Vaccination rates in some Texas schools are currently low enough to allow for large measles outbreaks, according to data published in JAMA Network Open.

Investigators simulated measles transmission in the Texas population using an agent-based decision analytic model and a synthetic population of Texas, derived from the 2010 United States Census. The 2018 vaccination rates from schools were used to simulate hypothetic equivalents of these schools. Single cases of measles were introduced and daily activities and interactions were modeled for each population member. The number of infections over the course of 9 months was then counted for 1000 simulated runs in each Texas metropolitan area.

Using the 2018 vaccination rates, the median number of cases related to an outbreak in large metropolitan areas was demonstrated to range from 1 to 3 cases, which is consistent with previous Texas outbreaks in 2006 to 2017. In the Austin and Dallas metropolitan areas, however, the upper limit of the distribution of plausible outbreaks (the 95th percentile, associated with 1 in 20 measles introductions) exceeded 400 cases. The simulations also identified 2 schools with vaccination rates of 70% and 85% that may be linked to a large measles outbreak.

These results are similar to the largest real-life outbreaks in the US since measles was eliminated in 2000.  Exponential increases in the potential size of outbreaks were associated with decreases in vaccination rates in schools with under-vaccinated populations in 2018. Specifically, a 5% decrease in vaccination rate was associated with a 40% to 4000% increase in potential outbreak size, depending on the metropolitan area. A mean of 64% (standard deviation 11%) of cases occurred in students who refused vaccination, but a mean 36% (standard deviation 11%) of cases occurred in bystanders.

As with all models, the foundational assumptions need to be considered. For example, Texas public school districts only publish the vaccination rate of the district, preventing the simulations from revealing the risk associated with specific public schools with low vaccination rates. Investigators also made the assumption that seventh-grade vaccination rates provide a more accurate representation of schoolwide vaccination rates than kindergarten vaccination rates. The relative ratios of human interactions in schools, workplaces, households, and neighborhoods are also likely to vary by location. Further, simulations that forecast outbreak sizes with decreased vaccination rates did not include an estimate of when these decreases might occur. Population and school sizes will change over time, which may affect forecasts, as would using alternative methods and data sources other than census data and published values of the basic reproduction number of measles.

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The data from the simulations run using 2018 vaccination rates suggested that there is a significant chance of outbreaks totaling more than 100 cases in multiple geographic areas. Investigators cautioned that, “if the vaccination rate among students in Texas continues to decrease in schools with under vaccinated populations, the potential number of cases associated with measles outbreaks is estimated to increase exponentially.”

Reference

Sinclair DR, Grefenstette JJ, Krauland MG, et al. Forecasted size of measles outbreaks associated with vaccination exemptions for schoolchildren. JAMA Netw Open. 2019;2:e199768.