NC COVID-19 Threat-Free Index for the week ending March 29, plus a discussion of herd immunity

By Jon Sanders

Raleigh, NC – Here is the NC Threat-Free Index for the week ending March 29:

  • As of March 29, there were 876,108 North Carolinians presumed to be recovered from COVID-19
  • Active cases comprised just 2.5% 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 about 0.2% (two-tenths of one percent) of NC’s population (note: active cases are lab-confirmed cases of COVID-19 minus recoveries and deaths)
  • More than 24 out of every 25 (96.2%) of NC’s total cases were recovered, meaning they are no longer infectious
  • Only about 0.1% of people in NC had died with COVID-19 (regardless of the actual cause of death)
  • Meanwhile, about 91.5% 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 2020, as far as testing is concerned)
  • All things considered, about 99.8% 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 relative growth in lab-confirmed cases vs. recoveries, and it is likely understand because it does not account for vaccinations)
Photo by Mick Haupt

Herd immunity discussion

As of March 29, DHHS listed that 2,885,099 people had been at least partially vaccinated, and that 1,648,686 people had been fully vaccinated. The rough estimate of herd immunity for Covid is when 70% of the population is immune. I don’t know how much overlap there is between recoveries and vaccinations. If people in NC who have recovered from the virus (i.e., people with natural immunity, or in a backwards way of looking at it, people who have had a natural vaccination) were a completely separate population from those who have been vaccinated, that would suggest that as of March 29, up to 23.7% of the state population was immune.

Even that statistic would likely be vastly undercounting state immunity, given how many mild infections (never diagnosed), exposures fought off due to preexisting immunity, and SARS-CoV-2 infections that people had prior to the virus being officially identified (diagnosed as a “influenza-like illness”) there must have been.

With respect to immunity via previous Covid-19 infections and vaccinations, newly published research (March 19, 2021) in the Journal of the American Medical Association confirms expectations: antibodies either from recovering from an infection or from the vaccine not only rapidly appear and persist for months and months, but also they ward off virus infections from new variants.

Furthermore, natural immunity from an infection may be lifelong, as discussed in this Wall Street Journal piece by Dr. Marty Makary, professor at the Johns Hopkins School of Medicine, Bloomberg School of Public Health and Carey School of Business:

Some experts claim they don’t talk about natural immunity because we shouldn’t trust it. But a recent Public Health England study found that less than 1% of 6,614 healthcare workers who had Covid-19 developed a reinfection within five months—even though many of them work with Covid patients. Other experts believe natural immunity is powerful.

“Natural immunity after Covid-19 infection is likely lifelong, extrapolating from data on other coronaviruses that cause severe illness, SARS and MERS,” says Monica Gandhi, an infectious-disease physician and professor at the University of California.

To Gandhi’s point, lifelong really is lifelong. The Mayo Clinic points out that “those who survived the 1918 flu (influenza) pandemic were later immune to infection with the H1N1 flu, a subtype of influenza A.” For how long was that natural immunity still going strong? The H1N1 flu was during the 2009-10 flu season, over 90 years later.

New research posted on March 29 — which has not yet been peer-reviewed — finds that vaccination provides community-level protection, extending also to the unvaccinated:

[A]nalyzing vaccination records and test results collected during a rapid vaccine rollout for a large population from 223 geographically defined communities, we find that the rates of vaccination in each community are highly correlated with a later decline in infections among a cohort of under 16 years old which are unvaccinated. These results provide observational evidence that vaccination not only protects individual vaccinees but also provides cross-protection to unvaccinated individuals in the community.

Again, this study is pending peer review, and also it is observational. But the correlation is strong, and its implications are highly encouraging:

In this study, we identified a strong negative association between vaccination rate at the community level and the risk of infection for unvaccinated members of the community. We find that higher vaccination rates were associated with a later lower infection rate among the unvaccinated cohort.