Not three months ago, the coronavirus had so ravaged South Dakota that its packed hospitals were flying patients to other states for treatment. An analysis of data collected by Johns Hopkins University had shown that the mortality rates from Covid-19 in North and South Dakota were the world’s highest. In one Montana county, the rate of hospitalization for the virus was 20 times the national average.

As in some earlier hot spots like Arizona and Florida, the surge mushroomed as most leaders and residents in these states resisted lockdowns and mask mandates for months. In South Dakota, no statewide mask mandate was ever issued.

The spike in these states was as brief as it was powerful. Today, their rates of new cases are back roughly to where they were last summer or early fall. In North Dakota, which mandated masks at the height of its surge in mid-November, the turnaround has been especially dramatic: the daily average deaths per person is now the country’s second lowest, according to a New York Times database.

By some measures, the three-state hot spot’s trajectory has mirrored the nation’s. After the daily U.S. average for new cases peaked on Jan. 9, it took 37 days — until last Monday — for the rate to drop by two-thirds. It took South Dakota and Montana 35 days to reach the same mark after cases peaked in those two states in November. (North Dakota did it in 24.)

Deaths remain high nationally, because it can take weeks for Covid-19 patients to die. The country continues to average more than 2,000 deaths each day and is on pace to reach 500,000 deaths in the next week.

Experts say the spikes in the Northern Great Plains ebbed largely for the same reason that the U.S. caseload has been falling: People finally took steps to save themselves in the face of an out-of-control deadly disease.

“As things get worse and friends and family members are in the hospital or dying, people start to adjust their behavior and cases go down,” said Meghan O’Connell, an epidemiologist in South Dakota and an adviser on health issues to the Great Plains Tribal Leaders Health Board, which represents Native American populations in the area. Native Americans, who represent about 5 percent to almost 10 percent of the population all three states, have been infected by the virus at far higher rates than the general population.

During the outbreak’s worst weeks, from early November to late December, mask use rose 10 to 20 percentage points in South Dakota and 20 to 30 percentage points in North Dakota, according to survey data from the University of Maryland.

Since then, the U.S. vaccination drive has been gathering speed. North Dakota ranks fifth among states for giving its residents at least one shot; South Dakota is seventh and Montana is 11th.

Some experts see the coronavirus’s race through these states as a rough test of the widely rejected idea that the pandemic should be allowed to run its course until the population gains herd immunity.

While the region did not reach herd immunity, it may have come closer than anywhere else in the United States.

The outbreak in November vaulted North and South Dakota to the top of the list in cases per person, where they remain, according to a New York Times database, with 13 and 12.5 percent of their residents known to have been infected. Montana, at about 9.2 percent, is close to the middle of the national pack.

Just over 8 percent of Americans — about 27.9 million — are known to have had the coronavirus, but for many reasons, including that asymptomatic infections can go undetected, the Centers for Disease Control and Prevention suggests that the real rate is 4.6 times that.

By those measures, as least six in 10 Dakotans — and most likely more — could have gained some immunity to the virus by the end of 2020, according to Jeffrey Shaman, a Columbia University professor of environmental health sciences who is modeling the future spread of the virus. And in some places, he noted, the share could be even higher.