SANTA FE, N.M. — As the coronavirus rages across the United States, mainly in large urban areas, more than a third of U.S. counties have yet to report a single positive test result for COVID-19 infections, an analysis by The Associated Press shows.

Data compiled by Johns Hopkins University shows that 1,297 counties have no confirmed cases of COVID-19 out of 3,142 counties nationwide. The number of counties without a positive coronavirus case has declined rapidly, dropping from over half as the AP was preparing to publish. Of the counties without positive tests, 85% are in rural areas — from predominantly white communities in Appalachia and the Great Plains to majority Hispanic and Native American stretches of the American Southwest — that generally have less everyday contact between people that can help transmit the virus.

At the same time, counties with zero positive tests for COVID-19 have a higher median age and higher proportion of people older than 60 — the most vulnerable to severe effects of the virus — and far fewer intensive care beds should they fall sick. Median household income is lower, too, potentially limiting health care options.

The demographics of these counties hold major implications as the Trump administration develops guidelines to rate counties by risk of the virus spreading, empowering local officials to revise social distancing orders that have sent much of the U.S. economy into free fall. President Donald Trump has targeted a return to a semblance of normalcy for the economy by Easter Sunday, April 12.

Experts in infectious disease see an opportunity in slowing the spread of coronavirus in remote areas of the country that benefit from “natural” social distancing and isolation, if initial cases are detected and quarantined aggressively. That can buy rural health care networks time to provide robust care and reduce mortality.

But they also worry that sporadic testing for coronavirus could be masking outbreaks that — left unattended — might overwhelm rural health networks.

“They’ll be later to get the infection, they’ll be later to have their epidemics,” said Christine K. Johnson, a professor of epidemiology at the University of California, Davis. “But I don’t think they’re going to be protected because there’s nowhere in the U.S. that’s isolated.”

Counties that have zero confirmed COVID-19 cases could raise a red flag about inadequate testing, she said.

“I hope the zeros are really zeros — I worry that they’re not doing enough testing in those regions because they’re not thinking they’re at risk,” she said.

In New Mexico, a state with 2 million residents spanning an area the size of Italy, Democratic Gov. Michelle Lujan Grisham has moved aggressively to contain the coronavirus’ spread with a statewide school shutdown and prohibition on most gatherings of over five people.