Herd Immunity or Abandoned Flock? Dr. Changa Kurukularatne, MD, FAMS
Infectious Diseases Specialist
Infectious Diseases Specialist
‘Herd immunity’ is one of the most widely used terms in the context of the COVID-19 pandemic. Yet, the term is often poorly understood — and therefore misused — by physicians, scientists, public health officials and lay people alike.
The origin of the term can be traced to the pre-World War I outbreak of ‘contagious abortion’, an infection caused by the bacterium Brucella abortus, among American livestock, causing spontaneous miscarriage in cattle and sheep. A veterinarian from Kansas, George Potter, argued against the prevailing practice of killing the infected animals, referring to the concept of herd immunity in his 1918 writing: “Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down”.1 Potter advocated that the affected cows be allowed to clear the infection on their own and continue to nurse their calves until a sufficient proportion of cows retained immunity against the illness. As long as new, non-immune cattle were not introduced into the mix, the contagion would naturally die down.2
The term saw re-purposed usage again in the 1970’s with vaccine use to eradicate smallpox. The concept of herd immunity was used to understand the number of people that needed to be vaccinated in order to eradicate the disease from human populations.
The fundamental principle of herd immunity is the protection of pockets of non-immune, susceptible individuals from a contagious illness by an interstitium of immune individuals acting as a barrier to deny the continued spread of the contagion within the population. The basic reproductive number, or R0 (R-naught), is the average number of secondary infections caused by an infected primary source in the absence of immunity and external control measures. Intuitively, where R0 is >1, epidemics will worsen; when R0 <1, epidemics will subside.
R0 can be used to estimate the threshold of herd immunity through the (oversimplified) equation of 1 - 1/R0. Applying known R0 values provides an estimate of the necessary immune population thresholds needed to protect the population.
The older strains of SARS-CoV-2 were noted to have R0 of around 3. Applying the oversimplified equation of 1-1/R0, we see that 67% of the population must develop immunity to effectively curtail the pandemic. However, newer variants such as Delta (B.1.617.2) have a much higher R0, ranging from 3.2 to a whopping 8, with a mean R0 of around 5, beating out SARS, MERS, Ebola, smallpox, seasonal influenza and pandemic influenza.3 For context, seasonal influenza has an average R0 of 1.3, and measles, an unusually cruel 15-18.4 As our oversimplified equation 1-1/R0 tells us, the herd immunity threshold for the Delta variant with an R0 of 5 becomes 80% - up from the 67% for older variants.
Even well-meaning doctors, scientists and public health officials who truly believe that vaccination does not prevent SARS-CoV-2 infection or eliminate transmission will in the same sentence speak of achieving herd immunity thresholds through vaccination campaigns. This cerebral uncoupling must be faced head-on.
Assumptions of all forms are problematic;5 those surrounding herd immunity threshold calculations are no different. The formula assumes random mixing of people and a stable R0 across the entirety of a susceptible population. In reality though, human interactions are non-random, and R0 varies considerably in different communities within a population.
An even more significant assumption is that vaccination confers 100% durable immunity. We know this not to be true. Vaccines do not confer 100% protection from infection with SARS-CoV-2, nor do they confer a zero-percent transmission risk. As one paper took great pains not to mis-speak, “Vaccinated individuals are likely to be less likely to transmit the virus”.6 Being likely to be less likely to transmit the virus is not what we need to achieve herd immunity thresholds. Even well-meaning doctors, scientists and public health officials who truly believe that vaccination does not prevent SARS-CoV-2 infection or eliminatetransmission will in the same sentence speak of achieving herd immunity thresholds through vaccination campaigns. This cerebral uncoupling must be faced head-on. Simply put, if one can still contract the contagion and transmit the contagion despite vaccination (and indeed be re-infected following natural infection), then the concept of herd immunity has trouble finding foothold in the context of COVID-19.
Let us recall what COVID-19 has taught us so far. One - you can contract COVID-19 more than once after natural infection.7 Two - re-infection probability appears to be higher with a different strain than the original.8 Three - prior infections, be it asymptomatic or symptomatic, are poor predictors of population protection.9 If these weren’t discouraging enough to anyone attempting to calculate herd immunity thresholds for COVID-19, we also know that new strains are capable of selling crowd-wowing head-fakes, ankle-breaking cross-overs and bowl around the legs of natural as well as vaccine-induced immune responses.10 Therein lies the problem.
An R0 by any other name? Throw a healthy amount of physical distancing for long enough between infected and non-immune people, ensure good compliance with face masks, instil responsible behaviour in your population, implement robust test-trace-isolate programs and voila! Your R0 will start to drop. Is this herd immunity? Certainly not, immunity being the operative word. The moment distancing practices are abandoned, large gatherings are held with face masks hanging off of chins and ears and people start frolicking again, so will R0 rise again. Sounds familiar?
Manaus, in northern Brazil, was devastated by a particularly vicious outbreak of COVID-19 in 2020. Serologic and case data estimated 76% of Manaus’ population - higher than the theoretical herd immunity threshold of 70% based on R0— was exposed to SARS-CoV-2. As hospitalisation rates subsequently declined, herd immunity was hailed as the saviour - until infections, hospitalisations and deaths dishearteningly rose again, presumably due to waning immunity, a new P1 strain, or other unknown population factors.11
Time will tell. What we do know is the principal tenets of herd immunity are simply not applicable in the case of the elusive SARS-CoV-2, its supercharged variants, our immune responses, and the currently available vaccines. I am not the sole bearer of bad news; see ‘Five reasons why COVID herd immunity is probably impossible’.12
We hope that natural immunity, specifically, the cell-mediated immune response, will convey durable, long-term population protection, even after neutralising antibody levels wane.13 We hope future vaccination strategies will elicit durable immune responses without escape mechanisms for newer variants. We also hope against hope that SARS-CoV-2 will somehow lose its infectivity, virulence and lethality over time, and learn to live amongst us in relative harmony, much like its cousins, the cold-causing Coronaviridae.
None of these hopes, let it be known, involve ‘herd immunity’ as we know it.
1Jones D, Helmreich S. “A history of herd immunity” Lancet 396 no.10254 (2021):810-811. doi:10.1016/S0140-6736(20)31924-3
2McDermott A. “Core Concept: Herd immunity is an important—and often misunderstood—public health phenomenon”. Proceedings of the National Academy of Sciences 118 no.21 (May 2021): e2107692118. doi: 10.1073/pnas.2107692118
3Liu Y, Rocklöv J. “The reproductive number of the Delta variant of SARS-CoV-2 is far higher compared to the ancestral SARS-CoV-2 virus”. Journal of Travel Medicine taab124 (2021): doi:10.1093/jtm/taab124
4McDermott A. “Core Concept: Herd immunity is an important—and often misunderstood—public health phenomenon”.Proceedings of the National Academy of Sciences 118 no.21 (May 2021): e2107692118. doi: 10.1073/pnas.2107692118
5Jackson Samuel L. "Everybody knows, when you make an assumption, you make an ass out of you and umption.” The Long Kiss Goodnight (1996).
6Vitiello A, Ferrara F, Troiano V, La Porta R. “COVID-19 vaccines and decreased transmission of SARS-CoV-2” Inflammopharmacology 1 no.4 (July 2021) doi:10.1007/s10787-021-00847-2
7Vitale J, Mumoli N, Clerici P, et al. “Assessment of SARS-CoV-2 Reinfection 1 Year After Primary Infection in a Population in Lombardy, Italy”. JAMA Intern Med. (May 2021): doi:10.1001/jamainternmed.2021.2959
8Wang J, Kaperak C, Sato T, et al. “COVID-19 reinfection: a rapid systematic review of case reports and case series”. Journal of Investigative Medicine 69 (2021):1253-1255.
9McDermott A. “Core Concept: Herd immunity is an important—and often misunderstood—public health phenomenon”. Proceedings of the National Academy of Sciences 118 no.21 (May 2021): e2107692118. doi: 10.1073/pnas.2107692118
10Garcia-Beltran WF, Lam EC, St. Denis K, et al. “Multiple SARS-CoV-2 variants escape neutralization by vaccine-induced humoral immunity”. Cell 184, no.9 (2021):2372-2383. doi:10.1016/j.cell.2021.03.013.
118Buss LF, Prete CA, Abrahim CMM, et al. “Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic”. Science 371 no. 6256 (2021):288-289. doi: 10.1126/science.abe9728
12Aschwanden C. “Why herd immunity in COVID is probably impossible”. Nature 591(2021): 520-522
13Cromer D, Juno JA, Khoury D, et al. “Prospects for durable immune control of SARS-CoV-2 and prevention of reinfection”. Nat Rev Immunol 21 no.395(2021): 404. doi:10.1038/s41577-021-00550-x