Hoping to Understand the Virus, Everyone Is Parsing a Mountain of Data

CHICAGO — The latest count of new coronavirus cases was jarring: Some 1,500 virus cases were identified three consecutive days last week in Illinois, and fears of a resurgence in the state even led the mayor of Chicago to shut down bars all over town on Friday.

But at the same moment, there were other, hopeful data points that seemed to tell a different story entirely. Deaths from the virus statewide are one-tenth what they were at their peak in May. And the positivity rate of new coronavirus tests in Illinois is about half that of neighboring states.

“There are so many numbers flying around,” said Dr. Allison Arwady, the commissioner of the Chicago health department. “It’s hard for people to know what’s the most important thing to follow.”...