Don’t overreact to high positivity rate, experts say
Christ Community Health Center medical staff collect nasal swabs as hundreds of Memphians line up for COVID-19 testing at their site in Hickory Hill on Tuesday, July 14, 2020. Experts caution weekly positivity rates are more meaningful than daily ones, and even there observers should watch over time. (Mark Weber/Daily Memphian)
If you’re relying on the daily positivity rate to assess community transmission, you’re in for a bumpy ride.
On Thursday, July 30, for instance, the daily rate was 53.4%, horrendous for a community aiming for 10%.
The daily rate reflects the ups and downs at the laboratories and testing centers. So, for instance, if a big batch of tests come in one day from a ZIP code with lots of infection, the daily positivity rate will be high.
And if labs have slowdowns, several days of tests may come back at once, which also skews the daily rate.
The better gauge is the weekly positivity rate. And if you need some good news, the weekly rate is 14.1%, down from 16.4% for the week ended July 18.
“So, the fact that that’s going down, that is what’s really important,” said Dr. Jeff Warren, a general practice physician who is also a City Council member and a member of the local task force helping guide the local response to the pandemic.
“If those averages continue, then we are making progress.”
He stresses the importance of getting the big picture and watching the numbers over time to see the trends.
These and other local COVID-19 statistics are on the Shelby County Health Department’s COVID-19 dashboard.
In the health department’s briefing with reporters Thursday, director Alisa Haushalter praised the community for its great work, “particularly around masking,” saying that research from the University of Memphis on mask use proves more people are wearing masks.
She also said closing limited-service restaurants — which have a higher percentage of alcohol sales than food sales — has made a difference.
If you like statistics, the ones to pay attention to are the number of new daily cases, hospitalization rates, which show both the use of regular beds and ICU beds, and the weekly positivity rate.
Dr. Manoj Jain, the infectious disease expert who is advising Mayor Jim Strickland, looks at these statistics first and in this order.
He also likes the transmission rate statistic, which shows how fast the disease is spreading. It is sometimes referred to as the reproduction rate.
That information is available at covidactnow.org.
In Shelby County, the transmission rate Thursday was 1.04, which means the virus is still spreading, but slowly. On June 23, it was 1.21.
The health department reports lots of daily data to the media, including total case numbers, total numbers tested, deaths and deaths in the last 24 hours.
“It helps us determine the impact on our system,” Haushalter said. “How much contract tracing can we do and so on. But it also lets us know whether there are issues with the lab testing system, and whether or not there were delays in reporting.
“Unfortunately, we have 273 deaths, and that is both confirmed deaths, as well as probable deaths.”
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Statistics to watch Alisa Haushalter Dr. Manoj Jain Dr. Jeff Warren positivity rateJane Roberts
Longtime journalist Jane Roberts is a Minnesotan by birth and a Memphian by choice. She's lived and reported in the city more than two decades. She covers business news and features for The Daily Memphian.
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