Abstract
Vaccine exemptions, which allow unvaccinated children to attend school, have increased by a factor of 28 since 2003 in Texas. Geographic clustering of unvaccinated children facilitates the spread of measles introductions, but the potential size of outbreaks is unclear. To forecast the range of measles outbreak sizes in each metropolitan area of Texas at 2018 and future reduced school vaccination rates. An agent-based decision analytical model using a synthetic population of Texas, derived from the 2010 US Census, was used to simulate measles transmission in the Texas population. Real schools were represented in the simulations, and the 2018 vaccination rate of each real school was applied to a simulated hypothetical equivalent. Single cases of measles were introduced, daily activities and interactions were modeled for each population member, and the number of infections over the course of 9 months was counted for 1000 simulated runs in each Texas metropolitan area. To determine the outcomes of further decreases in vaccination coverage, additional simulations were performed with vaccination rates reduced by 1% to 10% in schools with populations that are currently undervaccinated. Expected distributions of outbreak sizes in each metropolitan area of Texas at 2018 and reduced vaccination rates. At 2018 vaccination rates, the median number of cases in large metropolitan areas was typically small, ranging from 1 to 3 cases, which is consistent with outbreaks in Texas 2006 to 2017. However, the upper limit of the distribution of plausible outbreaks (the 95th percentile, associated with 1 in 20 measles introductions) exceeded 400 cases in both the Austin and Dallas metropolitan areas, similar to the largest US outbreaks since measles was eliminated in 2000. Decreases in vaccination rates in schools with undervaccinated populations in 2018 were associated with exponential increases in the potential size of outbreaks: 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 (SD) of 64% (11%) of cases occurred in students for whom a vaccine had been refused, but a mean (SD) of 36% (11%) occurred in others (ie, bystanders). This study suggests that vaccination rates in some Texas schools are currently low enough to allow large measles outbreaks. Further decreases are associated with dramatic increases in the probability of large outbreaks. Limiting vaccine exemptions could be associated with a decrease in the risk of large measles outbreaks.
Highlights
Measles is a highly infectious disease that can result in severe health outcomes, including pneumonia, brain damage, and death.[1]
At 2018 vaccination rates, the median number of cases in large metropolitan areas was typically small, ranging from 1 to 3 cases, which is consistent with outbreaks in Texas 2006 to 2017
Decreases in vaccination rates in schools with undervaccinated populations in 2018 were associated with exponential increases in the potential size of outbreaks: a 5% decrease in vaccination rate was associated with a 40% to 4000% increase in potential outbreak size, depending on the metropolitan area
Summary
Measles is a highly infectious disease that can result in severe health outcomes, including pneumonia, brain damage, and death.[1]. In the United States, the Centers for Disease Control and Prevention recommends that children receive a first vaccination dose at ages 12 to 15 months and a second dose at ages 4 to 6 years (before school entry), some children are not vaccinated because of contraindications.[2] Vaccination does not confer immunity after 2 doses in 3% of people.[2] Approximately 92% to 96% of a homogeneously mixed population requires immunity to achieve herd immunity,[3] vaccination coverage requirements should be adjusted for local demographic conditions.[4]. Measles is no longer endemic in the United States,[5] but regular outbreaks still occur when individuals exposed in other countries enter the United States,[6,7] causing tens to hundreds of cases annually in recent years.[8] The 3 largest outbreaks since elimination infected hundreds of people each,[9,10,11] the last of which led to mandatory vaccinations ordered in New York, New York.[12]
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