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.