Pass virus on now, 900 will get it over next 2 months

By , Daily Memphian Updated: June 09, 2020 2:33 PM CT | Published: June 08, 2020 5:13 PM CT

The novel coronavirus is complicated in its disease pattern and racial disparities. What’s uncomplicated is its reproduction rate – the number of people whom a single infected person can infect.

As the contagion spreads, the reproduction rate goes up. It’s up now in Shelby County to 1.2, which means it is spreading at a rate comparable to every infected person every day infecting another person.

“That means 30 in 30 days,” said Dr. Jeff Warren, a member of the City Council and Memphis-Shelby County COVID-19 Task Force. “In 60 days, it’s 900.”

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It’s exponential math, which means each case doubles and quadruples over time like cells in a developing fetus.

Based on the reproduction rate and other measures, the Shelby County Health Department and county Mayor Lee Harris said Monday phase 2 restrictions would stay in place until at least June 15.

“Unless there are extraordinary changes in the metrics between now and June 15th, we expect to be able advance to Phase III on that date, but in the meantime, the Health Department will continue to monitor new cases, positivity rate, reproduction rate, testing capacity, hospital capacity and public health capacity,” health department officials said in a statement Monday, June 8.

In early March, before schools and businesses closed, the disease was spreading at reproduction rate of 5 – one person infecting five others.

Shortly before the stay-at-home orders went into effect on March 23, the reproduction rate was 2.88. At that level, the disease would have swept rapidly through the whole community, says Warren, who uses the analogy of a lily pad expanding in a pond.

“At a reproduction rate of 2, in 30 days, the pond will be completely full of lily pads,” he says.

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But the unnerving part, he said, is it will be half full on Day 29.

“It’s building up exponentially, but you don’t see the effect of it until it’s too late. Then you are overwhelmed,” said Warren, who for several weeks has been promoting a mandatory face mask ordinance on the City Council.

The vote was postponed last Tuesday. Council members will discuss the ordinance at a Tuesday, June 9, executive session open to the public. The council currently does not have a voting meeting scheduled for Tuesday to actually vote on third and final reading. If one isn’t set, the matter would have a final vote at the June 16 council session.

During the stay-at-home period, the reproduction rate hovered at 1.09. At 1, a person who recovers is replaced by one sick person. Below 1, the disease begins to die out.

During the two weeks of the phase 1 reopening, the reproduction rate inched up to 1.02. In phase 2, which Mayor Jim Strickland extended to June 15 last week, reproduction rate levels have measured a median of 1.20., a modeling site the task force has been using, considered reproduction levels over 1.1 as high.

On Monday, it noted cases in Shelby County were growing at an unsustainable rate.

The weekend had a more urgent feel at Baptist Hospital-Memphis, where a larger number of COVID patients were admitted, “some of them quite sick,” said Dr. Stephen Threlkeld, an infectious disease expert.

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“This would be about the time we would see an upturn in cases that could be related to Memorial Day and the relaxing of social distancing.”

On Monday morning, Baptist-Memphis had 36 cases of COVID-19, up 31 from last Friday.

Coronavirus has a high contagion rate. In clinical terms, a person who is infected might be expected to infect 3.8 to 8.9 others. The common cold, also contagious, has a reproductive rate of 2-3.

Beyond contagion, the risk is compounded by the number of people who are asymptomatic, Threlkeld said.

“You don’t know that someone else has it next to you when you’re breathing the same air up close or shaking hands and touching your nose,” he said.

Recent reports suggest nearly half or more of coronavirus cases may be traced to people who had no symptoms, a stark difference from many other infectious diseases, including Ebola.

“People with Ebola don’t pass the infection on until they get sick. It’s not easy to miss,” Threlkeld said.

Monday, the World Health Organization said asymptomatic people are not driving the spread of the disease, saying that while transmission from asymptomatic people does occur, it is rare.

While there is political debate, Threlkeld said, about how much good social distancing has done, “I don’t think there’s any question that most people in medicine would agree that we have benefited from the suffering that we’ve done.

“It’s been hard on people and their jobs and our elderly folks who are isolated. There’s a cost to pay with all that. But I think it’s clear that that’s benefited us in terms of the number of cases and very likely has saved a lot of lives.”

While that political debate rages on, Warren says the virus has no political affiliation.

“It is going to grow based on the amount of food you give it. That is how a pandemic works. It grows based on the number of people who have it and the number of people they expose.”

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Dr. Jeff Warren Dr. Stephen Threlkeld reproduction rate 1.2
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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