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:
- The PCR test cycle threshold in North Carolina is far too high
- Doctors aren’t told how many cycles were used to arrive at a positive test result
- A positive test result can’t tell how much or even how viable the viral material it detected is
- Using the cycle threshold limit supported by research consensus (30 cycles) would have prevented up to 90% of “positive” test results in several other states from being considered positive
- Severe economic and personal restrictions are being leveled in North Carolina on the basis of positive test results as well as the percentage of test returning positive
… 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:
- As of Jan. 25, there were 635,543 North Carolinians presumed to be recovered from COVID-19
- Active cases comprised just 11% of NC’s total case count (note: a case of COVID isn’t a permanent infection, and only someone with an active case of the virus can conceivably transmit it to you)
- Active cases represented just 0.7% (seven-tenths of one percent) of NC’s population (note: active cases are lab-confirmed cases of COVID-19 minus recoveries and deaths)
- More than seven out of every eight (87.8%) of NC’s total cases were recovered, meaning they are no longer infectious
- Only 0.08% of people in NC had died with COVID-19 (regardless of the actual cause of death)
- Meanwhile, 93.2% people in NC had never had a lab-confirmed case of COVID-19, despite the PCR test cycle threshold set so high as to produce a large amount of false positives (note: this proportion will always decline, but we have been living with this virus since February, as far as testing is concerned)
- All considered, about 99.2% of people in NC posed no threat of passing along COVID-19 to anyone — a virus most had never had and the rest had recovered from (note: this proportion will fluctuate based on the relative growth in lab-confirmed cases vs. recoveries, and is likely understated because it does not account for vaccinations)