Residents of Nashville and 12 surrounding counties appear to have effectively starved the coronavirus to the point of shrinking the outbreak, but the virus continues to spread through most of Tennessee, according to new research from Vanderbilt University.

Vanderbilt researchers said Thursday they are now confident that many infected people in Nashville region are no longer passing the coronavirus to anyone else, and the overall number of people who are carrying the virus has begun to shrink.

John Graves, a Vanderbilt professor who is studying the virus, said this progress is the result of Nashvillians embracing social distancing and reducing their movements, as shown by traffic camera and cell phone tracking data.

Even a simple trip to the grocery has been transformed, he said.

“People need to know that what they’ve done has made a huge difference,” Graves said. “It wasn’t a futile effort. We didn’t just shut down the economy for no reason.”

This new research comes from an evolving model of the coronavirus outbreak designed by Graves and others at Vanderbilt’s Department of Health Policy. The model largely focuses on the virus “transmission rate,” which measures how many people get the virus from each infected person. If the transmission rate is reduced below one for a significant amount of time, then the virus will recede.

Vanderbilt researchers said last week they believed the statewide transmission rate was 1.4 as of April 6. The rate has dropped significantly since then, but researchers are still not confident it has fallen below one. At this time, the Nashville metropolitan area is the only region in the state where researchers are confident this threshold has been reached.

In Tennessee’s Mid-South region, which contains Memphis and a few neighboring counties, the transmission rate could be slightly below one or as high as 1.25, based on the Vanderbilt research.

In the region around Knoxville, the transmission rate is believed to be at one or below, but researchers are not confident in this measurement due to limited data on virus testing and cases.

Finally, the highest transmission rate appears to be in the state’s “south central” region — 12 mostly rural counties in the bottom half of Middle Tennessee. The new research suggests an infected person in this region would spread the virus to as many as two other people. Graves said Thursday researchers are least confident about this region due to the small number of cases and tests in these counties.

“There are so few cases in some of these regions that we really really need to emphasize how uncertain it is,” Graves said, adding later: “A small change – four or five people getting infected – could send these numbers moving in those areas.”

The analysis from Vanderbilt aligns with prior reporting by The Tennessean that the statewide outbreak has slowed but not yet begun to shrink. In recent weeks, about a dozen doctors and public health experts have urged residents to maintain social distancing efforts to protect the progress that has already been made.

"The steps you are taking (are) truly making a difference," said Dr. Alex Jahangir, chairman of the Nashville coronavirus task force, on Thursday. "So please keep it up, stay at home and go out only if it's absolutely necessary. If you do go out, please take precaution."

Coronavirus: 3 scenarios for the outbreak

The new Vanderbilt research builds upon virus predictions that were released by the university last week. Based on the fluctuation of the transmission rate, researchers forecast three scenarios for the outbreak statewide.

Status quo scenario: In this scenario, Tennessee keeps current social distancing strategies in place and the transmission rate of the virus remains where it is today. It is predicted the virus would peak in June with about 5,000 hospitalizations, which would stretch but not shatter the capacity of hospitals.

In this scenario, Tennessee keeps current social distancing strategies in place and the transmission rate of the virus remains where it is today. It is predicted the virus would peak in June with about 5,000 hospitalizations, which would stretch but not shatter the capacity of hospitals. The optimistic scenario: Improvements to social distancing lowers the transmission rate below 1, which means the virus peaks in mid-May with only 2,000 to 3,000 hospitalizations. Hospitals should be more than capable to handle the patient load.

Improvements to social distancing lowers the transmission rate below 1, which means the virus peaks in mid-May with only 2,000 to 3,000 hospitalizations. Hospitals should be more than capable to handle the patient load. The reversal scenario: If Tennessee were to abandon social distancing now, the transmission rate of the virus may rise back to where it began. Each infected person would infect as many as five others, and as much as 13% of the population would be infected. By mid-May, 50,000 people would be hospitalized.

Tennessee is not locked into any of these scenarios. Researchers have said repeatedly that slight changes in social distancing could send ripples through the state, either slowing the virus or causing it to rebound.

“What we are emphasizing here is how fragile this progress is and how quickly it can unravel. And we are seeing examples of that all around the world,” Graves said Thursday. “It really underscores that to ease out of social distancing you really need additional mitigation strategies in place to put out the brush fires as they flare up.”

Reporter Yihyun Jeong contributed to this report.

Brett Kelman is the health care reporter for The Tennessean. He can be reached at 615-259-8287 or at brett.kelman@tennessean.com. Follow him on Twitter at @brettkelman.