Current Texas schools’ vaccination rates could allow for measles outbreaks as large as 400 cases in the Dallas-Fort Worth and Austin metropolitan areas.
Even slight decreases in immunization rates could result in exponentially larger outbreaks, the authors say, noting that other states have population centers at similar risk.
“Lower vaccination rates imply that outbreaks may occur with greater frequency, because there are both more people who can become exposed to measles when away from their metropolitan statistical areas and more people who can be infected by an exposed individual from elsewhere,” write David R. Sinclair, PhD, of the University of Pittsburgh in Pennsylvania, and colleagues.
“In addition, refusers may be locally grouped, sharing schools and communities, creating a greater risk of measles introductions spreading to a large number of unvaccinated students.”
Nonmedical exemption rates in Texas have been increasing every year since 2003, when state legislation made it easier to obtain nonmedical exemptions. In fact, exemptions have increased 28-fold, from 2300 in 2003 to 64,000 in 2018, likely driven at least in part by politically active anti-vaccine organizations such as Texans for Vaccine Choice. And it doesn’t help that Andrew Wakefield, whose fraudulent research inspired two decades of unfounded fears about the MMR vaccine and its links to autism, lives in Austin.
“Texas is the largest state by population that allows vaccine exemptions for religious or personal reasons. “It has some of the largest metropolitan areas in the country, but also population centers that are typical of many areas of the US.”
The researchers used 2010 US Census data to create a model to simulate possible measles transmissions in the Texas population. The model incorporated 2018 vaccination rates for Texas schools and typical daily activities and interactions for 1000 simulations in each metropolitan area of the state. Each hypothetical scenario tracked infections over 9 months.
They estimate that the median number of cases in Texas cities would likely range from 1 to 3 cases if 2018 vaccination rates are maintained during an outbreak, which is in line with case numbers seen in outbreaks that occurred between 2006 and 2017.
However, the number of cases could climb above 400 in the Austin and Dallas-Fort Worth metro areas, according to the model’s upper limit predictions. That number of cases would rival the largest outbreaks since US measles elimination in 2000.
The combination of high population density and multiple undervaccinated schools would drive these outbreaks, the authors note. Specifically, 35 schools in Austin-Round Rock, 13 schools in Dallas-Fort Worth, and 15 schools in Tyler have vaccination rates below the estimated herd immunity lower threshold of 92%.
When the researchers investigated the effects of even lower immunization rates than seen in 2018 — if existing trends were to continue in currently undervaccinated schools — they found exponentially greater outbreaks. For example, a 5% reduction in immunizations was correlated with a 40% to 4000% larger outbreak than would be seen with the 2018 vaccination rate, depending on the city.