Stage Set for Big Measles Outbreaks in Texas With Current Vaccination Rates

Tara Haelle

August 21, 2019

Current Texas schools' vaccination rates could allow for measles outbreaks as large as 400 cases in the Dallas-Fort Worth and Austin metropolitan areas, according to a study published online today in JAMA Network Open.

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, Sinclair told Medscape Medical News. 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," Sinclair told Medscape Medical News. "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. 

The model estimates that 64% of those infected would be students whose parents had refused vaccination, whereas the other 36% cases would be "bystanders," such as individuals too young or too ill to be vaccinated, those in whom the vaccine is contraindicated, and those in whom the vaccine had waned or was ineffective.

The authors noted that even small networks could have big impacts, such as two schools in Tyler, Texas, whose vaccination rates are 70% and 85%.

"In the event of an outbreak in schools with undervaccinated populations, interventions targeted at these schools may be especially effective," Sinclair and colleagues write. "Undervaccinated close-knit communities present an increased risk of outbreaks; mandating that schools with low vaccination rates plan for outbreak scenarios may help reduce outbreak sizes."

With nonmedical vaccine exemptions available in 45 US states, large measles outbreaks could potentially occur in many parts of the country, Sinclair told Medscape Medical News.

"Geographic clustering of unvaccinated children is also important — if unvaccinated children are clustered into select schools, large measles outbreaks could still occur even if the overall vaccination rate in a city is high," he said.

And that clustering may exist in the places people wouldn't expect, according to Tara Smith, PhD, professor of epidemiology at Kent State University in Ohio. Smith pointed to a 2018 study that took stock of clusters of high exemption rates throughout the United States. The study highlighted four Texas cities with clusters of high rates — Houston, Fort Worth, Plano, and Austin — and several cities already associated with high levels of vaccine hesitancy, such as Seattle, Spokane, and Portland. But other potentially problematic cities were scattered across the Southwest and Midwest, especially Phoenix, Salt Lake City, and several Michigan cities. And Idaho and Wisconsin had pockets of high nonmedical vaccine exemption rates.

"Anywhere those pockets exist, we need to be careful and keep an eye on those," Smith said. Smith and Maimuna S. Majumder, PhD, MPH, of Harvard Medical School in Boston, Massachusetts, address two prongs of prevention in an editorial accompanying the study: actions during outbreaks and maintenance of high immunization rates to avoid outbreaks. "During outbreaks of measles, cases are typically isolated, and those who have been exposed are subject to quarantine, they write. But those measures were insufficient for New York's 2018 outbreak, when unvaccinated children were banned from school and mandatory vaccination orders were implemented. Outbreaks might also spur catch-up vaccination.

"The more challenging aspect of prevention is maintaining vaccination rates in the absence of ongoing epidemics, which is complicated by the continued spread of misinformation about vaccines," Smith and Majumder write. "Countering such antivaccine rhetoric is not a simple task. Research into the factors driving vaccine hesitancy has not led to any easy answers or readily scalable interventions," leading public health officials to turn to legislation limiting nonmedical exemptions.

"Anti-vaccine sentiment definitely runs across the political spectrum from libertarians who just don't want government involved in their vaccine choices to the far left, who are against pharmaceutical companies and don't trust modern medicine," Smith told Medscape Medical News. "Any of the legislation that has been passed to reduce the number of vaccination exemptions, to take away any of the philosophical or religious exemptions, has largely been opposed by the more libertarian and right-leaning groups based on the idea of medical freedom and freedom from intervention from government," she said.

At the same time, she pointed to Robert F. Kennedy, Jr., one of the most vociferous critics of US vaccine policy and a member of one of the most prominent US political families on the left, as an example of the bipartisan nature of vaccine refusal.

Since vaccine policy is typically made at the state level rather than the federal level, Smith encouraged providers to stay up to date with legislation being considered in their state and in their local communities.

Providers should also take note of populations who are unvaccinated or undervaccinated because of logistical reasons rather than hesitancy or refusal, Smith said. "That's the piece that gets left out of a lot of this, those not getting vaccinations because of lack of access, lack of transportation, lack of time to bring their children into the clinic" and similar issues associated with "inequality" rather than "privileged choice," Smith said. Setting up after-hours clinics or working with public health departments are ways providers can help improve vaccination rates independent of vaccine attitudes.

The research was funded by the National Institutes of Health. Two authors, John Grefenstette and Donald Burke, codeveloped the Framework for Reconstructing Epidemiological Dynamics (FRED) software tool, for which they may receive licensing royalties, and cofounded the company Epistemix Inc based on the software. Study author Mark Roberts is an unpaid technical advisory board member of Epistemix Inc. Smith and Majumder have disclosed no relevant financial relationships and used no external funding.

JAMA Network Open. Published August 21, 2019. Full text, Editorial

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