The COVID Report with David Blake: Backdating of new cases

Thursday, Sep. 10

At the Georgia Dept of Health Status Report, https://dph.georgia.gov/covid-19-daily-status-report, they make daily reports of the numbers of new cases.

If you scroll further down, you can see the new cases by day in a bar plot titled “COVID-19 Over Time”. If you select cases and Georgia and Date of Onset you can see what is called an epidemic curve. It plots the times of symptoms starting for each new case over time, as proxied by the time of testing.

If you instead choose Date of Report for that plot, it shows the provisional plot, in which the data is displayed dated to the time the test result was reported.

If you are following all that, there is a delay between testing and reporting. This delay is the test turnaround time.

If you take the epidemic curves from the Status Report on two consecutive days, and subtract them by date, you can measure the delays between testing and reporting for that day’s entries.

That is this Sept 9 plot, of the number of days new cases are backdated from the reporting day. 

The median of this distribution is 7 days, and the mean a little over 9. These measures are a really important attribute of our pandemic response, and a truly underappreciated one. Faster is better, and we are not fast enough in Georgia. As a result, our testing is almost useless for fighting transmission of COVID-19.

Let’s suppose you wake up today with a dry cough. You call Public Health, and you get a testing slot for tomorrow. You are tested, and 6 days later the test result is reported and you are a positive. However, today is Thursday.

You were exposing people Tuesday and Wednesday. The report came in next Thursday. Those people you exposed are now 8- and 9-days post exposure, and if they began to become infections after three days, they have already exposed most of the contacts they will ever expose.

This is the median scenario in Georgia.

Now, let’s pretend you were in Phoenix this week. You woke up with a dry cough on Thursday. Got tested. Two days later, Saturday, you were told you were positive. You inform your Tuesday and Wednesday contacts 3 and 4 days after they were exposed to you that you were positive, and they can isolate before they expose others.

Your state used testing to block COVID-19 transmission, and rapid testing is a very powerful tool.  Arizona did have rapid testing during its summer peak, but it improved in the past month.

As much as we talk about the power of testing, slow testing will erode all of that power in blocking infection transmission.

In Georgia, only 6 percent of the tests are returned within 48 hours, and we are unable to use testing to block the next generation of infections. This is a problem that, if taken to heart by the state, can rapidly be resolved (less than a month), and allow Georgia to have a much faster recovery.