Thinking Out Loud: This is What Herd Immunity Looks Like

The MMWR reports on a “typical” portion of American society, blood donors; their donation provides a sample of the “seroprevalence” of COVID-19 based on the presence of antibodies. Before jumping to the findings, a quick word about the demographics of blood donors.

In a study of 390,000 US blood donations reported by donors between 16 and 69: 77% were white, 16.3% Black, 2.3% Hispanic with 11 donations/1000 persons among whites, 6/1000 persons among Blacks, and 3/1000 persons among Hispanics. A study of German blood donors showed no gender prevalence; most donors were motivated by “helpfulness” and had few co-morbidities. A majority of blood donors are aged 25 to 64.

MMWR Data

Beginning in July 2020, the seroprevalence of COVID-19 has been estimated from blood donations. All blood donors were tested for antibodies against the spike, found in those infected or vaccinated, and for antibodies against the nucleocapsid proteins, only present after an infection. This data could then identify those with vaccine-induced, infection-induced, or hybrid immunity. The nationwide cohort included 142,758 individuals; roughly half had vaccination data available for cross-referencing.

By the third quarter (July to September) of 2022, 96.4% of persons aged ≥16 years had COVID-19 antibodies from previous infection or vaccination.

• 22.6% from infection – the percentage was higher amongst the young rather than the old
• 26.1% from vaccination
• 47.7% from hybrid immunity – the hybrid immunity was lowest among adults aged ≥65 years.
• “Among persons with no previous infection, the incidence of first infections during the study period (i.e., conversion from anti-N–negative to anti-N–positive) was higher among unvaccinated persons” - The difference declined from 40% to 5.4% over the period, but remained statistically significant.  

With the end of the COVID-19 public health emergency, weekly COVID hospitalizations in the US were 7,212,  weekly deaths of 607, and an overall vaccination rate of 17%. There were 153,000 hospitalizations at their peak the week of January 20, 2022; weekly deaths peaked January 9, 2021, with 26,000.

What exactly is herd immunity?

Looking back on our general understanding of herd immunity during the pandemic’s early days, I believe we saw it as “protection or exemption” – more in keeping with a legal definition than a biological one. If only we took specific measures, behavioral (masks, lockdowns, social distancing), and pharmaceutical, i.e., vaccines, we would be protected or exempt from COVID-19. That was an incorrect understanding in general, and of the novel COVID-19 virus specifically. The reality of herd immunity is that

• It slows but does not end transmission.
• It is acquired through exposure to the virus, but at a significant cost to our health, and through vaccination. The CDC data show that older individuals, who most frequently sought vaccination, were protected just as much as younger individuals, who achieve immunity the old-fashioned way by becoming infected (symptomatic or not). Many gained enhanced immunity through both exposure to the virus and the vaccine.
• Immunity may be more impactful in reducing the severity of illness than its occurrence.

As the number of individuals with greater resistance to infection and lesser symptoms in the presence of infection rises, we achieve what we are calling herd immunity. The earlier notion that we might somehow calculate the necessary percentage of the population that needs to be immune by simply using a pathogens R₀  was incorrect. [1] It treated everyone as susceptible, which is not the case, especially as more individuals acquire protective antibodies. And it failed to consider the severity of the resulting disease, a real-world concern not modeled in a system that categorizes all infections as equal. The continuing emergence of variants with differing infectiousness and severity of illness was not predicted or considered. 

The latest CDC data show us what herd immunity may look like – far fewer cases, less severe disease, and fewer deaths. These results are the product of behavior by the hosts and the pathogen. Vaccination works far more effectively for measles than influenza; that is a biological fact based on the virus and our response immunologically – one cannot be separated from the other. The mistake we made in discussing herd immunity early in the pandemic was that we did not know what we didn’t know. The pandemic gave us a better understanding; it would be hubris on our part to believe we have anything near complete understanding of the term herd immunity that we so often casually cast about.

[1] As a reminder, the R₀ measures the average number of secondary infections a single infected individual can cause in a susceptible population.

Source material Estimates of SARS-CoV-2 Seroprevalence and Incidence of Primary SARS-CoV-2 Infections Among Blood Donors, by COVID-19 Vaccination Status — United States, April 2021–September 2022 Morbidity and Mortality Weekly Reports.