people get sick during an outbreak depends in large part on the community's overall vaccination rate. According to Majumder, research suggests that a community might need a vaccination rate "as high as 96 percent" to achieve true "herd immunity."

    The Arkansas outbreak

    Since August 2016, an outbreak of mumps in Arkansas has sickened nearly 3,000 people across 33 counties. The outbreak caught the attention of Majumder and her colleagues, who started digging into the data to try and understand how the outbreak took off. They recently published their findings in The Lancet Infectious Diseases.

    The Arkansas Department of Health released "semi-regular" reports on the outbreak, which included case counts by age, county, and school district—but there were no "historical archives that could give us a sense of how the outbreak had grown over time," Majumder writes.

    To fill in the blanks, Majumder said her team used "the HealthMap Digital Surveillance System, which collects social media reports and news about public health issues around the world and turns them into usable epidemiological data." Since August 2016, the tool showed that the total number of mumps cases rose to about 2,900 by March 15.

    But what did the data say about vaccination rates in affected communities? On the one hand, the Department of Health data show that among those who contracted the virus, the vaccination rate averaged 70 percent. However, according to Majumder, "studies have shown that self-reported vaccination rates are typically 1.5 to two times greater than actual vaccination rates"—meaning the real rate of vaccination among those who became ill was probably somewhere between 35 and 46 percent.

    Majumder and colleagues then used mathematical models to estimate the overall vaccination rate in affected communities. In their estimation, the rate was no higher than 89 percent and as low as 70 percent—far short of the 96 percent rate that is required for herd immunity.

    "And this is why herd immunity is so important," Majumder concludes. "When we vaccinate, we protect not only ourselves but the most vulnerable members of our communities, too" (Majumder, "Shots," NPR, 3/22).

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