A commonly heard refrain has been the call for COVID testing as a preamble to reducing social distancing. The concept underlying this is that people will be screened to identify infected individuals, and then they and their contacts will be identified and isolated. This will control transmission and reduce the number of infections.
While this is an attractive concept, I don’t believe it is feasible given the characteristics of the testing and the nature of COVID infections. In addition, many people calling for this mass testing don’t truly understand the tests.
The tests to identify active infection identify molecular (genetic) evidence of the virus in the throat of an individual. These “swab” tests were developed based on the concept of identifying the infection in symptomatic people with a high likelihood of infection. In this population, there would be large numbers of truly infected people (true positives), which would improve the value of the test by reducing the effect of false positives. In addition, if you are identifying large numbers of true positives you are removing the bulk of the infected individuals from the population. This would have a significant impact on transmission.
Unfortunately, COVID infection in the great majority of young healthy people is asymptomatic. Therefore, to identify infections you must screen large numbers of asymptomatic, healthy individuals. Currently in most state virus testing, only 3-5% of medically selected individuals are positive for virus by molecular tests. Looking at even larger numbers of people to find even fewer infected individuals is incredibly inefficient.
It’s also important to realize that because these individuals are asymptomatic, they will have to be screened frequently to identify if they become infected. It is likely you would have to do screening at least once a week to try and impact COVID transmission.
Test factors also work against using these assays to screen large populations. Most of the tests take a significant amount of time (at least an hour) and require a reference lab facility. The one test with rapid results (Abbott ID NOW) has concerns about a significant number of false negatives. This is a huge issue with a screening test because if you’re screening hundreds of individuals to find only 5 or 10 infections and missing 20% of them, this will not reduce viral spread.
There is now a test identifying viral proteins in the throat, but the utility of this test as a screen has not been determined. You could also screen people for COVID antibody and release them from further testing, but that also would require clinical trials to show they would not be infectious.
I believe it is more important to use tests for the presence of virus as they were intended in order to identify symptomatic patients requiring care; not to screen the entire population.
Given these limitations of screening, the best way forward is to protect at risk individuals as the primary goal. These are the people who would get sick, require hospitalization, and be more likely to die from infection. If infections were limited in this population, you would truly reduce the medical and social impact of this infection.
Don’t screen the healthy. Protect the at risk and use reasonable measures to reduce transmission such as masks in large and crowded places.
Pandemic Pondering 