What is herd immunity?

Editorial Team

Editorial Team

February 7th, 2013

Editorial Team

‘Herd immunity is a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.’

It arises when a high percentage of the population is protected through vaccination against a virus or bacteria, making it difficult for a disease to spread because there are so few susceptible people left to infect.

This can effectively stop the spread of disease in the community. It is particularly crucial for protecting people who cannot be vaccinated. These include children who are too young to be vaccinated, people with immune system problems, and those who are too ill to receive vaccines (such as some cancer patients).

The proportion of the population which must be immunised in order to achieve herd immunity varies for each disease but the underlying idea is simple: once enough people are protected, they help to protect vulnerable members of their communities by reducing the spread of the disease.

However, when immunisation rates fall, herd immunity can break down leading to an increase in the number of new cases. For example, measles outbreaks in the UK and pertussis outbreaks in the US have been attributed to declining herd immunity.

Using animation, this video helps to explain how herd immunity works and what happens when herd immunity breaks down. It is designed to be used as an educational tool, ideally supported by an experienced trainer.


  1. Jemima


    August 31st, 2013

    It appears that this outdated college theory is still being used to trot out new vaccines….

    Herd Immunity: Three Reasons Why I Don’t Vaccinate My Children… And Why Vaccine Supporters Shouldn’t Care That I Use Vaccine Exemption Forms

    “Herd Immunity. This hypothesis was plucked out of an old college textbook. It states that the more people are immune to an infectious agent, the less likely an immune-compromised individual is to come in contact with it. In other words herd immunity serves as a human shield – a type of immunity – for “at-risk” individuals. But remember, it’s only a hypothesis.

    My background as a medicinal chemist taught me to rely on proven research. I learned to be less sensitive to emotional arguments and more sensitive to facts supported by reproducibility. This is one of the main principles of the scientific method. It refers to the ability of a test or experiment to be accurately reproduced.”


    If the herd immunity theory actually WORKED, those who have already been vaccinated should have nothing to worry about.

  2. Gary Finnegan

    Gary Finnegan

    September 3rd, 2013

    Thanks for your post. We answer your point below. But first we should note that the article you link to seems to be for a US-based audience whereas Vaccines Today is for EU residents. There are some cultural differences between European and the US when it comes to immunisation policy. For example, the reference to ‘exemption forms’ is not relevant to people living in Europe.

    However, you also take issue with the scientific consensus on how herd immunity works. This video animation explains the basic theory of how herd immunity protects those who are too young or too sick to be vaccinated. https://www.vaccinestoday.eu/stories/what-is-herd-immunity/

    Here’s another source explaining the value of herd immunity in more detail

    This TED talk by Prof Adam Finn provides an interesting case study (pertussis) showing what happens when vaccination rates are high – as well as what happens when vaccination rates fall below the level required to achieve herd immunity.

    In terms of published evidence, we would draw your attention to this comprehensive scientific review on the topic http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3171704/