by Jon Sanders
Director of the Center for Food, Power, and Life, Research Editor | John Locke Foundation
After the holiday hiatus, I am bringing back the NC Threat-Free Index. There are a few items I’d like to point out first:
Vaccinations. I can’t adjust for vaccinations without a way to disaggregate people who’ve been vaccinated from people who’ve already had an infection. So that would mean the threat-free index would provide a lower estimate than the actual amount of people posing no threat to pass along the virus. The more vaccinations, the more the index will be undercounting.
Population. The index now uses the estimated population for NC for July 2021 (last year used estimates for July 2020).
Infections. The index relies on counting people who have been diagnosed with laboratory-confirmed cases of COVID-19. That’s a measure greatly influenced by the number of cycles used by the PCR tests predominantly used to determine infections. Beyond a certain level of cycles (the research consensus is 30), a “positive” test may not have actually found viable, active virus.
The PCR tests are notorious for false positives, as explained here:
… careful interpretation of weak positive results is needed (1). The cycle threshold (Ct) needed to detect virus is inversely proportional to the patient’s viral load. Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different NAT technology.
WHO reminds IVD users that disease prevalence alters the predictive value of test results; as disease prevalence decreases, the risk of false positive increases (2). This means that the probability that a person who has a positive result (SARS-CoV-2 detected) is truly infected with SARS-CoV-2 decreases as prevalence decreases, irrespective of the claimed specificity.
In other words, the longer it takes (i.e., the more cycles required) to detect the virus, the more likely the finding is a false positive.
Most PCR assays are indicated as an aid for diagnosis, therefore, health care providers must consider any result in combination with timing of sampling, specimen type, assay specifics, clinical observations, patient history, confirmed status of any contacts, and epidemiological information.
That means a “positive” test result is not enough information to confirm an infection. How many cycles did it take? When was the test taken? Are you showing any signs of being sick? Have you recently been around someone infected? Is another test needed? Doctors need to take all these factors into consideration.
For the index, there’s no way of knowing how many false positives inflate NC’s case count, but this known problem strongly suggests another way in which this index is undercounting the actual amount of people posing no threat to pass along the virus.
All that said, here is the Threat-Free Index for the week ending January 25: