Hospital experts explain: Why does social distancing push model's COVID-19 peak later?

2020-04-18 | The Charlotte Observer

CHARLOTTE, N.C. _ Charlotte hospital systems Atrium Health and Novant Health first told Mecklenburg County the COVID-19 peak would likely hit the area between mid-April and mid-May.

Now, Mecklenburg officials expect to see that peak on June 8. That's because social distancing efforts are starting to work, Public Health Director Gibbie Harris says.

So why would effective social distancing push the peak of COVID-19 in Mecklenburg County further away?

That's the point of social distancing, Atrium and Novant experts say. Effective social distancing means the virus spreads throughout the community more slowly and infects fewer people at the same time, "flattening the curve" of infections over time.

Confirmed coronavirus cases in Mecklenburg are now doubling every six days, instead of a previous rate of every 2.85 days, hospital leaders told County Manager Dena Diorio in a letter Wednesday. At least 1,136 county residents have tested positive for the coronavirus and 24 have died as of Friday afternoon, officials said.

The term "flattening the curve" comes from graphs of the coronavirus' impact on patients and hospitals. Slowing the spread of the virus means fewer people will be infected at the peak of the outbreak, keeping hospitals from being overwhelmed with a surge in patients.

"Our goal here is to have slow spread," Atrium chief physician executive Dr. Scott Rissmiller told the Observer. "We're not going to be able to stop it at this point, but we want slow spread so that people are exposed to it and develop immunity at a pace that our communities and health systems can support and keep up with."

Harris presented a similar "flatten the curve" chart to county commissioners on April 14. The county is seeing roughly 40% to 50% social distancing, putting the peak at June 8 with a demand of 2,756 hospital beds on that day, she said.

With only 30% social distancing, the peak would have been on May 22 with nearly 3,000 more hospital beds needed on that day.

"We've talked about flattening the curve, but no one talked about we're not changing the area under the curve," said Michael Thompson, an associate professor of public health sciences at UNC Charlotte. "If we flatten the curve, it has to go out longer. We've met the challenge."

It's good the peak is further away, because that means public health efforts are working and there could be fewer people infected in the long run, Harris says. But that does mean people will have to practice social distancing for a longer period of time.

"We are looking at less of a strain on our hospital system, less of a challenge of meeting the needs of those who are ill in our community, but an extension in the amount of time that we're looking at for the need for social distancing and our stay-at-home order," Harris told county commissioners Tuesday.

If social distancing is relaxed to soon, there could be another surge in patients, Novant infectious disease expert Dr. William Harley II told the Observer.

Mecklenburg County's stay-at-home order is now set to expire April 29, at the same time as Gov. Roy Cooper's statewide order. But the county could extend that order past the end of April if needed.

"It's not the time to take our foot off the gas," Rissmiller said. "We have the stay-at-home order in place through the end of this month, and I would encourage everybody to keep vigilant and keep practicing, because it is working."

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Both of Charlotte's hospital systems have their own COVID-19 projection models. And the county is using a model from the University of Pennsylvania that incorporates data on how susceptible people are to the coronavirus, as well as the total number of infected and recovered individuals.

But since Mecklenburg does not have precise infection data due to limited COVID-19 testing capacity, Thompson said public health officials are forced to make assumptions based on "best guesses."

"These models are flawed ... a model is only as good as the data it has," Thompson said. "This University of Pennsylvania model is more helpful for local decision makers, and it's more intuitive. You're inputting things like (case) doubling rates, and death rates, and the known infectious periods you're measuring locally."

Harris told county commissioners that Mecklenburg looked at three different models before deciding to use Pennsylvania's. Many people across the country have been using the University of Washington's model instead.

The Washington model, which places heavy weight on effective social distancing, has a much more favorable outcome for North Carolina, Harris said. Pennsylvania's model was chosen because it was more cautious and presented a middle-of-the-road projection, Harris said.

Compared to the Pennsylvania model, the Washington model put North Carolina's peak much earlier _ hitting peak resource use April 17, with only 713 beds needed in the state.

Harris said the conservative model from Pennsylvania was more in line with Novant's and Atrium's models.

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It's important to pull from multiple models, Rissmiller said.

"We're in unprecedented territory here _ so no one knows exactly how this is going to play out," he said. "Having multiple models that you can run ... really gives you a much better feel for what to prepare for, and what the realm of possibility is."

County officials and hospital leaders have not disclosed what parameters are used in the modeling to project the peak in cases and strain on critical healthcare resources, such as available ICU beds and ventilators. And elected officials say they have not been briefed on the information either, despite repeatedly urging Harris and Diorio to offer more transparency during the coronavirus pandemic.

"I think you can build support with people if you give more information on those models that are going to affect their lives and businesses," county Commissioner Pat Cotham said in an interview this week. "If you don't explain it, people are getting frustrated."

County Commissioner Mark Jerrell said he does not know the variables influencing Mecklenburg's model _ or who's in charge of inputting and analyzing the projections.

"This time is very stressful, and as we're delivering information, we have to be extremely deliberate," Jerrell said. "If there are changes, people need to understand how the changes came about and why the change is happening."

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