Henrico’s 7-day moving average of virus positivity drops – but what exactly does that mean?

The percentage of Henrico residents tested for COVID-19 who have tested positive during a sliding 7-day period increased steadily late last month before beginning a three-day descent May 26.

The county’s 7-day positivity percentage moving average had dipped to 13 percent as of May 28, the most recent day for which such data is available, according to the Virginia Department of Health. That number is slightly higher than the statewide average of 11.8 percent reported the same day.

The metric is considered to be the most realistic snapshot view of how the virus is growing, because determining that percentage on a day-by-day basis is virtually impossible. But as the Citizen discovered this week, even that week-long snapshot is not an exact science. That’s the result of a number of factors.

VDH calculates the moving average by using the most recent week of announced data: testing encounters and positive test results. Each day, officials divide the number of positives reported that day by the number of testing encounters reported that day, then multiply by 100. Then, they add that number to the same percentage obtained during the previous six days and divide by seven, VDH spokeswoman Julie Grimes told the Citizen.

Though VDH attributes a newly confirmed case to the date on which the patient first experienced COVID-19 symptoms – not to the day on which that person was tested or on the day those test results were announced – it does not backdate that data for the purposes of the 7-day moving average, Grimes said.

“The lab data, from which the percent positivity is derived, and the case data are two independent data sources, and there is not a one-to-one correlation,” she explained.

So, for example, the 7-day moving average from May 10 to 16 would include both a test taken by a patient May 11 and his positive result reported May 14 – even if that patient’s case was separately attributed back to May 3, the date on which he began showing symptoms. Using a 7-day window, instead of a daily one, is the VDH’s way of mitigating skewed data that could result from this type of scenario played out tens of thousands of times statewide.

The state’s 7-day percent positivity average has fallen steadily since its peak of 22.2 percent April 19; Henrico’s peaked at 33.1 percent April 4.

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