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.