So yes, I hear you silently cursing with the question: “okay so what is herd immunity and why should I care about it now anyway?” Well, it turns out that herd immunity is a hot topic at the moment. Okay, maybe not in our part of the world (Ghana, Africa). But it is all the rage here in the USA where we have excelled at playing ostrich with the deadly realities of COVID-19.

Some weeks ago, a group of infectious diseases epidemiologists and public health scientists, issued the Great Barrington Declaration (can be found here: https://gbdeclaration.org/). The Declaration was authored by three Professors from Harvard, Oxford and Stanford Universities respectively and signed by 592,160 concerned citizens, 11,428 medical & public health scientists and 32,447 medical practitioners as of October 29, 2020.

Their main point: “Those who are not vulnerable should immediately be allowed to resume life as normal. Simple hygiene measures, such as hand washing and staying home when sick should be practised by everyone to reduce the herd immunity threshold... The most compassionate approach that balances the risks and benefits of reaching herd immunity, is to allow those who are at minimal risk of death to live their lives normally to build up immunity to the virus through natural infection, while better protecting those who are at highest risk. We call this Focused Protection.”

Simply put, they are advocating for the abolishing of social distancing, masking and quarantining.

That then begs question, what is herd immunity… and herd immunity threshold?

“Herd immunity, also known as indirect protection, community immunity, or community protection, refers to the protection of susceptible individuals against an infection when a sufficiently large proportion of immune individuals exist in a population. In other words, herd immunity is the inability of infected individuals to propagate an epidemic outbreak due to lack of contact with sufficient numbers of susceptible individuals” (Omer, SB et al. JAMA, October 2020). Herd immunity is an old Public Health concept going back over one hundred years. Herd immunity can be achieved in one of two ways: vaccination (artificial) or natural infection and recovery from the infection. The two are as different as day and night. Vaccination achieves herd immunity through provision of preformed antibodies or by using live but weakened virus (or bacteria) to induce an “artificial infection” that leads to immunity. The second way of achieving herd immunity, natural infection, well, is just that, natural infection.

The herd immunity threshold is defined as the proportion of individuals in a population who, having acquired immunity, can no longer participate in the chain of transmission. If the proportion of immune individuals in a population is above this threshold, current outbreaks will extinguish, and endemic transmission of the pathogen will be interrupted. The herd immunity threshold varies by disease and depends on the infectivity of the virus. The higher the infectivity the higher the threshold. For the most highly contagious diseases, like measles, the herd immunity threshold is approximately 94% – that is at least 94% of the population must become immune to the measles virus (either via vaccination or natural infection) – in order for herd immunity to be achieved. For COVID-19, scientists and the World Health Organization estimate the percentage as between 50% and 70%.

For most diseases in recent history — from smallpox and polio to diphtheria and rubella —vaccines have been the route to herd immunity. Did I hear you mutter about vaccine side effects? Well, much has been made of side effects of vaccines based on isolated incidents to discredit vaccines. Polio vaccine bashers ignore the fact that polio is eradicated because of the vaccine that we are all too eager to discredit now. Other examples like an increased incidence of Guillain-Barré syndrome followed influenza immunization for swine flu in 1976-77. Subsequent studies have not confirmed an association between influenza vaccine and Guillain-Barre syndrome.
What about egg allergy you asked? When this one was subjected to epidemiologic analysis, there were 33 cases of anaphylaxis out of 25.1 million doses. Moreover, modern day vaccines no longer have significant egg residues.

However, for a disease like COVID-19, that has no vaccine, the only way to achieve herd immunity (short of rigid social distancing-masking) is to allow natural infection to slash through the population for those who are lucky enough to recover to achieve natural immunity. This is the strategy the Great Barrington Declaration is advocating for – an infection-based herd immunity approach – letting the low-risk groups become infected while “sequestering” the susceptible groups.

Such a strategy though is fraught with risks because a new pathogen will result in substantial mortality because most, if not all, of the population would not have immunity to the pathogen. Sequestering the high-risk populations is impractical because infections that initially transmit in low-mortality populations can spread to high-mortality populations (from children to parents/grandparents in this case). It should be noted also “vulnerable” groups – proportions of the elderly, minority, low socioeconomic income, disabled – differ by population. Moreover, so far, there is no example of a large-scale successful intentional infection-based herd immunity strategy.

There are only rare instances of seemingly sustained herd immunity being achieved through national infection. The most recent and well-documented example relates to Zika in Salvador, Brazil. Early in the COVID-19 pandemic, as other countries in Europe were locking down in late February and early March of 2020, Sweden made a decision against lockdown. Initially, some local authorities and journalists described this as the herd immunity strategy: “Sweden would do its best to protect the most vulnerable, but otherwise aim to see sufficient numbers of citizens become infected with the goal of achieving true infection-based herd immunity.” By late March 2020, Sweden abandoned this strategy in favor of active interventions; most universities and high schools were closed to students, travel restrictions were put in place, work from home was encouraged, and bans on groups of more than 50 individuals were enacted. Evidence that Sweden’s open-door policy was unwise is that as of October 29, 2020, the death toll in Sweden was 5,938 compared to its Scandinavian neighbors (implemented social distancing programs from the start): Denmark with 719, Finland with 358 and Norway with 282.

Using the USA (because I do not have data on Ghana) as an example, and 50% – 70% as the herd immunity threshold, at the time of writing this article (on the morning of October 31, 2020), the total number of infections is 9,047,792 with 229,708 deaths. At this rate, in order for the USA to achieve herd immunity, it would require at least 165.5 million to 231.7 million infections with 4.2 million to 5.9 million deaths. Deaths are only one part of the equation. Individuals who become ill with the disease can experience serious medical and financial consequences, and many people who have recovered from the virus report persistent lingering health effects, including headaches, generalized weakness, vision and hearing changes, loss of taste or smell, memory loss, and cognitive impairment for up to 3 months. This may occur even in patients with mild COVID-19. (NIH, October 12, 2020. [Also related to this, see the New York Times story linked below on persistent "brain fog" in COVID-19 survivors: https://www.nytimes.com/2020/10/11/health/covid-survivors.html].

There is also the matter of science not knowing as yet all the long term effects of the virus on especially children, pre-teenagers and teenagers (the so-called “low-risk”). We do know for example that viral infections like measles, chickenpox and Herpes have long term consequences in terms of neurologic sequelae later in life.

Then comes the final question of whether herd immunity by natural infection is even achievable in the first place. With less than 10% of the population being infected so far, with an infection-induced immunity lasting 2 to 3 years (duration unknown, may be less), infection-induced herd immunity is not realistic at this point to control the pandemic based on the calculations. In other words, it is a much more complex scenario than the simplified version of “get infected-recover-build-immunity-hooray!” Beyond this simplified version, there is more pain, suffering and gnashing of teeth that must be factored in – prolonged disability, loss of income.

Finally, there is the question of duration of immunity and what happens after immunity wanes – being cognizant of the fact that we have no idea when. Do we simply repeat the entire cycle of free-for-all-infection all over again? On the contrary, artificial herd immunity via vaccination is a carefully controlled process from start to completion. The vaccine dose is titrated to the minimum amount of vaccine required to achieve the highest level of balance between optimum immunity and safety in a defined population and demography. The exact duration of immunity so achieved is known and maintained through a comprehensive vaccination schedule.

So, what do we do? Which strategy? How do we implement it?

At the individual level, decisions tend to be contradictory for any given life situation. This is because decisions depend on basal considerations (what matters to them the most). In other words, humans seldom make rational or logical decisions. Human decisions are therefore unpredictable.

Using my hapless self as an example, as a medical doctor working on the frontlines taking care of COVID-19 patients, on the one hand, I place more premium on the constant danger of getting infected on the job and the explosive workload created by COVID-19 – so naturally, I am in favour of quarantine-isolation-masking; at the same time, my children are online schooling – so I am against quarantine-isolation-masking. On the other hand, I can imagine my children’s schoolteachers also preferring schools to remain closed for their (teachers’) safety while being conflicted with childcare worries of their own.

While this description of human decision-making may appear counterintuitive, it is anything but. Such deviations from logical decision-making are quite normal and to be expected. At least according to the science that purports to study human behaviour and choice, Economics.

It is for good reason that the field of Economics begins all its theories and statements with the caveat “all things being equal” – “ceteri paribus” – may be more familiar to some. The problem though is that nothing in real life is ever “equal”. Enter the Prospect Theory, invented in 1979, it is one of the most groundbreaking theories in Economics and deservedly won the Nobel Prize in Economics in 2002. It seeks to explain human behaviour in terms of economics and finance but has been widely applied to all sciences that work to explain how individuals make decisions when faced with uncertainty.

The theory discovered that individuals believe that improbable events are more likely to occur than they are in practice (in this case believing that COVID-19 will kill all of us if we do not implement one or the other strategy, even though it may not), and conversely that probable events are less likely to occur than they are in practice (believing that I will never get COVID-19, even though I might).

Even more pertinent is the discovery by the prospect Theory that individuals view outcomes of decisions/actions in terms of gains and losses, with the latter weighted more than the former. Thus, social distancing tends to be evaluated by individuals in terms of lost (jobs lost, income lost, school years lost) rather than gain (lives gained, deaths prevented). Wearing a mask and avoiding social gatherings is equated to being “forced” by scientists and central government to lose one’s right to not wear a mask and the taking away of the right to freedom of movement.

From our discussion on human behaviour and choice above, it might be quite safe to declare that decisions for the social good, must not be left to individuals. Regardless of the strategy, decisive centralized decision-making and leadership is a requirement for effective systematic implementation and uniform adherence.

Regardless of the strategy, decisions must be devoid of political interference; decisions must be based on science, the veil of ignorance concept, humanism and what works for the greater society. Humans evolved from hunter-gatherer-nomadic societies into modern highly organized societies bound by centralized laws for a reason. Even in the animal kingdom, clans are ruled in an organized law-and-order hierarchy by alpha males and females. Our pathetic situation in the USA is not that bad decisions were made, it is that NO decisions were made at all by our national leadership. The result being that individuals have made decisions – and rightfully so – based on personal, political, economic and other parochial considerations.

In the end, these may just be rumblings from faraway lands that may never reach our part of the world (Ghana, Africa). But to borrow from my last article, if your neighbour’s beard happens to be on fire, do not just go sit by water, go ahead and preemptively immerse your entire beard in the river!