Almost 10 years after health officials declared measles was eliminated in the U.S., falling vaccination rates pose a threat to herd immunity.

Now, a new study suggests some Texas schools could soon face large measles outbreaks.

The findings appear in the journal JAMA Network Open.

Measles is highly infectious and can cause severe health problems, including pneumonia, brain damage and death.

The two-dose measles, mumps and rubella vaccination is 97% effective, but herd immunity is hampered by a small section of the population that declines vaccinations for various reasons.

As of June 2019, 45 states allow vaccine exemptions for religious or personal reasons, in addition to medical exemptions. Texas, the largest among them by population, has seen exemption rates multiply since 2003 from 2,300 to 64,000.

Researchers ran 1,000 simulations for each of Texas's metropolitan statistical areas over a 270-day school year. The simulations were based on data from 2010 U.S. Census and actual Texas schools.

Lead author David Sinclair of the University of Pittsburgh says Austin–Round Rock and Dallas–Fort Worth–Arlington could see outbreaks involving 400-plus cases under the worst scenarios, which accounted for roughly 1 in 20 scenario runs.

• See measles simulations

That finding is based on 2018 vaccination rates, which in some schools fell below 92%.

If rates drop another 5%, that number could climb sharply. In Dallas, the worst outbreak sizes could reach 550, while Austin could approach 1,100.

"What we did find across the whole state was that the potential size of outbreak will rise exponentially if vaccination rates decrease," Sinclair said.

In Arizona, personal belief exemptions apply to kindergarten through 12th grade only, and religious exemptions are allowed solely for childcare, preschool and head start programs.

Statewide student immunization rates for measles have dropped to 93%, down 1.2% from three years ago.

Herd immunity requires around 92-96% of individuals be immunized.