Correcting COVID-19 PCR Prevalence for False Positives in the Presence of Vaccination Immunity

Halem, M.

Many public health authority reports on COVID-19 cases confound positive test results with population prevalence. As the population prevalence approaches the PCR test false positive rate (FPR), for example during a vaccination campaign, it is necessary to adjust the the raw test results for the false positive rate. This paper provides a technique for estimating the test false positive rate and making the correction to test population prevalence in the absence of accurate and definitive specificity. Using current data providing by the Public Health England as of the most recent complete data, a false positive rate of 1.16% (95% CI 1.09 - 1.23% ) was found for the PHE PCR test for the period 1 January through 29 March 2021. During this period, the test population prevalence is decreasing, starting at a decay rate estimated as 3.0% per day (CI 2.79 - 3.14%). This rate of decay increased to an estimated 14.7% by the end of the period (CI 13.30 - 16.16%) Finally, mean test population prevalence was estimated at 14.3% (CI 13.75 - 14.87%) on 1 January and is estimated to have declined significantly to 0.06% (CI 0.00 - 0.13%). If PCR test positivity are used without the application of the false positive rate, the percent positive PCR tests will eventually "flatline" at the false positive rate, and produce a false positive bias even if test population prevalence should fall to zero.