How best to measure a pandemic? Scientist questions Pritzker’s use of COVID-19 positivity rate


CHICAGO — The COVID-19 “positivity rate” is a term we’ve heard countless times during the pandemic, as it’s the number Governor JB Pritzker uses to guide his public health decisions like restricting access to bars and restaurants.  

But a top scientist is raising questions about whether that number is an appropriate measure of how widely the virus is spreading, or should be used as the most important metric to determine if businesses close or not. 

At the outset of the pandemic, Pritzker faced a problem: how to make a complex and complicated subject understandable to the general public, and use that understandable data to guide his decisions. 

Among other key numbers like deaths, hospitalizations, and tests, the “test positivity rate” —which is the percent of COVID-19 tests which come back positive — emerged as a key guidepost to measure the spread of the novel coronavirus.

“You can’t make a judgement about how widespread the transmission is occurring, whether there’s true community spread unless you’re looking at positivity rate,” Pritzker said. 

The governor said his decisions were guided by models done by research experts which show when we might see spikes, including one developed by University of Chicago scientist Sarah Cobey.

While she wasn’t available for an interview Thursday, Cobey was recently quoted in the downstate Belleville News-Democrat questioning the governor’s reliance on the positivity rate.

“I have been very critical of their use of this metric and basically almost all of the metrics they’ve proposed so far because they are not scientifically founded,” the paper quoted Cobey as saying. “They’re roughly right, but they’re not metrics you want to hang your hat on.” 

In the article, Cobey argues the metric is only useful if there is enough testing happening in a region. A high positivity rate in an area which averages very few tests may not be indicative of how widespread the virus has become.

During Thursday’s daily coronavirus briefing, University of Chicago Dr. Emily Landon praised Cobey as a world-class scientist, but said the governor makes his decisions based on a variety of information.  

“In a pandemic, no one has a monopoly on expertise,” Landon said.   

Republican lawmakers are also asking the governor to be more transparent about the data he’s using. 

Still, the positivity rate is one of the few “real-time” indicators available to elected officials, as other things which could be measured like hospitalizations or deaths connected to COVID-19 are considered “lagging indicators” in that they could occur weeks after an outbreak.  

“We want to make sure we’re measuring not just the number of people who are getting the virus, but how fast is it moving,” Pritzker said. 

The governor says anyone who is curious can check infection rates data themselves, as all of the data used to make mitigation decisions is available to the public through the Illinois Department of Public Health. 

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