Q: What is herd immunity?

Manish Sadarangani: Herd immunity describes how a population is protected from a disease after vaccination by stopping the germ responsible for the infection being transmitted between people. In this way even people who cannot be vaccinated can be protected. For example, the bacteria meningococcus and pneumococcus can cause blood poisoning (septicaemia) and meningitis. In most people the bacteria live harmlessly in the throat and do not causes disease, but sometimes they get into the bloodstream leading to these severe infections. They can live harmlessly in the throat of one person but if they spread to someone who is particularly susceptible (such as a young baby) they can cause severe disease. By being vaccinated an individual is not only protected from being infected themselves but they then also cannot pass this infection onto other people, where it may cause severe disease. However, for herd immunity to work a large proportion of the population need to be vaccinated.

People are shown as circles. Infectious agents (germs) spread between the people in orange, although they do not get severe disease. When the infection reaches people who are highly susceptible (red) they get the disease and can be very sick or die.

In the lower panel, the people in green have been vaccinated. This now protects those in yellow as well, who had previously got the infection and possibly the disease. Although the figure only shows a few people being vaccinated, in reality many people have to be vaccinated for herd immunity to work.

Q: Why is it important that children are vaccinated if they can be protected through herd immunity?

MS: For many diseases children, and in particular young children, are at the highest risk of the disease and also have the most severe illness. The best way to protect someone against a disease is to vaccinate them directly, rather than rely on ‘indirect’ protection through herd immunity. If someone who is unvaccinated does meet the germ responsible for that disease they will be completely susceptible. There will always be some children who cannot be vaccinated for medical reasons – most commonly because they are too young or because they have a problem with their immune system. It doesn’t take much of a drop in the number of people vaccinated in a population for herd immunity to be less effective and this isn’t then a reliable way of protecting a child.

Q: How many children need to be vaccinated for herd immunity to work?

MS: This varies depending on the germ and how contagious it is. The more contagious it is then the more people need to be vaccinated for herd immunity to work. For example, measles is very contagious. Before the use of the measles vaccine, every person with measles would infect another 10-15 people and so the disease would spread very quickly. To achieve herd immunity for measles at least 90-95% of the population need to be vaccinated. A disease like polio is less contagious, and 80-85% of the population would need to be vaccinated for herd immunity to work. Although this is lower it is still a very high proportion, especially given that some people cannot be vaccinated for medical reasons.

Q: Does herd immunity work for all diseases?

MS: No. Herd immunity only works for diseases that are spread directly between people (i.e. are ‘contagious’), like measles. One example where it would not work is tetanus. The bacteria which cause tetanus lives in the soil, so anyone who is not vaccinated would be susceptible and could easily be infected if they were exposed to bacteria in the soil, such as through a dirty wound, even if everyone else around them was vaccinated and protected.

Q: The theme of this year’s Immunisation Week is ‘Closing the Immunisation Gap’. What do you think we can do to close this gap?

MS: We need to identify reasons why some populations in the world have lower immunisation rates than others and then try to come up with solutions to address them. Many children every day get sick and die from diseases which we already have vaccines for but in some populations many children are not receiving them. It is likely that different problems will be present in different areas and solutions need to be tailor-made to the problems in each case. We also need more research to know how much disease there is throughout the world, particularly in developing countries where it is more difficult to do this research. This will help prioritise vaccine development programmes and also identify which populations need the vaccine most. This is more easily said than done, and needs ongoing collaboration on an international scale with political will and financial backing. This needs to be made a global health priority given that vaccines are the most effective and cost-efficient healthcare intervention we have.

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