Apr 4, 2022 - COVID

Confusing COVID counting could skew positivity rates

Illustration of magnets on a school locker in the shape of a contact tracing chart.

Illustration: Maura Losch/Axios

We recently explained that Chicago and Illinois officials do not count positive COVID-19 cases if they are detected through home tests.

  • This seemed odd given that the data could affect positivity rates, which help guide policy.

Driving the news: Axios learned Friday that about half of all testing data the city does use to gauge positivity comes from weekly routine testing of about 57,000 CPS students.

  • These are not kids who suspect they're sick, just those signed up for weekly surveillance. Still, their data is thrown into the general pot of positivity results.

What they're saying: "We never look at test positivity in isolation," CDPH medical director Isaac Ghinai tells Axios.

  • "We also look at case numbers, hospitalization usage, severe disease like ICU admissions and deaths. We also have new surveillance metrics, like wastewater surveillance."

Zoom in: CDC wastewater data has consistently shown 1,000% COVID growth in recent weeks in samples representing millions of Chicago households.

  • But as Ghinai notes, those CDC reports don't show baseline data, which was low before the exponential growth.
  • He recommends CDPH's wastewater reports, which show baseline numbers but with a significant lag time. CDPH's latest analysis digs into data from … early February.

Between the lines: Ghinai says he understands that excluding home test data and including thousands of routine school tests each week can skew metrics, which is why city officials keep those factors in mind when reviewing the numbers internally.

The bottom line: World Health Organization officials say the recent reduction of COVID monitoring "inhibits our collective ability to track where the virus is, how it is spreading and how it is evolving: information and analyses that remain critical to effectively end the acute phase of the pandemic."


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