Here's how to interpret daily covid-19 case counts - TribLIVE

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As coronavirus cases mount in Allegheny and surrounding counties, questions often arise about how the data should be interpreted. Viewed without context, the numbers can lead to some misconceptions.

Allegheny County health officials reported 158 new cases Thursday, for instance, along with 12 hospitalizations — but those infections didn’t happen overnight.

“Some people get reported at a later time and there’s a lag,” said Dr. Jong Jeong, professor and interim chair of the Department of Biostatistics at the University of Pittsburgh’s Graduate School of Public Health. “We don’t know exactly what happened today. Some report can come in that happened a few weeks ago.”

The positive cases reported by Allegheny County on Thursday, for example, are from tests administered between June 25 and July 7. Because test results can take weeks to return, or as quickly as a few days, it’s difficult for experts to know exactly what’s happening on any specific date. Day by day, the snapshot changes, Jeong said.

Reports of deaths and hospitalizations are even more complicated. Of the 36 deaths reported in Pennsylvania on Thursday, 10 occurred in April, May and June. Information about the exact dates Thursday’s hospitalizations occurred could not be found, but county health officials have reiterated in the past that hospitalization data often trails behind daily case counts. It can take up to several weeks for an infected individual to get tested, get the results and become sick enough to be hospitalized.

Because of the reporting delays, experts and officials likely won’t know the most accurate record of the virus until it’s already passed, Jeong said.

The number of tests administered each day also fluctuates, which can affect the raw number of positive cases detected. In recent weeks, President Donald Trump has said increased testing capacity explains rapid spike in cases across the U.S. — a claim that health experts have called misleading, as the rate of infections far exceeds the proportionate number of tests.

For more clarity, Jeong said it’s important to look at the percentage of positive cases out of tests administered, to see if the number of infections is disproportionate.

Dr. Amesh Adalja, a Pittsburgh-based infectious disease and critical care physician, has said in the past that the ideal percentage for tests coming back positive for covid-19 — aside from the unlikely 0% — would be below 5%.

But the positivity rate in tests across the U.S. has risen since late June to around 8%, according to an analysis by Johns Hopkins University. In Allegheny County, positive cases have made up 8-10% of tests administered for several days now, Health Director Dr. Debra Bogen said at a news conference Tuesday. For much of May, the rate was only 3-4%.

While the data does lag, it is still helpful to monitor the trajectory of the virus in the region, Jeong said. Watching case counts rise, even if the rise really occurred weeks ago, helps statisticians and scientists create models and make predictions. It’s important for looking at the big picture.

The caveats can be frustrating and confusing, Jeong said, but taken in full, that data provides the most accurate picture possible at this point in the pandemic.

Teghan Simonton is a Tribune-Review staff writer. You can contact Teghan at 724-226-4680, tsimonton@triblive.com or via Twitter .

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