Search Herd Immunity: How does it work?
General Public Engagement 26 April 2016 Dr Manish Sadarangani explains how herd immunity works and why vaccinating children is important.
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.
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.
Infectious agents (germs) spread between the people in orange, although they do not get severe disease.
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, 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 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?
Herd immunity only works for diseases that are spread directly between people (i.e.