Herd Immunity for Dummies – what you really need to know

Have you heard that ’herd immunity’ is mass murder, genocide, euthanasia, eugenics?  This is what many journalists, some of the finest non-scientific minds in academia and on social media say.  Don’t be fooled. The failure to understand the idea of herd immunity  is a bit like  denying  the idea of gravity or evolution.

Herd immunity has more than 40,000 references on google scholar.   It  is even taught  in school biology classes.

The Myth of Vaccines and Herd Immunity

Herd immunity does not depend on vaccination.  The concept  predates vaccination programmes.  The history of smallpox vaccination is a misleading guide to the story  of vaccination. The biggest vaccine gains have   occurred since world war 2. (1)  Herd immunity was discussed decades before these. And having the concept helped make vaccination planning more effective.

Epidemic Disease and the Virgin Population

Infectious disease spreads fast amongst virgin populations. In the Americas in the sixteenth century smallpox possibly killed tens of millions.  Coronavirus is not that dangerous but its transmission – the famous R rate – is rapid.

Herd immunity

Look at the diagram. The first part   shows that  in a virgin population disease spreads  from one person to two to four to eight and so on   (although not everyone will get it perhaps because of some chance genetic immunity).

But notice over time, even if there is no vaccine, there will be a reduction in transmission as immunity grows. Someone with immunity will not get it again and  that person will also not  transmit it. The reproduction rate goes down. Now remember that before vaccines this is what could happen over a long period and at serious  cost if the disease was highly dangerous.

The Discovery of Herd Immunity

Even before the idea of a germ theory of disease and ideas of immunity observers puzzled over why epidemics varied in size. At the turn of the twentieth century a debate took place between those who said this was because of  changing disease virulence and  those who said it was blockages in transmission. It soon became apparent that these blockages – which came to be called  ‘herd immunity’ – were immensely important. They helped to explain how plant diseases, animal diseases and human diseases progress. (2)

Using Herd immunity with Vaccines

The broadening possibility of vaccination then allowed the possibility of creating herd immunity. Of course, everyone should be vaccinated. But herd immunity meant that you did not need to vaccinate everyone to have a huge effect. The idea of herd immunity also helped you to plan your vaccination programmes to get the biggest hits while vaccination rates were still low. (3)

Today  a coronavirus vaccine will solve all problems. But billions of vaccine doses will not be available at once so we will still need to think of using the idea of herd immunity to help deploy them.

Herd Immunity and the  Non-Vaccine Scenarios

flatten-the-curve-smaller

1. The Baseline projection – herd immunity alone.

I think it is tosh to say that this was ever seriously proposed. But you need a good idea of what is involved here to make other programmes work. It is a bit like asking what will happen if you design a plane in a way  that ignores the laws of gravity. It doesn’t mean that you want to ignore these laws – you need to ask the questions to overcome them.

The simple answer is that if the R rate is high an epidemic will spread very quickly and herd immunity will develop very fast but at great cost.

2. Flatten the Curve

The next scenario is the famous  flatten the curve. Here you try to reduce the spread so that your health service is not overwhelmed until  a  treatment   or   vaccine can be found.  You still need to aim for the same level of herd immunity as in the baseline scenario – you are just   going to take longer to do it and reduce the cost.

How long? You need to avoid overloading your medical system. Suppose you have 10,000 intensive care beds and 10% of those who get infected need them –  you can only allow 100,000 to be infected at any one time. If the need rate is 1% then you could allow 1 million. This shows why the infection rate is so important. Equally  if you increased intensive care beds to say 20,000 you could double those you allow to be infected. You will need to manage this as long as it takes to get you to the herd immunity level.

But there is an additional complication. The measures you take will save the lives of those with coronavirus and they may also save other lives e.g. by reducing pollution. But they will also increase other aspects of the death rate. So you will need to think about trade offs. Do you want to save  one  person with coronavirus by killing perhaps 1.5 without it. This is why what is called all cause mortality and the overall pattern of excess mortality is so important.

How do you do it? Lockdowns, test and trace, allowing low risk groups to get it? That is what is being discussed but they all involve using the idea of herd immunity.

3. Minimal Herd Immunity

This is a third possibility. You try to suppress the disease wherever it occurs. This means you want the line to be as flat   as possible. I am not clear if anyone is really following this strategy but elements of it seem to be influencing policy in China and perhaps South Korea. However if you are not generating much immunity at all then you cannot allow the same type of free movement that occurred before the coronavirus hit or  you will be back to square one. If you suppress the disease within  –  you need to have permanent barriers without to stop it returning until  a vaccine is found.

So which ever way we turn we need the concept of herd immunity.  It is time  to embrace science not reject it out of ignorance and political opportunism.

(1) Plotkin, Stanley. “History of vaccination.” Proceedings of the National Academy of Sciences, 34 (2014): 12283-12287.

(2) Fine, Paul EM. “Herd immunity: history, theory, practice.” Epidemiologic reviews, 2 (1993): 265-302.

(3) Fine, Paul, Ken Eames, and David L. Heymann. ““Herd immunity”: a rough guide.” Clinical infectious diseases, 7 (2011): 911-916.

3 comments

  1. I’m always amazed at discussions about herd immunity that avoid the concept of ‘endemic disease’ which is what many infectious organisms can achieve when they settle into a large population when either no vaccine is avaliable, the available vaccines have only moderate effectiveness or the disease organism mutates rapidly, requiring constant updating of the vaccine. Natural herd immunity (i.e. w/o vaccine) is a lousy strategy to GET RID of an infectious disease unless the population is reasonably small and isolated.

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  2. Thanks. I am just an economic and social historian who before covid hit had read some of the past and present literature on infectious diseases. I think it was only on fringes that people said let rip. I hope in blog to spread good knowledge. Your comment on endemic disease is well made. It is little discussed in popular accounts. Heard one epidemiological suggestion that in UK an endemic equilibrium would = number of car accident deaths. This would make no sense to those who imagine zero covid. But strange how they do not go for 0 anything else. Yes – you are right about vaccines.

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