Herd immunity or abandoned flock? The unique problem with COVID-19
‘Herd immunity’ is one of the most poorly understood and misused terms in the context of COVID-19. The fundamental principle of herd immunity is the protection of susceptible individuals within a population with no immunity against a particular contagion, by individuals with immunity against that contagion. To better understand this concept, consider three categories of individuals within a population vis-avis immune status: Alpha,infected (spreaders); Beta,non-infected (susceptible); and Gamma,immune to that particular agent. When the proportion of Gamma individuals is higher than the herd immunity threshold, they form a protective interstitial barrier to prevent an Alpha from directly infecting a Beta.
The term herd immunity dates back to the pre-World War I outbreak of ‘contagious abortion’ among American livestock. The Kansas veterinarian George Potter argued against the practice of killing infected animals, stating “Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down”. Potter advocated that infected cows be allowed to naturally clear the infection and continue to nurse their calves. As long as new, non-immune cattle were not introduced (Potter’s ‘new fuel’), the contagion would naturally die down.
The term saw re-purposed usage in the 1970s to estimate the number of people within a population that needed to be vaccinated in order to eradicate smallpox.
The concept
The basic reproductive number, or R0 (R-naught), is the average number of secondary infections produced by one infected individual within a fully susceptible population. As intuitively apparent, where R0 is >1, epidemics will worsen; when R0 <1, epidemics will subside. R0 is utilised to estimate herd immunity thresholds by using the (oversimplified) equation of 1-1/R0.
The older SARS-CoV-2 variants had an R0 around 3, meaning, each infected person would infect 3 others. Applying the oversimplified 1-1/R0, 67% of the population must have developed immunity to protect the non-immune. Newer variants such as Delta (B.1.617.2) have a much higher R0, ranging from 3.2 to a staggering 8 (mean R0, approximately 5), beating out SARS, MERS, Ebola and influenza. For context, seasonal influenza has an average R0 of 1.3, and measles, an unusually cruel 15-18. Applying the oversimplified 1-1/ R0, we see the herd immunity threshold for the Delta variant is 80% — a jump from 67% for older variants.
The problem with oversimplification
Assumptions in life are problematic, and 1 - 1/R0 is no exception. The formula assumes that immunity conferred (through vaccination or natural infection) is complete and durable. It further assumes a uniformly stable R0 and random movement of people within the population.
In reality, human movements are non-random, and R0 varies considerably within a population.
COVID-19 vaccines do not confer 100% protection from infection, nor a zero transmission risk. Vitiello et al. (2021) took great pains not to mis-speak in their adverb-laden conclusion “Vaccinated individuals are likely to be less likely to transmit the virus”.
Vaccines being ‘likely to be less likely to transmit the virus’ brings us nowhere near achieving herd im-munity. Scientists and public health officials who understand that COVID-19 vaccines neither fully prevent infection nor eliminate transmission will speak of achieving herd immunity through vaccination campaigns — cerebral uncoupling indicative of today’s buzzword culture. Simply put, if one can still contract and transmit the virus despite vaccination, and indeed be re-infected following natural infection, then herd immunity as a concept has trouble finding any foothold in COVID-19.
Abandoned flock?
Let us recall what COVID-19 has taught us so far. One - you can contract COVID-19 more than once (uncommon, but it happens). Two - the likelihood of re-infection increases with new variants. Three - prior infections within a population, be they asymptomatic or symptomatic, are poor predictors of future protection. If these weren’t discouraging enough to anyone attempting to calculate herd immunity thresholds, we also know that newer viral strains are capable of evading vaccineinduced immune responses, selling the ol’ head fake to hastily assembled antibodies and bowling around the legs of sluggish, clueless immune cells.
An R0 by any other name? Implement physical distancing, good compliance with face masks, responsible population behaviour, efficient test-trace-isolate programmes, effective early homebased treatment, and voila! Your R0 will drop, guaranteed. When distancing practices are conveniently forgotten, face masks hang mindlessly off people’s chins and ears, and large gatherings resume with reckless abandon, the R0 will surely rise as does the morning sun from the East; ergo, not herd immunity.
If not herd immunity, then what?
Manaus, in northern Brazil, was devastated by a particularly vicious outbreak of COVID-19 last year. An estimated 76% of Manaus’ population was exposed to SARS-CoV-2 - higher than the herd immunity threshold of 70% based on its R0. As deaths and hospitalisations declined, herd immunity was hailed as the saviour — until new infections, hospitalisations and deaths dishearteningly rose again, presumably due to waning immunity, a new P1 strain, and other unanticipated population factors.
Here is what we must accept, no matter how bitter the pill: the principal tenets of herd immunity are simply not applicable to the elusive SARS-CoV-2, its slippery and supercharged variants, our unpredictable immune responses, and the currently available vaccines. This idea is not without company; see Asch-wanden’s ‘Five reasons why COVID herd immunity is probably impossible’ published in Nature just a few months ago, appropriate adverbs and all.
The COVID-19 vaccines are very good at reducing deaths and preventing hospitalisation due to severe disease. Hospitals must function at a manageable capacity. If this is not an important benchmark, just visualise 10% of 20 million people flooding our hospitals over the next 2-4 weeks, and try to access care for an injury or illness that plagues you or your family. When hospitals exceed their capacity threshold for treatment, people die of heart attacks, strokes, blood poisoning, internal bleeding and trauma that otherwise could and would have been saved.
Regardless of how good the COVID-19 vaccines are at reducing mortality and preventing severe disease (translation: get vaccinated), they lack the infection prevention oomph and transmission elimination fire-power of, for instance, Max Theiler’s 1937 Yellow Fever vaccine. Boasting lifelong immunity with just a single dose, seroconverting a whopping 98% and inducing high levels of neutralising antibodies in just 10 days, the Yellow Fever vaccine has been instrumental in quelling every single outbreak since its introduction. Viruses vary greatly, as do vaccines. The COVID-19 vaccines are simply not in the same league when it comes to preventing infection and transmission. Meaningful reductions in both these metrics are observed — just not to the level required for herd immunity.
Here is what we must accept, no matter how bitter the pill: the principal tenets of herd immunity are simply not applicable to the elusive SARS-CoV-2, its slippery and supercharged variants, our unpredictable immune responses, and the currently available vaccines.
So where do we go from here?
We must simply — and simply must — employ interventions with demonstrated track record in fighting COVID-19. We hope the proven recipe of testing-tracingisolation will be utilised to its fullest potential in every country in a meaningful and effective manner, devoid of non-scientific interference and idiocy. We hope our border security and testing capabilities will be sufficiently enhanced to identify new variants as early as humanly possible. Where early home-based treatment is critical, we hope personal gain or political advantage will not collude to prevent the rigorous scientific analysis of Ivermectin as a preventative and therapeutic strategy. We hope that natural immunity, specifically, the cell-mediated immune response, will come through in the long run, conveying durable protection long after neutralising antibody levels have waned. We hope future vaccination strategies will elicit robust immune responses without such clearly marked neon escape hatches for newer variants.
We also hope against hope that SARS-CoV-2 will somehow lose its infectivity, virulence and lethality over time, and agree to live amongst us in relative harmony, much like its cold-causing cousins.
None of these hopes, let it be known, involve ‘herd immunity’ as we know it. Let us not use herd immunity as vaccine propaganda; COVID-19 vaccines are already credible enough in benchmarks that truly matter. Instead, let us get on with the business of making a real difference in saving lives and livelihoods through sound leadership and good governance, as responsible citizens. Compassion, unlike herd immunity, is achievable in COVID-19. Compassion will enable us to eliminate the stigma so distastefully linked to this illness, and to face its challenges with dignity and humanity.