What is herd immunity?
The simple answer
‘If you don’t catch it, you can’t pass it on’
Most infectious diseases are caused by germs that are spread from person to person. In many cases, these germs can only survive, thrive and multiply if they can find vulnerable ‘host’ people to inhabit. If people are protected by vaccination, then there aren’t any available ‘hosts’ for the germs to live and thrive in.
In this way, if a vaccine is effective enough, and if enough of the population get vaccinated, the amount of disease in the community gets less and less, until if there are only a few people who are not vaccinated, then they are protected too.
A simple way of thinking of this is “If you don’t catch a disease, you can’t pass it on”.
This concept is known as ‘herd immunity’ or ‘community immunity’, and is such a powerful mechanism in the right circumstances it can lead to the control or eradication of disease. It is through this mechanism that smallpox has been eradicated, and polio is virtually unheard of.
Herd immunity does not apply to all diseases, nor all vaccines. Tetanus for example, lives in the soil so you don’t catch it from a person, and so it is only your own vaccination status that confers any protection (the vaccination status of those around you has no influence on your own risk). Herd immunity is also a concept considered only in the control of disease by vaccine, not by allowing uncontrolled spread of disease.
(Map taken from Surveillance data, measles epidemic in the Netherlands, 2013)
A more detailed explanation
When enough of the population is immunised, germs that cause vaccine preventable disease cannot find anyone to thrive and multiply in. If this happens the germs cannot spread, cannot find anyone to infect, and cannot cause disease.
This then means that everyone in the community is protected, even those who cannot be immunised because they are too young or too sick. It also protects those who have not yet completed the whole course because of their age.
This graphic demonstrates this.
In some cases, where humans are the only known reservoir or host of a particular vaccine preventable germ, herd immunity can mean that a disease can be totally eradicated, and then no one would ever be at risk of that disease again. ie if a virus or bacteria can only live in humans and is not found in soil or other animals, eradication of that disease is possible. Humans are the only known hosts of smallpox, measles and polio.
And it is via this mechanism that smallpox no longer exists. We no longer need to get vaccinated against it because of a global vaccination campaign and the science behind herd immunity.
Through this mechanism there is a very real possibility polio could be eradicated within our lifetimes. For more information about polio eradication, have a look at these 10 facts described by the World Health Organization. And IF we could vaccinate enough of the population against measles, there is a change we could also rid the globe of that disease too.
Different diseases and different vaccinations have different levels of ‘herd immunity’, because it varies according to how effective the vaccine is, how long the vaccine is effective for, and how contagious the disease is.
For most diseases, somewhere between 80-95% of the population need to be vaccinated to protect the vulnerable. This is known as the herd immunity threshold. The more contagious the disease, the greater the proportion of the population that need to be vaccinated to achieve herd immunity.
Vaccines do not work as a total ‘force field’ around someone. Instead, they prime the body’s immune system to recognize germs, so that if they are encountered in real life, the body is ready and strong and boosted and prepared to fight off disease. Each dose of a vaccine strengthens this response, and some vaccines need a course of doses to ensure adequate protection. If you are incompletely vaccinated, or immunocompromised, whether through choice or through age or through illness, you are dependent on herd immunity to protect you.
For most vaccine preventable disease, it is vital that it is children that are amongst the most highly immunised. Day care, pre schools, schools, kindergartens, and play groups are all places where people meet up and mingle in groups. And children share their germs far more than adults do, which is why we all know that coughs and colds (for example) ‘spread like wildfire’ through pre-schools.
It is also in childcare centres and kindergartens that there are lots of children who are incompletely vaccinated because they are too young to have completed the course, and it is there that kids have baby siblings who are too young to have been vaccinated. In the event of an outbreak of vaccine preventable disease, unvaccinated children may be asked to stay away from school to prevent further spread of disease, and to protect those who are undervaccinated or susceptible to disease through no choice of their own.
If enough of the population refuses to immunise themselves and their families, it is a chink in the armour of herd immunity, and you can get clusters, or pockets, of disease that can start up.
In the Northern Rivers of Australia, the number of people who have refused to vaccinate their families has meant that we are below the herd immunity threshold for some diseases. This means we are now at real risk of epidemics of preventable infectious disease, and already have higher than necessary rates of vaccine preventable diseases in the area, as demonstrated by this data from 2009.
Here are some other ways that herd immunity can be demonstrated;
- Here is a Chain of Protection video.
- An easy to understand animation from Harvard Medical School.
- An animation demonstrating how measles spreads through communities with different vaccination rates.
- Surveillance data, measles epidemic in the Netherlands, 2013
- Chain of Protection – herd immunity
- Harvard Health herd immunity animation
- The Science of Immunisation booklet
- Smallpox eradication
- 10 facts about the eradication of polio
Page reviewed January 2021