Residents of Arkansas have been under siege by a viral threat that is typically preventable through vaccination. Since August 2016, more than 2,000 people have been stricken with mumps, an infection of the major salivary glands that causes uncomfortable facial swelling.

The disease is highly contagious but can usually be prevented by making sure that children (or adults) have had two doses of the measles-mumps-rubella (MMR) vaccine. But strangely, about 70 percent of people in Arkansas who got sick with mumps reported that they had received their two doses of the MMR vaccine.

So, members of the HealthMap lab, led by Chief Innovation Officer and director of the Computational Epidemiology Group at Boston Children’s Hospital, John Brownstein, PhD, asked, “Why did this outbreak take off?”

Social media shows real-time, on-the-ground activity

“We turned to a tool called 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,” writes Maia Majumder, a HealthMap researcher, in a recent NPR Shots blog post.

“For this particular outbreak, digital disease surveillance allowed us to map the total number of cases at various points in time, data that wasn’t publicly available from the Arkansas Department of Health,” says Majumder. “Moreover, digital disease surveillance actually brought the outbreak itself onto our radar. Until our study, the vast majority of coverage regarding the outbreak was buried in local news streams.”

In a new paper, published in The Lancet Infectious Diseases, the HealthMap team reveals just how these Arkansas communities were left so vulnerable to this vaccine-preventable disease.

Vaccination for the individual and the community

The interesting thing about the MMR vaccine is that 10 percent of people who get both doses will still be liable to catch mumps. In order to keep it from spreading, analysis shows that 96 percent of people in a given “herd” have to have received their CDC-recommended two doses of the MMR vaccine.

Many people (70 percent, in fact) in Arkansas who got sick said they had received two doses of the MMR vaccine. Although the HealthMap team knew from previous studies that people tend to over-report their own vaccination history — sometimes even by twice as much — that could mean 35 percent of those who got sick had received two doses of the MMR vaccine, still way above the expected 10 percent. At the overall population level, something was amiss.

Tracking the outbreak online

To find out what was going on, the team first needed to understand how the mumps outbreak had spread. The HealthMap team turned to their Digital Surveillance System to get a better picture.

Luckily for their data gathering purposes, people are usually not shy about sharing personal information online.

Using social media data, the HealthMap team tracked the spread of mumps through Arkansas communities since August 2016. From there, the team used mathematical modeling to determine that the overall vaccination rate in the affected Arkansas communities was very likely less than 89 percent. In fact, it could even be as low as 70 percent.

Herd immunity matters

The HealthMap team’s analysis showed that the affected Arkansas communities had failed to establish strong herd immunity against mumps. The large percentage of unvaccinated people allowed mumps to spread quickly, even infecting an unusually high number of people who had received their two doses of the MMR vaccine.

Preventable diseases like mumps are usually not deadly for healthy people. But for infants and people with compromised immune systems — due to cancer or immunosuppressant use, for example — being in a herd that’s susceptible to these infections might actually prove fatal.

Majumder sums it up at the end of her NPR post: “When we vaccinate, we protect not only ourselves but also the most vulnerable members of our communities.”

Explore real-time disease activity around the world using HealthMap.