The NC Threat-Free Index Explained

• Since July 2020, the NC Threat-Free Index has provided contextualized looks at North Carolina’s Covid-19 numbers that have been lacking from media and government updates
• All of the statistics generated for the NC Threat-Free Index are based on numbers provided by government sources
• In recent weeks the index has expanded to include estimates of herd immunity and how few people have been reinfected or contracted Covid-19 after being vaccinated

In July 2020 I began providing contextualized Covid-19 updates for North Carolina that soon became the NC Threat-Free Index. With the index I sought to fill a need sorely lacking in government updates and media coverage of the pandemic: perspective.

Big numbers are hard to follow, and the always-increasing case counts tend to inflame fears. As I noted last year, “Normally, government officials and media consider it their public duty to calm people and tamp down unhealthy, irrational fears.” In this case, however, they were actively engaged in fearmongering — still are.

When people hear there were hundreds of new cases today, they’re not told the other side: hundreds of people also completed their recovery today and are no longer infectious. They also don’t consider the relatively small amount of people with active cases of Covid-19 against the very large number of North Carolina’s total population.

As I wrote last November about why we need the NC Threat-Free Index,

Round-the-clock virus coverage in media, authoritarian government restrictions against people and businesses, and questionable state health data have all contributed to a climate of fear and suspicion far out of scope with what the virus warrants and without any regard to severe and mounting unintended consequences in other aspects of people’s lives. …

People are left with the impression that we’re surrounded by a contagion that’s far more widespread and deadly than it really is. Among other things, it makes them extremely vexed when someone says something like “far more widespread and deadly than it really is,” as if seeking to quantify how infectious and deadly this virus is constitutes a kind of COVID blasphemy. …

Is this to say we should not respect the virus? Of course not. It is not to downplay the seriousness of COVID-19 or the lives lost because of it. But it is also not to make out as if it is the plague.

In recent months, the NC Threat-Free Index has also sought to estimate how many North Carolinians have some immunity to Covid. It includes accounting for North Carolinians with naturally acquired immunity from prior infection, which a large body of research shows is even stronger, broader, and more durable than vaccine-induced immunity.

I started making these estimates because media and politicians have chosen, for whatever reason, to ignore this other half of herd immunity. At best, they’re missing a huge chunk of North Carolina’s herd immunity story. At worst, they are needlessly endangering the rights of millions of North Carolinians. They even ignored research findings by the Centers for Disease Control and Prevention (CDC) from Sept. 2, 2021, that had estimated the share of people with antibodies from either prior infection or vaccination in the Central and Western North Carolina Region at 81.2% (which, incidentally, was higher than my estimate for the state).

With the state Department of Health and Human Services now providing data on reinfections and post-vaccination infections, I have been able to provide contextualized looks at those aspects, too, showing how few people have been reinfected or contracted Covid-19 after being vaccinated.

All of the statistics generated for the NC Threat-Free Index are based on numbers provided by government sources. Following is an explanation of where each statistic is found and how each estimate is made.

• North Carolinians presumed to be recovered
  The number of North Carolinians presumed to be recovered from Covid-19 is published weekly by the North Carolina Department of Health and Human Services (DHHS) under guidance from the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).
• Active cases
  The number of active cases of Covid-19 is derived by subtracting presumed recoveries and deaths provided by DHHS from the total case count provided by DHHS. Active cases as a percent of the total cases count is derived by dividing the number of active cases by the total case count provided by DHHS. Active cases as a percent of the population is derived by dividing the number of active cases by the current-year North Carolina population estimate provided by the North Carolina Office of State Budget and Management (OSBM).
• Percentage of total cases that are recovered
  The percentage of total cases that are recovered is derived by dividing the number of presumed recoveries by the total case count provided by DHHS.
• Percentage of North Carolinians who have died with or from Covid-19
  The percentage of the North Carolina population that died with or from Covid-19 is derived by dividing the deaths provided by DHHS by the North Carolina population estimate provided by OSBM.
• Percentage of North Carolinians who posed no threat of passing along Covid-19
  The percentage of North Carolinians who posed no threat of passing along Covid-19 is derived by the following: the North Carolina population provided by OSBM minus total cases provided by DHHS and adding presumed recoveries provided by DHHS, divided by the North Carolina population provided by OSBM minus deaths provided by DHHS.
• Percentage of adult North Carolinians with some immunity to Covid-19
  Finding the percentage of adult North Carolinians estimated to have some immunity to Covid-19 requires several steps. Immunity is either through vaccination or naturally acquired from an infection. The percentage of adult North Carolinians (18+ years of age) vaccinated with at least one dose is provided by DHHS.It is more difficult to estimate the percentage of people with natural immunity for reasons explained here: “Unlike with people who are vaccinated, there aren’t official records of everyone who has encountered SARS-CoV-2 and fought off a Covid-19 infection. There will be records of those who have tested positive, yes, but as the CDC acknowledges, only about “1 in 4.2 COVID–19 infections were reported.” That estimate makes sense when you consider mild infections, cases in which one member of a family is known to be infected but all members feel some effects, and infections that occurred in the months before Covid-19 was officially tracked.

  Only the cases of adult North Carolinians must be counted, which means subtracting from the total case count provided by DHHS the number of cases for people ages 0-17 provided by DHHS as well as the number of deaths provided by DHHS.

  Since immunity flows from infections, not whether a case is documented or not, the number of cases of adult North Carolinians is then multiplied by the 4.2 infections multiplier given by the CDC. This estimate of actual infections among adult North Carolinians becomes the estimate of adult North Carolinians who have natural immunity. Dividing that number by the state’s estimated adult population (found by subtracting the number of people ages 0–17 from the total North Carolina population estimate provided by OSBM) gives the percentage of adult North Carolinians with natural immunity.

  To avoid double-counting immune adults, this percentage is discounted by the vaccination rate, using a conservative assumption that the naturally immune are vaccinated at the same rate as the rest of the adult population. Finally, that discounted percentage of naturally immune adults is added to the percentage of adults vaccinated with at least one dose per DHHS to derive the percentage of adult North Carolinians with some immunity from Covid-19.

• Percentage of North Carolinians with some immunity to Covid-19
  A similar procedure is used to estimate total immunity in North Carolina. Here, of course, the case count and population are as given, without subtracting case or population counts of those ages 0-17. The percentage of all North Carolinians vaccinated with at least one dose is provided by DHHS. The percentage of North Carolinians with naturally acquired immunity is estimated from the total case count provided by DHHS, multiplied by the 4.2 infections multiplier from the CDC, and discounted by the vaccination rate provided by the DHHS. That discounted percentage of naturally immune North Carolinians is added to the percentage of North Carolinians vaccinated with at least one dose per DHHS to derive the percentage of North Carolinians with some immunity from Covid-19.
• Reinfections and the reinfection rate
  The number of reinfections is provided by DHHS, and the reinfection rate is derived by dividing the number of reinfections by the total case count provided by DHHS.
• Post-vaccination cases and the post-vaccination infection rate
  The number of post-vaccination cases is provided by DHHS, and the post-vaccination infection rate is derived by dividing the number of post-vaccination cases by the number of fully vaccinated people provided by DHHS.
• Percentage of new cases that are post-vaccination cases
  The percentage of new cases in North Carolina that are post-vaccination cases is provided by DHHS.
• Percentage of North Carolinians without some immunity to Covid-19
  The estimate of North Carolinians with neither vaccine-induced nor natural immunity is derived by multiplying the percentage of North Carolinians who have not been vaccinated (i.e., subtracting from 100% the percentage of North Carolinians with a least one dose per DHHS) by the estimated percentage of North Carolinians who have not had an infection (subtracting from 100% the percentage of North Carolinians with naturally acquired immunity that was estimated from the total case count provided by DHHS and multiplied by the 4.2 infections multiplier from the CDC). Obviously, adding this estimate to the estimated number of North Carolinians with some immunity to Covid-19 should sum to 100%.