A new data analysis by researchers at Georgetown University pinpoints a number of undervaccinated clusters of the United States that pose a significant threat to the nation’s—and potentially the world’s—gradual progress against the Covid-19 pandemic, particularly given their potential to serve as “factories” for extremely contagious variants such as the now-dominant Delta strain.

The five most significant clusters identified by the Georgetown researchers are largely located in the southern U.S., in states such as Georgia, Alabama, Arkansas, Tennessee, Mississippi, and Louisiana—all of which are currently experiencing a rise in coronavirus cases as Delta rips through communities concentrated with people who have yet to receive a single vaccine shot. Those clusters include more than 15 million people.

“The group of counties in each cluster… together have lower vaccination coverage than expected, and make up a large population size. All of the top five clusters are focused in the southeastern U.S.,” the researchers note.

“The more geographically clustered unvaccinated individuals are,” the analysis continues, […]