Where Will Measles Next Land in the U.S.?

High rates of international air travel in conjunction with high rates of non-medical vaccination exemptions may help predict which U.S. counties are next in danger of a measles outbreak, researchers argued.

U.S. counties at highest risk not only had high rates of non-medical exemptions, but also proximity to airports with high volumes of international air travel, according to a modeling study by Sahotra Sarkar, PhD, of the University of Texas in Austin, and colleagues.

Writing in The Lancet Infectious Diseases, the authors said their model correctly predicted outbreaks in Washington, Oregon, and New York. They cited data from the CDC and various state health departments, which said that at least 45 U.S. counties reported measles cases in 2019. Of these, 30 were either in their top 25 at-risk counties or spatially adjacent to one of these counties, the authors said.

Sarkar and colleagues’ quantitative model takes into account international air travel volume and non-medical exemption rates in the county, plus county population, and incidence rate of measles at travel origin. All of these factors are combined to help calculate the expected relative size of a measles outbreak in a county.

Indeed, the authors said that based on their model, the top five counties at risk for measles outbreak in 2019 are:

• Cook, Illinois (Chicago)
• Los Angeles, California
• Miami-Dade, Florida
• Queens, New York (New York City)
• King, Washington (Seattle)

Peter Hotez, MD, PhD, dean of Baylor’s National School of Tropical Medicine in Houston, has previously performed an analysis of anti-vaccination “hotspots” in the U.S., and called this research “a nice advance over our 2018 paper.”

“[This study] has an extra layer of information by folding in both additional states and consideration of air travel and airport hubs,” Hotez, who was not involved with the research, told MedPage Today. “This could become a dominant theme for this summer.”

Sarkar and colleagues listed counties that have not yet had a measles outbreak, but because they are close to major international airports, they are at risk of imported measles cases. They include:

• Travis, Texas (Austin)
• Maricopa, Arizona (Phoenix)
• Clayton, Georgia (includes Atlanta’s international airport)
• Honolulu, Hawaii
• Wayne, Michigan (Detroit)

Based on their model, the authors also identified the countries that are contributing the most to measles risk across the U.S. India contributes the most risk, followed by China, Mexico, Japan, and Ukraine.

“Critically, we recommend that public health officials and policymakers prioritize monitoring the counties we identify to be at high risk that have not yet reported cases, especially those that lie adjacent to counties with ongoing outbreaks and those that house large international airports,” said co-author Lauren Gardner, PhD, of Johns Hopkins University in Baltimore in a statement.

The authors said that their model can also be applicable to other vaccine-preventable diseases, where non-medical exemption rates and air travel are relevant to disease spread, including potentially pertussis, mumps, and rubella.