Virus reproduction rate is the number to know; it’s now a promising 1.14 in Shelby County

By , Daily Memphian Updated: May 12, 2020 9:06 AM CT | Published: May 12, 2020 4:00 AM CT

David Sweat, head of epidemiology at the Shelby County Health Department, is poring over a short list of different COVID-19 models right now, reading them like a map for the road the disease will take from here.

There isn’t just one model for predicting the coronavirus surge because none are accurate in all facets, he says. But has been reliable about the reproduction rate, the average number of people infected by one positive person.

Mid-South COVID-19 Regional Response Fund grants $62,500

In the early days of the pandemic, the average positive-testing person here infected 4 to 5 others, based on a model built by Vanderbilt University. The number in Shelby County is now thought to be closer to 1.14, a testament to the effectiveness of social distancing and stay-at-home orders, Sweat said.

“The longer you can sustain a low reproduction rate of the virus, you are still going to have an epidemic, but it is not ever going to be acutely so high (that) it kills your hospitals.”

Sweat, also head of the Memphis/Shelby County COVID-19 Task Force’s “epi team,” is intently focused on the rate in Shelby County, and Monday, May 11, for the first time, he updated the task force on rates in neighboring counties.

“Fayette and Tipton are having a highly active epidemic for the first time,” he said in an interview. “There are other counties in Tennessee flagging red, and Marshall County in Mississippi is 1.44, a concerning rate.”

Local testing reaches record high as positive rate declines

“People from those places, if they are going to be hospitalized, they are most likely going to be hospitalized here,” he said.

Shelby County is using about 70% of normal hospital bed capacity and slightly higher (73%) in ICU beds. Both numbers, according to the task force data, are in the yellow or cautionary zone, but not critical.

As late as last week, about 25% of hospital beds in the county were being used by out-of-county residents.

“It is really important to know where there are a growing number of cases,” said Dr. Manoj Jain, the infectious disease expert advising the task force. “Often, we are not aware by just looking at the absolute number of cases. When we see outbreaks in certain regions, we can make interventions.”

Officials from the surrounding counties participate in the task force meetings.

“The reproduction rate tells us if the epidemic is increasing, steady or declining. If you have a reproduction or transmission rate of 1, that means the number of cases coming in each day is the same as the day before. However, if the numbers are increasing, there is a growing number, and the epidemic is going to continue further,” Jain said.

The reproduction rate has hovered at 1.14 for several days, which Sweat takes as a good sign. It has been in what COVIDACTNOW calls medium-stage transmission since mid-April.

<strong>Dr. Manoj Jain</strong>

Dr. Manoj Jain

“We are still growing. We still have the virus transmitting; it’s just not drastically transmitting.”

Since the first case was announced in Shelby County on March 8, Sweat and others on the task force have been watching a number of COVID models, including CHIME (COVID-19 Hospital Impact Model for Epidemics) from the University of Pennsylvania; University of Washington’s Institute for Health Metrics and Evaluation; COVIDACTNOW; and models built by Vanderbilt and Imperial College in London.

“In the early days, we scoured for models we thought either looked interesting or promising and gave us the ability to deconstruct the map and build local instances of those models,” Sweat said.

He says the team of data scientists comes from University of Tennessee Health Science Center, the city and county, St. Jude Children’s Research Hospital and Baptist hospital. “A lot of places are lending people to the computational effort.”

The IHME model at University of Washington and CHIME were both recalibrated two weeks ago because their findings were not running true with real-time data on the ground.

“The University of Washington model sort of predicted we were already past our peak and was vastly over-projecting deaths,” Sweat said. “At a time when we had 50 deaths, that model was saying we should have 150.

“We don’t have 150 deaths. We knew that model was not performing well on both metrics,” he said.

Significant drop-off of new tests maintains declining positivity rate

CHIME on the other hand, was created to predict the impact on hospital resources, including ICU beds. In Shelby County, it has turned out to be a more accurate measure of mortality rates.

“We all noticed that,” Sweat said. “This model is predicting about the number of deaths we’re actually observing.”

COVIDACTNOW, a public site, has been remarkably accurate in predicting the reproduction rate, although it does not have compelling data on hospital use or test positivity rates here.

CHIME predicts the peak in Shelby County will now happen around November. It earlier predicted August.

It’s one thing to know the peak might be in November and a completely different thing to know how many may be hospitalized or die, Jain says.

“If someone asks what will happen in the fall, it’s really hard to predict because what we do in two to three months will determine what will happen, not just in the next few months, but months and years afterward.”

Back in mid-April, the reproduction rate in Shelby County hovered below 1 for four or five consecutive days.

From March 24, when nonessential businesses closed, to April 25, the number of people in groups of 10 or more dropped to its lowest level in the pandemic so far, based on local cellphone data reported on

But, “We lost control at the end of April,” Sweat says.

Monday, Shelby County earned an F for having less than a 55% reduction in nonessential visits and less than 40% reduction in density encounters on

<strong>Dr. Jeff Warren</strong>

Dr. Jeff Warren

Dr. Jeff Warren, a Memphis City Council member and a primary care physician serving on the task force, recommends people pay attention to the reproduction rate.

“It gives people something they can follow and understand,” he said.

Late this week – on May 15 – Sweat expects to have data that show how much the gradual reopening of the economy on May 4 and May 6 may have affected the reproduction rate.

He is not saying it will go up due to the loosening of restrictions in Phase I.

“Everyone has to make their own decisions, business owners and customers. That’s what we are seeing right now. People are making their own choices about what they feel safe doing or what makes business sense for them to do with the requirements they would have to operate under,” he said.

Based on its own data now, the Shelby County Health Department expects the coronavirus to surge in June. But it could come as late as August.

The field hospital renovations being done by the U.S. Army Corps of Engineers in the former Commercial Appeal building will be complete on Thursday. Equipment will arrive shortly after, and the facility will “be staged for quick activation,” according to notes the task force received Monday.

The hospital is scheduled to function for 18 months.

“We’re still in the first wave of this epidemic,” Sweat says. “We don’t know, in the total epidemic, how long it will take until it is done.

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“But for the foreseeable future, our hospital resources are not overwhelmed. Our ICU beds are not overwhelmed. The trajectory of the epidemic is lower than it was on the health care system. We are managing this epidemic. The social impacts and the economic impacts, all those are very, very good questions. I don’t have an answer for those.

“But every day that we keep the reproduction rate of the virus low is a day of good news.”


David Sweat Shelby County Health Department COVIDACTNOW Dr. Manoj Jain Dr. Jeff Warren
Jane Roberts

Jane Roberts

Longtime journalist Jane Roberts is a Minnesotan by birth and a Memphian by choice. She's lived and reported in the city more than two decades. She covers business news and features for The Daily Memphian.


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