I have been having e-mail and Twitter discussions about the “herd immunity threshold” (HIT) and what it means for policy. Typically, the conversations centre around four items: (i) the number of people who have been infected with the virus and are therefore potentially immune to the disease (these two things are not the same). We seem to be arriving at significantly different numbers; it would have been an academic discussion was it not for the claim that (ii) we are nearing the HIT, (iii) the drop in new cases is due to a large proportion of immune individuals rather than lockdown, and hence (iv) we can all relax and return to “normal”. There is a variation on these points, suggesting that even if we are not close to HIT, we should simply let the disease run through the population while the vulnerable are “protected” by vaccination). I will try to address these four points briefly below.
(i) With the two large waves of COVID-19, we clearly have built up some “natural” immunity – by this, I do not mean cross-immunity from other coronaviruses or BCG vaccines – but immunity from having COVID-19. The question is how much. A number of studies point to the levels being not higher than 20-30% in the general population; the evidence comes from antibody testing as well as model outcomes. These numbers can be higher locally, for example, some evidence points to 50% in Delhi and maybe as much as 70% in Manaus (although this last claim has proven controversial).
The levels of antibodies in the UK are between 10-20% and the models which “reconstruct” the prevalence predict about 15-30%.
(ii) There is quite a lot of renewed interest in the estimation of the “herd immunity threshold” (HIT). Simple models predict 60-70% based on R=3 as observed in Wuhan in the early stages of the epidemic; for the new strains this might be higher, but then we never observed them without any non-pharmaceutical control measures.
We, modellers, do know this is an overestimate, as it does not include various sources of heterogeneity. The question, however, is by how much it is overestimated. Some researchers originally came up with 20%, although they have been revising the numbers (I think ca 30% is the latest estimate); there are very solid papers that point out errors in this analysis. The problem is that each model makes assumptions that often are very difficult to validate.
(iii) I found the arguments by the “low HIT” camp unconvincing and so I think we are still some way from HIT, perhaps except some locations (some places in India come to mind).
As I understand it, Manaus is a good example of the need to be cautious. The antibodies estimates in Autumn 2020 found about 70% population positive and the researchers claimed that the first wave stopped because of the HIT. Unfortunately, Manaus then experienced the “second wave” of disease, suggesting that either the antibodies levels were lower, or the protection was overcome.
One thing to realise is that the models predict that once we start approaching HIT when the infection levels are high (as now), the reduction should be relatively slow. The “threshold” is not really that dramatic.
(iv) I have the main issue with people who claim that we are already close to or at the HIT and so we can all relax, drop all precautions and just return back to “normal”. This is a recipe for disaster if – as I believe – we are not past the HIT and the consequences are terrifying. Why? It is because I do not believe the “third” lockdown will work – people will stop complying and we will have a wave of the infection going through the population again, with huge loss of life and long-COVID.
