While the details are extremely complex, most people understand the general principle of immunity to disease in individuals and that two of the main ways in which immunity can be acquired are through vaccination or “natural” exposure-to-an-infectious-agent-that-does-not-result-in-death. Arguments tend to be about how good, and how long lasting, such immunity might be in the case of the SARS-CoV-2 virus (responsible for Covid-19 infections), and that is something we just do not know yet.
The notion of immunity within a population (or “herd”) is a more imprecise notion which seem to be at the heart of a burgeoning culture war on social and traditional media.
Let us try and unpick the various issues:
We do not need to speculate too much about what happens when contagious disease (i.e. disease that spreads from person to person) disperses unchecked through a population with no immunity because we have plenty of examples within living memory.
Prior to 1954, measles might arrive in a small relatively isolated community and spread rapidly through it. Some individuals might die, others might suffer serious injury[i], but most would recover and most of these individuals would thereby acquire immunity to the measles virus – immunity that would usually (though not always) remain with them for the rest of their lives.
What is of note here is that the measles virus would not necessarily need to infect every individual in the isolated community before cases of measles disappeared in that community. Measles is highly contagious, but once a certain threshold of immunity (around 91-94%[ii]) is reached within a randomly mixing community[iii], the virus will normally cease its spread and disappear from that community.
The 6-9% who never became infected are now safe – protected by all those who did become infected and survived. Moreover, even if a new infected person arrives in the community, it is unlikely (though obviously not impossible) that they will pass on their infection to anyone in the community.
The protection (such as it is) from measles for that isolated community may last for a generation, but as more and more people die of old age and more and more new children are born without immunity, that community protection weakens. Eventually, a new epidemic of measles will devastate the community.
This is one reason why infections may come in waves.
Those bad old days are behind us and, pace the activities and influence of various anti-vaccination fanatics, we can achieve 90% or more immunity against measles across a population by vaccinating everyone we can.
So far so good, and we have not even mentioned the boo term yet …
Mass vaccination is the context in which the term “herd immunity” (AKA “community immunity”) is most often encountered these days, but, as this definition from the Centers for Disease Control and Prevention makes clear, use the term is not restricted to immunity that is acquired from vaccination programmes:
Community immunity: A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community. Also known as herd immunity.
But even this definition leaves some loose threads untied. As Fine, Eames, and Heymann have put it[iv]:
The term “herd immunity” is widely used but carries a variety of meanings. Some authors use it to describe the proportion immune among individuals in a population. Others use it with reference to a particular threshold proportion of immune individuals that should lead to a decline in incidence of infection. Still others use it to refer to a pattern of immunity that should protect a population from invasion of a new infection. A common implication of the term is that the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals (this is sometimes referred to as “indirect protection” or a “herd effect”).
To illustrate the three usages using the measles example:
1. An example of “herd immunity1” might be “There is only 80% herd immunity against measles in the UK at the moment and that is too low.”
2. An example of “herd immunity2” might be “Cases of measles will only start to decline towards zero in the UK once 90%+ of the population have been vaccinated (or have recovered from the illness) and herd immunity is achieved.”
3. An example of “herd immunity3” might be “An epidemic of measles in Finland would be impossible because of the level of herd immunity there achieved through high MMR uptake.”
I should argue that usage “3” overlaps with “1” and “2”, but this is all getting a bit pedantic. I think we can make do with the “common implication of the term” that “the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals” and all agree that that is true.
I do not see any good reason for a culture war over terminology here!
So what about the SARS-CoV-2 virus? (I hear you all cry.)
There is still huge uncertainly about Covid-19. All scientists can do as we look ahead is to use complicated mathematical models to predict what might happen given certain assumptions.
Inputs to these models are things like how many people currently have Covid-19; how long they remain infectious; how many people they each infect; how likely they are to die and in what time-frame; and, yes, how many have recovered from and infection and thereby (it is to be hoped) gained immunity.
I say “it is to be hoped” because, as is often pointed out, the immune response that defeated the virus in a particular individual may not persist, or a new strains of the virus may evolve against which immunity previously acquired is no longer effective.
There is, I should wish to contend, a growing body of evidence that the people who gleefully point these things out are being unduly pessimistic. But time will tell.
On the other hand, there is (putting my pessimist’s hat back on) growing evidence that a lot of people who “recover” do not fully recover from Covid-19[v].
So, as an earlier famous Vladimir had it: “Что делать?” – “What is to be done?”.
It is strange – I often reflect – that someone’s views on our trade relationships with other European countries should be such an accurate predictor of their views on virology. But this seems to be very much the case.
There is a vocal body of people on social and traditional media who argue (alongside venting their spleen at the Belgians or whoever) that we should moderate or abandon our efforts to control the spread of the virus and let nature take its course. Such people often use the term “herd immunity” to describe the happy state that would emerge after the “unfortunate deaths of a few old people who would have died soon anyway”.
Other people appalled (rightly IMHO) at such a suggestion often seize on the term “herd immunity” and seem to have invented yet another definition:
- “herd immunity4”: “The belief that we should accept the death and disability of hundreds of thousands of people with equanimity.”
But do the more moderate members of the let-nature-take-its-course brigade have a point? Could we somehow arrange for everyone who would fully recover from Covid-19 infection to be infected while protecting all the more vulnerable people?
The short answer is “no!”.
The slightly longer answer is that if let Covid-19 spread freely across the globe (with or without attempting to protect people we assume are most at risk of death or disability) far too many people would end up dead or disabled – including perhaps people even "lockdown sceptics" care about.
The slight caveat here is that we are currently allowing as much normal activity as we feel is safe to allow at any one time and thus striking a balance between risks to vulnerable people and other considerations – including risks to people who are vulnerable to non-Covid related dangers[vi]. In doing this, it is, I submit, often reasonable and ethical to try and decrease risks to the more vulnerable even where this increases risks to the less vulnerable.
It is not wrong or evil to point out that there are, sadly, always difficult trade offs to be made in life.
In the meantime all we can do is try and get a test and trace system in place that actually works, continue our social distancing, continue targeted social measures – which will inevitably involve a lot of trial and error – and hope for a vaccine as soon as possible.
[i] Which is why everyone should vaccinate their children with the MMR jab.
[iii] Of course, real communities do not mix entirely randomly.