In a commentary on an analysis published in The Lancet Infectious Diseases, researchers confirm that the 2 primary factors contributing to the resurgence of measles cases in the United States are the reintroduction of the virus as a result of travel from countries that are experiencing measles outbreaks and the low vaccination rates fueled by the antivaccine movement and made possible by nonmedical exemptions.

These factors have led to a global increase in measles cases. In the United States, 704 cases of measles were reported across 22 states between January 2019 to April 2019, compared with 372 cases reported in 2018, 120 cases in 2017, 86 cases in 2016, 188 in 2015, and 667 in 2014. In fact, by April 2019, this year has demonstrated the highest number of measles cases reported since the virus was declared eliminated in the United States in 2000. These numbers indicate that the United States is at high risk for large epidemics of measles, such as those seen recently in Europe.

After confirming the hypothesis on the ease with which measles spreads through travel as it is highly contagious and transmitted through airborne respiratory droplets, researchers determined it is possible to perform a predictive risk analysis identifying which counties in the United States are most at risk for outbreaks. Investigators used a quantitative model using a spatial relative risk profile based on the following 4 factors: volume of international air travel to the destination county, rates of nonmedical exemptions in the county, the local population, and the measles incidence rate of the outbreak at travel origin.  

The analysis results were spatially consistent with reported cases. The Centers for Disease Control and Prevention and other state health departments report at least 45 US counties with reported cases of measles in 2019, and 30 of these are either on the predicted list of top 25 at-risk counties or spatially adjacent to one of those counties. A figure representing the counties most at risk is included in the article, as well as the countries that are most contributing to measles risk: China, India, Mexico, Japan, Philippines, Thailand, and Ukraine.

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Researchers highlighted the importance of directing surveillance toward counties with a high volume of incoming passengers from this list of countries, as well as taking steps to improve public health and increase vaccination efforts in these counties. They concluded that their analysis can be extended to other vaccine-preventable diseases for which air travel and nonmedical exemptions rates are relevant to disease spread, such as pertussis. The investigators plan to conduct these analyses in the near future.

Reference

Sarkar S, Zlojutro A, Khan K, Gardner L. Measles resurgence in the USA: how international travel compounds vaccine resistance [published online May 9, 2019]. Lancet Infect Dis. doi: 10.1016/S1473-3099(19)30231-2