Illinois and Chicago officials disagree on how to calculate the single most important benchmark for tracking the spread of the coronavirus, even as the number of confirmed cases of the virus continues to rise statewide.
After the number of positive tests for the coronavirus ticked up and hovered around 5% in Chicago, Mayor Lori Lightfoot moved on July 20 to impose new restrictions on bars, restaurants, gyms and salons, sounding the alarm about a sustained rise in the number of cases among young adults.
But Chicagoans who turned to the Illinois Department of Public Health for more information found data that painted a different picture, with state officials pegging Chicago’s positivity rate near 4% just before Lightfoot’s announcement.
The difference is due to the fact that state officials compare the number of positive tests for the coronavirus with the total number of tests conducted within a 24-hour period, while Chicago officials compare the number of positive tests with the number of individuals tested within a 24-hour period.
“It is not that one way is right and one way is wrong,” said Dr. Allison Arwady, the commissioner of the Chicago Department of Public Health. “But the way we are doing it is more standard. It gives us the best data to guide our response.”
Melaney Arnold, a spokesperson for the Illinois Department of Health, said “both values have different purposes and are both relevant to help us understand the entire COVID-19 picture.
“Case positivity helps us understand whether changes in the number of confirmed cases is due to more testing or due to more infections,” Arnold said.
In addition, state officials use the percentage of positive tests to determine whether Illinois is testing enough of the population “to make informed decisions,” Arnold said.
The difference between the positivity rate as calculated by state officials and the positivity rate as calculated by city officials has ranged between half a percentage point and a full percentage point. For example, on Tuesday, the positivity rate released by the city rose to 5.4%, while the data released by the state pegged the positivity rate at 4.5%.
The method used by Chicago officials eliminates multiple tests conducted on the same person on the same day, Arwady said. One person could take multiple tests on the same day if they are hospitalized and doctors and nurses are concerned an initial test delivered an inaccurate result, while others might seek out multiple tests on the same day if they work in a place with an increased risk, like a long-term care facility, she added.
It takes data scientists in the Chicago Department of Public Health “many hours a day” to “deduplicate” the data, in an effort to get “cleaner” data.
The positivity rate has gained in importance since Gov. J.B. Pritzker announced on July 15 that he would reimpose restrictions designed to stop the spread of the virus in each of Illinois’ 11 regions if those areas experience a sustained increase in positivity rates or if a region’s positivity rate reaches 8% or higher for three consecutive days.
The city will add the state-calculated positivity rate to its data portal in an effort to help alleviate confusion in the coming days, Arwady said.
“I don’t want people to be confused,” Arwady said. “We are being very transparent. We need people to trust the data.”
The state-calculated positivity rate for Illinois was 3.79% on Tuesday, according to the Illinois Department of Health.
A statewide positivity rate of more than 5% indicates a state may only be testing the sickest patients who seek out medical care, and are not casting a wide enough net to identify milder cases and track outbreaks, according to Johns Hopkins University, where officials have been tracking the pandemic.
“When states report the number of COVID-19 tests performed, this should include the number of viral tests performed and the number of patients for which these tests were performed,” according to the university’s guidelines. “Currently, states may not be distinguishing overall tests administered from the number of individuals who have been tested. This is an important limitation to the data that is available to track testing in the U.S., and states should work to address it.”
If properly calculated, data including positivity rates can help “leaders … make informed decisions about lifting social distancing and other transmission control measures,” according to the university’s guidelines.
In an ideal world, the U.S. Centers for Disease Control and Prevention should have set one method for calculating the positivity rate at the beginning of the pandemic, Arwady said. In the absence of that federal guidance, each health agency has been left to make its own decisions, Arwady said.
In part because other states are using different metrics to map the spread of the coronavirus, Chicago is using a more straightforward benchmark when it decides whether to add states to its quarantine order.
The 22 states included in Chicago’s quarantine order have an infection rate greater than 15 new COVID-19 cases per 100,000 residents per day. Chicago has an infection rate of fewer than nine new COVID-19 cases per day, per 100,000 residents, Arwady said.
“It can be confusing for people,” Arwady said.
That is why Arwady said she is encouraging Chicagoans concerned about the pandemic to focus on the total number of cases reported in Chicago. The city is now averaging more than 200 cases per day, which means that the virus is spreading at a moderate rate.
Major restrictions will likely be reimposed if the city sees an increase to approximately 400 cases per day, Arwady said.