The debate around screening asymptomatic individuals
The World Health Organization (WHO) and CDC did, however, recommend testing of asymptomatic people early in the COVID-19 pandemic, but the CDC revised this guidance in August of 2020 to recommend not testing asymptomatics even after potential exposure, only to reverse course again after public and expert pushback in the U.S. (Fang, M. 2020).
The US Food and Drug Administration (FDA) issued a strongly worded letter to healthcare providers in November 2020 warning about the potential for false positives from antigen testing, describing the problems associated with screening populations with a low background prevalence of COVID-19 (FDA 2020). The letter reminds practitioners that: “As disease prevalence decreases, the percent of test results that are false positives increase.”
CDC’s most recent (March 2021) guidance does, however, still recommend widespread screening, which necessarily includes testing of mostly asymptomatics, despite the widely known issues regarding such policies. CDC’s guidance states: “Rapid, point-of care serial screening can identify asymptomatic cases and help interrupt SARS-CoV-2 transmission. This is especially important when community risk or transmission levels are substantial or high.”
Many countries, including the UK and the U.S., have engaged in widespread population testing (Mercer and Salit 2021: COVID-19 testing is the “largest global testing programme in history, in which hundreds of millions of individuals have been tested to date.”).
Not surprisingly, there appears to have been significant internal debate about this important issue in these agencies. Those arguing for screening asymptomatic people seem to have overlooked the extremely high false positive rate that such testing necessarily entails with low background prevalence (Skittrall et al. 2020; Dinnes et al. 2021).
We explain below why it is almost always unwise to test asymptomatics on a widespread basis. Such testing can lead to extremely high levels of false positives even with highly accurate tests. Unfortunately, the available PCR and antigen tests are not very accurate. And inaccurate tests combined with widespread testing of asymptomatics can lead to catastrophically high levels of false positives, as explained below.