Vanderbilt reduces coronavirus hospitalization estimates from thousands to 300

Vanderbilt University researchers now estimate the coronavirus outbreak in Tennessee has plateaued at its current level of about 300 hospitalizations — a massive decrease from a prior prediction of a peak of thousands — according to revised virus modeling.

The shift, researchers say, is the result of statewide social distancing lowering the transmission rate of the virus faster than predicted in even their most optimistic scenarios. By staying at home to starve the virus, Tennesseans made a month of progress in less than a week, halting the outbreak before it could grow, they said. 

Despite this achievement, the virus is not defeated. Due to the incubation period of the virus, the Vanderbilt model does not yet reflect the impact of reopening businesses in Nashville and across the state. It is possible the outbreak could rebound as more Tennesseans leave their homes to dine and shop, but Vanderbilt is no longer projecting scenarios for the future of the outbreak.

John Graves, a Vanderbilt associate professor behind the modeling, said Wednesday the original projections were hampered by limited data about the number of people who were hospitalized at any given time and the length of those hospital stays.

“We can only model based on the data that we have at the time, and the data we had in our initial report was consistent with an epidemic that was exponentially increasing,” Graves said. “Tennessee was successful in slamming the brakes on that transmission and getting it to basically a simmer.”

The revisions with Vanderbilt's hospitalization model highlight one of the polarizing puzzles of the coronavirus outbreak. Government-mandated social distancing has a devastating impact on jobs and the businesses, and health experts have repeatedly warned that if this bitter medicine was truly effective, it may seem like it was never needed at all.

Predicting the path of the coronavirus pandemic has been a difficult but essential challenge for researchers both in Tennessee and beyond. Government officials need a forecast of the virus to guide decisions on testing and economic restrictions, but this strain of coronavirus is new and still not well understood. Doctors are still discovering new symptoms and complications from coronavirus, and research continues to explore just how pervasive, contagious and deadly the virus truly is.

Tennessee has also become a statistical outlier that bucks virus patterns in other states. For reasons that aren’t clear, the outbreak in Tennessee has a dramatically lower rate of hospitalization and death than the national average.

According to a revised modeling report released by the Vanderbilt Department of Health Policy on Wednesday, Graves and other researchers now say the statewide transmission rate is slightly below 1, which means the outbreak is slowly shrinking. Vanderbilt noted "distinct spikes" of new cases in the past two weeks, but these occurred as the state government expanded testing in prisons and nursing homes, while introducing drive-thru testing in many counties.

"This raises an important question," the modeling report stated. "Is this increase in cases because there is more widespread testing, because more people are getting infected, or both? This question remains difficult to answer with certainty, especially given widespread testing in congregate settings."

Vanderbilt originally predicted 2,000 to 5,000 hospitalizations

Vanderbilt’s original virus model, released on April 10, forecast multiple scenarios for Tennessee. These scenarios hinged on the virus transmission rate, which is a measurement of how many people catch the virus from each infected person. As of early April, Vanderbilt believed each infected person spread the virus to 1.4 others.

Vanderbilt predicted that in a "status quo" scenario where the transmission rate remained at 1.4, the outbreak would peak in June with about 5,000 concurrent hospitalizations, stretching the state’s hospital infrastructure to the brink.

Vanderbilt also described a more optimistic forecast, called the "continued progress" scenario," in which the transmission rate dropped below 1 and stayed there. The outbreak would then peak in May with only 2,000 to 3,000 hospitalizations at once.

But researchers believed it would take more than month for the transmission rate to drop this far, according to a statement from Vanderbilt. Instead, it took less than a week.

"The 'continued progress' scenario in the April 10 report assumed that the transmission number would gradually reduce to 1.0 by mid-May ..." Vanderbilt's statement said. "The state in fact achieved a statewide transmission number of 1.0 by mid-April — a month before the original 'continued progress' scenario anticipated.

"This effectively reduced the growth of COVID-19, as it meant that cases (and hospitalizations) were no longer growing exponentially."

Vanderbilt announced the statewide transmission rate had dropped to 1 on April 16, but researchers expressed a lack of confidence about their calculations in non-metro areas. The university said at the time that hospitalizations could remain stable or possibly decrease, but did not provide a new hospitalization estimate on that date.

Jake Lowary, a spokesman for the Department of Health Policy, said hospitalization estimates were revised on April 24 to a plateau of 250 to 300, although this estimate does not appear to have been made public until now.

Tennessee has roughly reached this plateau. As of Monday, 275 people were hospitalized with coronavirus, largely in Nashville and Memphis, according to Vanderbilt. About half of those patients were in an ICU. 

Nashville reopens restaurants, stores

Vanderbilt's new modeling comes as Tennessee is lifting restrictions on businesses in an effort to restart the economy and allow people to return to work.

Gov. Bill Lee has allowed restaurants, stores, gyms and barbershops to reopen across much of the state in the past two weeks, and Nashville Mayor John Cooper let restaurants and stores open on Monday.

Tennesseans are still encouraged to stay home as much as possible, but travel is almost guaranteed to increase as revived businesses tempt residents with a taste of normal life. The reopening does not appear to have caused an increase in coronavirus cases so far, but the true test of these decisions will occur over the next few weeks.

Graves, the Vanderbilt professor, said Wednesday that researchers could not yet estimate the impact of these changes, but they might be able to do so in coming weeks.

“Because of the transmission dynamics, we in no position right now where we can use any numbers, at least to date, to measure what the impact of lifting any of the safer at home policy has been,” Graves said. “What we are trying to do is at least update the data to establish a baseline from which we can evaluate any changes moving forward.”

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.