Posted July 12, 2020

The bottom line here is that more testing means fewer new cases – not the opposite as Trump and others keep claiming

Quotes form Trump and some Governors

Some amazing statements have been made by US some Republican officials along the lines that the current soaring number of Coronavirus cases is caused by the more extensive testing done in the US.Notorious examples include.

“Let me just make one statement, though. We do testing like nobody’s ever done testing. And when we test, the more you test, the more cases you find. Other countries, you know when they test? And I ask them all. They test when somebody’s not feeling well or when somebody walks into a hospital. So they don’t have tests” Donald Trump in July 9 interview with Sean Hannity¹

“If we were doing 80,000 tests a day in March or April, those numbers would’ve been dramatically higher in terms of case numbers from what we wound up having,”²

“As coronavirus cases spike in Florida and Texas this week, the state’s Republican governors attributed the alarming trends to increased testing . . .”³

The Facts

It is well know that testing has to be extensive enough that asymptomatic people are tested since they will become spreader if they go undetected. How many tests does this imply? It seems obvious that the higher the number of infected people per capita, the more tests it will take to uncover all infected individuals.

Table 1 below shows the the relation between testing and case growth in a number of representative countries, two US states and my province of Nova Scotia. These were selected to include four areas with high growth rate (the US, Florida, Brazil and Russia) and provides a comparison to others where the number of cases per day has been decreasing.

Table 1. Testing per detected case compared to case growth rate

Comments

Note that the table is arranged in order of the total case growth rate in descending order. With that in mind, one can see that there is almost and exact relationship showing that the more tests needed to detect one case, the lower the growth rate is – i.e. the more tests that are conducted, the less the spread of the disease. This is exactly the opposite to what Trump and others are spouting.

Some additional observations:

• The returns on more testing are not immediate. For example, if the number of tests were doubled when a high percentage of people are infected, the initial effect would be more cases reported but case counts would decrease thereafter as a higher percentage of infected people would self-isolate. 
• Serious reductions can then ensue when likely asymptomatic carriers are tested as a result of contact tracing. This only becomes realistic when new cases start to reduce to the point that it is feasible. For instance, Canada and some of the other countries have a few hundred new cases a day making contact tracing possible – certainly not the case on the US with 60,000 new cases a day.

References

1. Transcript of Interview with Sean Hannity, July 9, 2020
2. DeSantis blames expanded testing, younger Floridians for surge in COVID-19 cases, Tallahassee Democrat, July 6, 2020
3. Florida and Texas governors blame spike in cases on increased testing, CNN website, June, June 18, 2020