What is the background prevalence for COVID-19?
Even a test with a very high 99% specificity (1% chance of false positives), when used to screen asymptomatic populations with a low background rate of actual infection, will yield high levels of false positives.
The background rate of COVID-19 infection, even during high points, has always been relatively low. For example, Sadoff et al. 2021, the published results of the Johnson & Johnson vaccine clinical trial, including almost 40,000 participants in half a dozen countries, from late September 2020 to late January 2021, found a 0.5% PCR positive baseline (see Sadoff et al. 2021 supplementary appendix, p. 23).
Similarly, Baden et al., 2020, found a 0.6% background positive PCR test result in the 30,420 clinical trial participants for the Moderna vaccine, after initial testing. Study participants for this trial were selected based on being at higher risk for exposure to the virus and the testing was conducted from late July to late October 2020.
In the UK, the Government’s survey of the population in June 2020 found about 1 in 2,200 people with an active infection in the study window, which is 0.05%, an order of magnitude lower than the vaccine trials just mentioned (Connors and Williams 2020).
Voysey et al. 2021, the published results of the Astrazeneca vaccine trial, found 1.65% baseline antibody test positive results in their 20,675 study participants, but the antibody test (also known as serology testing) measures the presence of any past infection, not current infections, so this will necessarily be a significantly higher number than a snapshot in time of current infections, as is the case for PCR or antigen tests. The Astrazeneca trial study did not include PCR testing at baseline like the Moderna and J&J trials did.