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On herd immunity again

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.

Polsat TV – cd

Brief English summary: Four widely-accepted models are used to estimate the “true” number of infections from COVID-19. The cumulative number for Poland in February 2021 is 26% (ICL) and 14% (IHME); for comparison, the UK has 37% and 16%.

Po napisaniu poprzedniego blogu, poszukałem wyników modeli dla Polski, które umożliwiają ocenę “przwdziwej” ilości zachorowań. Szczegóły są na stronie internetowej:


ICL to Imperial College London (UK)
IHME to The Institute for Health Metrics and Evaluation, University of Washington (USA)
YYG to Youyang Gu, matematyk i informatyk z USA
LSHTM to The London School of Hygiene & Tropical Medicine (UK)

Używając wyników z tych powszechnie zaakceptowanych modeli, można wyliczyć, że do tej pory 26% (ICL) lub 14% (IHME) populacji przeszło przez chorobę w okresie do lutego 2021. Wyniki podane przez YYG i LSHTM nie sięgają do 2021, ale w sierpniu (0.7%) i pazdzierniku (3.8%) były zbliżone do ICL i IHME.

Nawet przyjmując pesymistyczne wyniki z modelu Imperial College London, mamy w Polsce 26% osób potencjalnie odpornych.

Herd immunity strikes back

With a bit of a shock and disbelief, I have read a new article in the Financial Times titled: UK scientists call for debate on allowing ‘big wave of infection’.

The article describes the idea that once all potentially vulnerable people are protected by the COVID-19 vaccine, we should allow the disease to run through the young population to achieve “natural” herd immunity. The title is a bit misleading as it is not scientists who are proposing this (at least not scientists mentioned in the article, like Mike Tildesley or Graham Medley, both of whom I highly respect), but politicians.

It is an interesting adaptation of the Great Barrington Declaration idea, only this time it uses the vaccination as the method to protect the “vulnerable”. There is a good discussion of the downsides of the GBD strategy on the COVID FAQ web site and the arguments there – particularly points 4-7 – stand whether the protection is achieved by locking down only the “vulnerable” parts of the society or by vaccinating them.

Whatever the arguments against this strategy, I am quite pessimistic about future developments. I suspect that unlike the original GBD proposal, the current one will gather a lot of support and – willingly or not – governments – and societies – will be tempted to go down that route.

Why is it so? I think that the societies in many countries including the UK and my native Poland are reaching a breaking point. The first lockdown was generally very successful in bringing down case numbers and it was generally broadly accepted and obeyed. I am still amazed at how rapidly and deeply the Google mobility data dropped back in March/April.

However, most countries (except Australia, New Zealand, Taiwan and similar) were not really focusing on zero-COVID approach. Fearing the economic meltdown and once the threat of health system collapse disappeared, the governments opened up. It all looked very rosy – the pandemic is (almost) under control.

We now know pretty well that Scotland almost eliminated the virus in summer 2020, only having it reintroduced later. The similar situation appeared in many other countries. The second and third waves came, with new, more infectious strains. But more importantly, the lockdowns came late, never really reached the depth of the first one, and were relaxed for Christmas and New Year.

While in most places the first lockdowns were painful, but simple and short, the second ones tended to be complex. Attempts to keep parts of economy going resulted in complicated rules. People stopped understanding why this part of the economy or social life is closed and the other is not. The economically vulnerable started feeling the pressure and essentially stopped obeying.

As the governments botched the response in mid-2020, they created a situation where more and more people, for different reasons, feel that the lockdowns do not work and should be abandoned. Trying – rightly – to convince more people about the merits of vaccination, we also created a conviction that vaccination will solve all problems. It will not, but it does not matter – the message is there.

So what are my expectations? The end to lockdowns followed by a large spring wave which – in contrast to the first one – will largely affect young people. Hopefully, there will not be too many hospitalisations and deaths but I doubt this will be the end of the virus.

Is this the best solution to the pandemic? Absolutely not; it is my firm belief that the optimal solution would have been an early, sharp, deep lockdown to bring the numbers down, followed by strict test-and-trace with extensive support for people who need to self-isolate, and strict border control requiring tests and quarantine.