Mecklenburg Projects Surge In COVID. But Model Creator Says 'Not Appropriate' To Use Model Now.
Mecklenburg County has released several projections in the last month that show a surge in COVID-19 cases is still on the horizon, with hospitals being overwhelmed with not enough ICU beds and ventilators.
But one of the creators of the University of Pennsylvania model that Mecklenburg uses said the county is likely past the point where the model is useful.
Michael Draugelis, chief data scientist at Penn Medicine, told WFAE the model should only be used at the start of an epidemic – not when a region has entered into what he calls a “plateau of cases.”
Mecklenburg County is generally considered to be in a plateau phase of the pandemic, with new infections having been level for several weeks.
“We made a decision to fit (our model) on the front side of the curve,” Draugelis said. “We will assume that people are using this as they are ramping up into the epidemic and towards the peak. So it’s never going to fit you on the other side of the curve.”
Because the model is built to show the spread of the virus at the start of an epidemic, it generally shows a rapid increase as it spreads from person to person. The model allows the user to see the impact of social distancing, which flattens the curve and spreads infections over a longer period.
He said the model will struggle to show what will happen once new infections are stable or start declining.
The model is called CHIME - COVID-19 Hospital Impact Model for Epidemics.
CHIME comes with a notice on the website: The model is “limited to short term forecasting. It is only applicable during the period prior to a region’s peak infections, and it accounts only for a single significant social distancing policy.”
Draugelis said the notice was posted to warn hospitals and health departments from using the model on the “back side of the curve.”
“That’s the reason for the notice,” he said. “It’s not appropriate to use past peak because the model will never fit past peak.”
The Mecklenburg County health department’s use of CHIME has produced dire forecasts that have been at odds with Gov. Roy Cooper’s plans to reopen. It's also created an unusual situation in which Mecklenburg County Public Health Director Gibbie Harris has said recently that the county is on the right track -- while the county's own projection from CHIME shows an explosion of cases that culminate in hospitals running out of beds and ventilators.
Mecklenburg’s first projection in April using CHIME showed hospitals overwhelmed in mid-May.
In fact, COVID-19 hospitalizations have declined from a high of 111 on April 9 to 58 on Wednesday. Here is a chart of Mecklenburg COVID hospitalizations:
Another projection showed hospitals being overwhelmed in mid-June. It’s too early to say whether that will happen, but that forecast showed hospitalizations should be increasing. They aren’t.
This week the county publicized a new projection that shows hospitals will be overwhelmed with a peak day of July 14:
When Mecklenburg runs a new model, usually only two factors change: The number of people in the hospital with COVID and the date of the new model.
Having fewer people in the hospital produces a flatter curve. Running the model on May 1 instead of April 1 pushes the peak back by a month.
When Mecklenburg runs new models that show the hospitalization peaks being pushed out from May to June to July, the county says that’s because it’s flattened the curve. But it’s also because more time has past since the previous model was run.
Mecklenburg deputy health director Raynard Washington said it’s still OK to use the CHIME model.
When asked about the CHIME notice about not using past peak infection, he said he doesn’t think Mecklenburg has reached that point. He said he interprets the CHIME definition of peak infection as “usually corresponding to the point where more people have been infected than are susceptible.”
That, however, could mean peak infection might not arrive until hundreds of thousands of county residents are infected.
He said the CHIME projections are more of a worst-case scenario of what could happen if restrictions are lifted.
“None of these models are excellent forecasts of what’s to come. ... But they valuably give us some sense of what could come as we relax stay at home and under more normal disease spread conditions (with people interacting more),” he said. “But again over interpretation will lead to misleading info.”
In a briefing with reporters on Thursday, Dr. Scott Rissmiller, Atrium’s chief physician executive, said the hospital is OK – despite the most recent CHIME projection that hospitals will be overwhelmed in July.
“Our most recent projection is still showing potentially an increasing volume in July,” he said, according to the Charlotte Ledger. “But every time we run the new projections, it gets flatter and it gets farther into the future, which is a good sign that what we are doing locally in the community is working.”
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