Vaccine Exemptions May Allow for Large Measles Outbreaks

More than 64,000 unvaccinated children attend Texas schools, thanks to legislation that allows for “conscientious exemption.” To learn more about the potential for outbreaks under these conditions, representatives of the Texas Pediatric Society—the state’s chapter of the American Academy of Pediatrics—contacted David Sinclair, a postdoc at the University of Pittsburgh. Sinclair and his colleagues use mathematical and statistical methods to predict how disease might spread, and they agreed to apply their tools to 2018 data on the vaccination rates in Texas public school districts.

The researchers ran 1,000 simulations for each of the 24 metropolitan areas, in which they forecasted what could happen if a single case of measles were introduced into the population. In most of the situations, three or fewer people got sick. But about 5 percent of the simulations for both the Dallas and Austin regions predicted that more than 400 people could be affected.

“We were seeing quite large potential outbreak sizes in some places and we were wondering, Have we miscalibrated our simulations somewhere?” Sinclair tells The Scientist, but the team members realized they were in the right ballpark when they started to see comparable numbers coming out of the real-life outbreak in New York City, where there have been 598 measles cases since January.

The models also revealed that, when more than 25 people were involved in an outbreak, about two-thirds of the cases were likely to be in unvaccinated kids, while the rest were predicted to occur in bystanders.

“The idea that people who don’t get vaccinated put everyone else at risk is really clearly articulated by their results,” says Natasha Crowcroft, a physician and public health researcher at the University of Toronto who did not participate in the study.

Not surprisingly, the team found an association between larger reductions in vaccination rates and increasing sizes of outbreaks. For example, with an 8 percent reduction in vaccination rates, outbreaks with a median of at least 10 cases were predicted to occur in 25 percent of the metropolitan areas. Yet with a 9 percent reduction, outbreaks of that size were predicted to occur in 71 percent of the metropolitan areas.

“The small percentage increase makes a huge difference to the risk,” says Crowcroft. “The positive message [here] is that you may think you’re just one person, but every person counts when it comes to getting good coverage. You really count as an individual, [so] when you get yourself or you get your kids immunized, you’re making a big difference.”

One limitation that Sinclair and colleagues acknowledge is that they ran simulations for each metropolitan area as an isolated unit, even though that’s not actually the case. “They’re probably underestimating . . . the total risk to all of the [areas] in Texas of any one outbreak starting because it could very well spread from one place to another,” explains Ferrari. “It’s a really complicated thing because we don’t know exactly how people are going to move around in an outbreak.”

Nevertheless, understanding the relative risk of an outbreak beginning and combining that with the likelihood of the disease spreading could help prioritize public health responses, he says. And in the future, models like these could be used to explore what kinds of interventions are most likely to be effective, which might make decisions during an outbreak easier for public health officials. 

Another caveat of the findings that the authors discuss in the paper is that vaccination levels for individual public schools aren’t available, so they used district-level data averaged out over each school. “If anything, these [models] will underestimate the likelihood of severe epidemics because you almost certainly have public schools that have much higher [exemptions] than the school district reported rate,” says Peter Hotez, a physician and researcher at Baylor College of Medicine. He published a perspective in 2016 warning of the potential for measles outbreaks in Texas due to increasing vaccine exemptions. “The next step is to get some transparency on the individual public schools and really expose the extreme risk that I think Texas schoolchildren face right now,” he adds.

D.R. Sinclair et al., “Forecasted size of measles outbreaks associated with vaccination exemptions for schoolchildren,” JAMA Network Open, doi:10.1001/jamanetworkopen.2019.9768, 2019.