Herd immunity has been talked about a lot in recent days, particularly here in the UK. I’ve been asked by a few people to explain what herd immunity is, and how it protects vulnerable members of a population. I won’t comment on the strategies being employed by any particular country or politician with regard to herd immunity to COVID-19. But I am going to try and explain what it is and how it works, so here goes.

When you are exposed to a pathogen (a micro-organism that causes disease), your immune system responds to fight it. Important weapons in the immune system’s arsenal are chemicals called antibodies. I won’t go into how they work – if folks are interested, I’ll do a separate post. Antibodies are specific – each antibody will only fight the particular pathogen it was designed for. When you are first exposed to a pathogen, it takes several days for your immune system to start making antibodies.

After you recover, white blood cells called memory B-cells remain in your bloodstream. They do exactly what the name implies – they ‘remember’ how to make antibodies to that pathogen. This means that if you are exposed to that particular pathogen again, your immune system will start making antibodies straight away, and will defeat the infection before you develop any symptoms. This is called immunity.

Immunity can be developed in two ways – naturally, through catching and recovering from an infection; and artificially, through vaccination. I’m going to talk about vaccination in another post, because a few people have asked about it in regard to COVID-19, so I won’t go into the details here. Basically, vaccination is away of provoking your immune system into making antibodies, and memory B-cells, without you having to get the illness; it usually uses a dead or weakened form of the pathogen.

So, how does herd immunity work? The diagram below will help explain:

The diagram shows the number of people in a population that are immune from a disease. On the left, we can see that the majority of the population are immune to the disease. The probability of those who are not immune coming into contact with the infection is low, and so is the probability that they will infect each other.

Now look at the population on the right, where there is a high proportion of individuals who are not immune. If just one of those individuals contracts the disease, the probability of them coming into contact with someone else who is not immune is high, and the disease will be able to spread through the non-immune members of the population.

How is herd immunity developed? In the case of common and mild infections, it occurs naturally. For example, there is significant natural herd immunity to some strains of flu that only cause mild illness. For more serious infections, herd immunity is developed artificially through mass vaccination programs.

In any population, there will be people who cannot be vaccinated or do not have immunity to a disease. Some vaccines can’t be given to people with compromised immune systems, such as those undergoing chemotherapy. Others may be allergic to one of the vaccine’s ingredients; for example, the flu vaccine can’t be given to people who are allergic to chicken eggs. There will also be people who have had the vaccine but do not have immunity. It takes time to build up immunity following a vaccine, so those who have only had the vaccine recently may not yet be immune. In addition, those who have been vaccinated but whose immune system is compromised may be vulnerable to the disease.

Let’s look again at the population on the left of the diagram. From the previous paragraph, you have no doubt realised that if this represents a population where mass vaccination is in place, the majority of people who have no immunity are also those who are most vulnerable to serious illness and life-threatening complications. Herd immunity protects them by minimising the risk of them coming into contact with the disease. On the right, however, there is no such protection.

Take the example of measles. Measles is a serious viral infection that can result in life-changing or even fatal complications. There is, however, a safe and effective vaccine, and where mass vaccination programs are in place, herd immunity protects the vulnerable. If uptake of the vaccine is poor, then this protection is no longer there and vulnerable individuals are at a high risk of catching the disease.

You’ve probably never heard of Serhiy Butenko, but his case illustrates this perfectly. Ukraine has a serious measles problem due to poor uptake of the vaccine; in 2018, there were approximately 54,000 cases. Serhiy Butenko was an 18-year-old medical student in Ukraine who had been vaccinated against measles as a child. In early 2018, he was recovering from a bad bout of mononucleosis (glandular fever) that had severely depleted his immune system. He was exposed to measles, and because his immune system was suppressed, he became ill despite having been vaccinated. He developed pneumonia, which is a common complication of measles, and in February 2018 he died after several days in intensive care. Serhiy is a good example of someone who would be protected by herd immunity.

So what about COVID-19? At the moment, there is no vaccine, so the only way to develop herd immunity is for the majority of the population to catch it and recover. Which is fine if you are one of the vast majority who would only experience mild illness, but not ideal if you are one of those who is at high risk of more serious illness. I said I wouldn’t comment on the strategy of any specific government with regard to herd immunity, but speaking as one of those at-risk individuals (I have asthma), I am not entirely convinced by this thinking. Still, I will wait to pass judgement until the scientific evidence supporting this strategy has been released, and I can evaluate it for myself.