Most public-health experts say it isn’t likely that migrants are contributing significantly to transmissions within the U.S., since nearly all are tested and quarantined before release, and because the Delta variant is already widespread.

Francis Collins, director of the National Institutes of Health, said on Fox News in August that while the border is a concern, it isn’t a driving factor of the pandemic in the U.S. He also said Covid-19 rates in Mexico and Central America were lower than in the U.S.

Darlene Bhavnani, an infectious-disease epidemiologist at the University of Texas, Austin, said she has seen little evidence to suggest that foreigners—border crossers or otherwise—are contributing meaningfully to the pandemic. The most significant factor behind recently surging infections remains low vaccine rates across the country, she said.

“Think about an entire city on fire and I was to walk in and drop a match,” Dr. Bhavnani said.