Nowhere are these regulatory disparities more counterproductive and jarring than in the border areas between restrictive and permissive states; for example, between Washington and Idaho, Minnesota and South Dakota, and Illinois and Iowa. In each pairing, one state has imposed tough and sometimes unpopular restrictions on behavior, only to be confounded by a neighbor’s leniency. Like factories whose emissions boost asthma rates for miles around, a state’s lax public health policies can wreak damage beyond its borders.

“In some ways, the whole country is essentially living with the strategy of the least effective states because states interconnect and one state not doing a good job will continue to spread the virus to other states,” said Dr. Ashish Jha, dean of the Brown University School of Public Health. “States can’t wall themselves off.”

A motorcycle rally in August in Sturgis, South Dakota, with half a million attendees from around the country spread COVID-19 to neighboring Minnesota and beyond, according to Melanie Firestone, an epidemic intelligence service officer for the Centers for Disease Control and Prevention, who co-authored a report on the event’s impact.

South Dakota “didn’t have policies regarding mask use or event size, and we see that there was an impact in a state that did have such policies,” Firestone said. “The findings from this outbreak support having consistent approaches across states. We are all in it together when it comes to stopping the spread of COVID-19.”