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Herd immunity

I heard ‘herd immunity’ mentioned on BBC Radio 4 this morning in the context of COVID-2019 pandemic. It is worth reminding ourselves what this means, as it is actually quite a controversial concept.

Its origins are from epidemiological modelling where people observed that a disease will not spread if the proportion of susceptible individuals (those who are still healthy and can contract the virus) falls below a certain value.

This can occur in different ways. The simplest one is when we have a large outbreak of a relatively benign disease so that nothing but palliative care is needed. The number of cases will at some point stop growing and eventually the epidemic will die out on its own. But the interesting point is that this will not lead to everybody having been through the infection. There will be quite a lot of people who simply do not get the disease even though they could have been infected. In a sense, they are protected by those who did get the disease and became immune. Thus, the ‘herd’ is becoming immune to disease, even though not all individuals are. Seasonal flu or measles outbreaks before vaccination era are examples of such a situation.

Another way of reaching the ‘herd immunity’ is by vaccination. You have probably heard about controversies associated with the MMR (measles, mumps, rubella) vaccine. Back in 1960s the US, the UK and other countries faced large measles outbreaks. The idea of massive vaccination was (and still is) to make enough people immune to the disease so that the virus cannot spread. For measles, this is estimated at about 90%, so that we expect that if the percentage falls below this number, new cases will start appearing. This is exactly what has been happening in many countries.

I am not going to go in this post into a discussion of merits or dangers of MMR vaccination – this is a subject for another post or posts. But I want to point out that the key element in both MMR and COVID-2019 idea of ‘herd immunity’ is that a large proportion of the population is asked to take a – usually very small – risk in either vaccinating (MMR) or going through coronavirus infection (COVID) for the public good. In other words, I am asked to take a risk to protect others from possibly facing a larger one.

This was made quite explicit this morning in the BBC Radio 4 programme:

‘Sir Patrick Vallance says aim is to create herd immunity and broaden peak of epidemic’

says Guardian this morning. The government’s idea is to slow down the epidemic but not to stop it so that the NHS is not overwhelmed, but enough people – possibly – get immunity to prevent future outbreaks.

There is a problem with this approach in that it has been shown that voluntary measures to create herd immunity simply do not work. People (consciously or subconsciously) evaluate risks of getting complications from mild coronovirus vs. benefits of protection against complications and at some point decide that this will not work for them. This point is given by a balance between perceived individual costs (falling ill, getting complications, passing on coronavirus to the elderly Mum) and perceived population benefits (not having an outbreak in autumn). When the first perceived cost is larger, people will not play the game and the ‘herd immunity’ fails (as it did for measles). There is a large body of evidence and theory using simulations (our own research) as well as the game theory that supports this behaviour.

This is a fascinating time for epidemiological researchers. As different countries implement different control measures, we start seeing differences in the way COVID-2019 epidemic is slowing down. One thing is certain, in the future the epidemiologists will need to very closely work with psychologists and economists. In our recent paper we said:

So, are we equipped to deal with the next pandemic? According to Blackburn et al. [98], the answer currently is a qualified ‘no’. However, the tools and processes may already be available to allow a more emphatic ‘yes’ to be the answer. Our future success in preventing and combating pandemics requires close collaboration across disciplines and systems.

So what is the solution? We are facing a crisis and there is a lot of unknowns. The key thing is for both politicians and experts to be very open about what they do and what they do not know about the outbreak. In the meantime, we need to do what we can to slow down the spread.

As usual, if you have comments, please send them to me at info@statisticallyinsignificant.uk