Study Predicts Potential Measles Outbreaks in Texas

A recent study suggests that vaccination rates in some Texas schools are low enough to allow for large measles outbreaks, but limiting vaccine exemptions could decrease this risk. Vaccine exemptions, which allow unvaccinated children to attend school, have increased by a factor of 28 since 2003 in Texas.

Measles, which can result in severe outcomes, such as pneumonia, brain damage, and death, can be avoided with vaccination. The CDC recommends that children receive their first measles vaccination between ages 12 months and 15 months, and a second dose between ages 4 years and 6 years (before entering school).¹

Two doses of the vaccine is about 97% effective, while a single dose is about 93% effective. It is extremely contagious: if 1 person has the virus, the CDC says up to 90% of people around him or her will also become infected if they are not vaccinated.²

Texas, the second largest US state by population, encompasses 24 metropolitan statistical areas (MSAs). MSAs are defined as a geographic area containing at least 1 urban area of at least 50,000 people and the surrounding counties from which people commute to those urban areas. Texas MSAs can be considered representative of many US population centers, based on population size.

The study found that 1 in 20 measles introductions was forecasted to be associated with outbreaks of more than 400 cases in the Austin and Dallas metropolitan areas, according to 1000 simulated outbreaks in each metropolitan area. Furthermore, 64% of cases were forecasted to occur in children for whom a vaccine has been refused and 36% in others. Approximately 0.2% of students in Texas are ineligible for the vaccination because of contraindications.

The study found that a 5% decrease in vaccination rate was associated with a 40% to 4000% increase in potential outbreak size, depending on the MSA.

While it varies by local demographic conditions, approximately 92% to 96% of a homogeneously mixed population requires immunity in order to maintain herd immunity.

As vaccination rates have decreased in Texas, however, the study found that large measles outbreaks of more than 400 cases could occur in the MSAs Austin-Round Rock and Dallas-Fort Worth-Arlington, based on their 2018 distribution of vaccination rates. In those areas, there are 35 and 13 schools, respectively, with vaccination rates less than 92%—the lower band of the estimated herd immunity threshold.

The study found that among the susceptible population (those who are unvaccinated), 30% of all transmissions occur in households, 33% occur in the neighborhood, and 37% occur in schools or workplaces

Outbreaks have increased across the United States during the past decade, though the potential for outbreaks in Austin and Dallas is higher when compared with other areas in the country. Larger outbreaks of 71,275 and 609 cases have occurred in Washington and New York States, respectively and are ongoing. Seventeen outbreaks occurred in the United States in 2018, compared with a median of 4 outbreaks per year from 2001 to 2012.

This decrease in vaccinations not only affects those who have not received the vaccine, but could also affect bystanders in the population as the herd immunity is weakened.

References

• Sinclair DR, Grefenstette JJ, Krauland MG, Galloway DD. Forecasted size of measles outbreaks associated with vaccination exemptions for schoolchildren. JAMA Network. August 2019. Doi:10.1001/jamanetworkopen.2019.9768
• Vaccine for Measles (MMR Shot) | CDC. Centers for Disease Control and Prevention. https://www.cdc.gov/measles/vaccination.html. Accessed August 20, 2019.