Vanderbilt: If transmission goes up as state reopens, Tennessee may need second lockdown

COVID-19 analysis by researchers at the Vanderbilt School of Medicine shows that Tennessee’s social distancing is working — but finds reopening too soon could undo all that work, even as Gov. Bill Lee announced that process will start Monday.

"We want to minimize the amount of time we all spend under restrictive social distancing protocols," John Graves, a Vanderbilt professor studying the virus, said in a briefing with reporters Friday.

The goal is to study whether Tennesseans would need to revert to stricter social distancing in response to new cases overwhelming hospital capacity.

If the order statewide was lifted on May 1, days after Lee’s plan has restaurants reopening, and the transmission rate rises to just more than 1.1, hospitals could be overwhelmed within 129 days, or the end of August, the study found.

That means that if, on average, infected persons are in contact with enough other people to spread the disease to just more than one other person each, the state could need to shut down again within four months to stop the spread, according to the findings of the study.

But if that transmission rate rises to 1.5 after a May 1 opening — or each infected person passes the virus to approximately 1.5 other people, on average — the hospital capacity statewide could be exceeded in just 46 days, or by June 9.

Study looks at transmission rate, hospital capacity

The study focuses on the coronavirus transmission rate, the average number of people infected by one person with the virus. The goal is to keep that number at or below 1.0, which would mean each person who tested positive was able to avoid passing the infection to more than one person, on average.

Tennessee’s transmission number remains around 1.0 in both metro and non-metro areas as of Friday — the same as reported last week.

“This indicates that social distancing has reduced transmission of the virus by limiting the number of contacts among Tennesseans,” a report from the university reads.

Researchers assume that rate will continue to fall if social distancing continues, to a 0.9 rate. 

"No one knows, of course, exactly how much the transmission number will change as social distancing decreases," Graves said. 

The study released Friday focuses on what the data collected throughout the pandemic shows about the likelihood for that transmission rate to go up once it becomes possible for people to gather in stores and restaurants as stay-at-home mandates are loosened across the state.

The findings parallel previous studies released by the university, even as Graves says the research is in "totally new, uncharted territory." 

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Staying at home longer means less likelihood of second lockdown

When a much later reopening date was put into the model, the time before a potential capacity overload was extended or even eliminated for the rest of 2020.

If the "Safer at Home" order remained fully in effect until June 1, and the transmission rate rose to 1.1, the economy could stay open through the end of the year, at least.

If the reopening was held to that later date and the transmission rate grew back to 1.5, the time it would take to hit capacity could be double what was modeled with a May 1 reopening — 92 days, or staying open until July 25.

“We stress that social distancing should not be lifted without strong public health measures — widespread testing and contact tracing — in place,” the report states. “The longer social distancing is continued, and the more transmission of the virus is reduced, the longer the economy could stay open before overburdening the state’s hospitals and risking the health of all Tennesseans who might need care, not just those suffering from COVID-19.”

Regional needs may differ from statewide capacity

Previous Vanderbilt reports show some regional variation in transmission rate, which may mean these projections will affect municipalities slightly differently. 

But there has been a statewide convergence toward that average 1.0 transmission rate so far, Melinda Buntin, a Vanderbilt health policy professor, said Friday.

The study assumes that if it took a set number of days after social distancing orders were put in place to decrease the transmission rate (say, from 1.5 down to 1.0), it would take the same number of days to ramp back up to that rate.

As of Wednesday, only 793 people have ever been hospitalized with the disease.

The capacity limit in the model was set at 1,000 concurrent hospitalizations, much higher than Tennessee has seen so far, at which point the state's hospital capacity for all patients would likely be overwhelmed. 

Overwhelmed hospitals could mean stricter social distance measures would need to be re-established.

It’s possible, though, that the 1,000 number is too high — the report indicates that a focus on the percentage of ICU capacity being used for COVID-19 patients in a region may be a better indicator of when hospitals are overwhelmed.

Graves and Buntin on Friday said researchers are "eager for input" from policymakers and health systems on that trigger point, and that a more regional focus may be effective in increasing the accuracy of the projections. 

"It's possible a lower number could be more realistic of the point where hospitals start to see stress," Graves said. 

The study assumes for the basis of the analysis that if an overwhelming number of beds were needed at one time, strict social distancing would be needed to slowly bring that transmission rate to 1.0 or below.

“Hospitalizations would continue to increase for a period of time because people already infected may need to be hospitalized over the next few weeks," the report said. "Additional infections and possible hospitalizations would also continue to mount until the effects of additional social distancing are realized several weeks later."

Data unclear on phased reopening

The study does not break down the impact of a phased reopening, because it remains unclear what such a process would do to the transmission rate over time.

“While it is currently unknown exactly how much the transmission number will change as ‘Safer at Home’ protocols are eased, we do know that the number of social contacts among Tennesseans will increase — even if certain aspects of physical distancing at businesses, mask-wearing and hygiene practices continue,” the study said.

If that transmission rate stays at or below 1.0 — a situation the study called a “fragile” success — even as people return to gathering places, it’s possible the capacity would never be met. 

Contact tracing and testing are needed to identify outbreaks and "break the transmission chain" through "shoe-leather" public health work, Graves said.

Graves did not say researchers felt one scenario was more likely than any other as the models look to an unprecedented future. 

"Our motivation with this model is to look at scenarios about how we might best ensure that our economy can reopen and we can all go back to doing what we used to enjoy, and where we can sustain that," Buntin said. "No one’s goal is to have a second peak or second wave of transmission of this highly infections virus."

For more details on what the capacity potential looks like with incremental differences in transmission rate, see the embedded report below:

Reach reporter Mariah Timms at mtimms@tennessean.com or 615-259-8344 and on Twitter @MariahTimms.