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.
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
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.