When a high percentage of the population is vaccinated, it is difficult for infectious diseases to spread, as there are not many people who can be infected. In the case of measles, for example, if someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone and it will quickly disappear. This is called ‘herd immunity’ (also referred to as ‘community immunity’ or ‘herd protection’)
Herd immunity gives protection to vulnerable people such as new-born babies, elderly people and those who are too sick to be vaccinated. However, herd immunity does not protect against all vaccine-preventable diseases. A good example of this is tetanus, which is caught from bacteria in the environment, not from other people who have the disease – no matter how many people around you are vaccinated against tetanus, it will not protect you from tetanus!
How does herd immunity work?
Herd immunity only works if most people in the population are vaccinated (e.g. 19 out of every 20 people need to be vaccinated against measles to protect people who are not vaccinated).
If you live in an area where vaccine coverage is low, and your child is not vaccinated, it is quite probable that many of the people they come into contact with will not be vaccinated either. If one of these people gets an infectious disease, they can easily pass it on to the other unvaccinated people around them; in some cases, the disease can then spread very quickly through the population.
It is important to note that, unlike vaccination, herd immunity does not give a high level of individual protection, and is therefore not a good alternative to getting vaccinated.
People who depend on herd immunity
There are some people in vulnerable groups who rely on herd immunity to protect them against life-threatening disease as they often cannot safely receive vaccines. These include:
- Immune-compromised people (on immune-suppressant medication, active chemotherapy treatment etc.)
- People with HIV
- New-born babies who are too young to be vaccinated
- Some elderly people
- People who are seriously ill
Read an interesting article about herd immunity – “Please Help Me Keep My Children Healthy”.