Recent measles outbreaks and infant deaths from whooping cough have seen a renewed public interest in vaccination. Usually, vaccination is seen primarily as an act of personal protection – getting yourself vaccinated against measles, for example, will (in most cases) protect you from catching the measles.

Vaccines also protect from disease through the action of herd immunity – the collective effect of having a large number of vaccinated individuals in the community which makes it harder for a disease to spread. For infections such as whooping cough, herd immunity is a critical goal of vaccination campaigns and is a major objective of renewed pushes to encourage vaccination, such as the Victorian Government’s free whooping cough boosters for parents of newborn babies.

Immunity to infection is established in an individual when they have either experienced a disease, or have been vaccinated, and their immune system is primed to prevent further infection. In this case, a critical threshold is passed within the body so that there are enough disease-specific defenses ready to be deployed if the same disease is encountered again.

Herd immunity, or community immunity, occurs when a population contains a large enough proportion of immune individuals to impede the spread of an infection. Herd immunity means that a disease can be kept at bay even if there are non-immune individuals in the population.

But herd immunity is more complex than just protecting non-immune people from disease when they are surrounded by enough immune people. Think for a moment about what the ‘herd’ is. What is your community? Your immediate family? Your workmates? The people in your neighbourhood? What about the people you travel on the train with? Or the people shopping at your supermarket? Our ‘herd’ or community changes all the time. So, of course, the extent of herd immunity and the risk of disease can also differ within these groups. For example, a vaccinated family with a unvaccinated newborn baby may have good herd immunity against whooping cough within their own home, but may live in an area where the overall herd immunity against whooping cough is poor.

The critical threshold for herd immunity differs between diseases, and depends on how easily a disease is passed between people (its transmissability), how prevalent the disease is and how effectively a vaccine (or natural infection with the disease) induces protection. For diseases such as tetanus, where even natural infection does not protect against subsequent infection, herd immunity does not develop with increased vaccination. Individual vaccination against tetanus will protect only the vaccinated individual. But for diseases such as whooping cough and measles, herd immunity provides an important layer of protection for unvaccinated and non-immune people within the community.

Let’s have a closer look at herd immunity against measles. Measles is very easily transmitted between people. It is estimated that in a non-immune population, one person infected with measles could infect around 18 others, and most people who come into close contact with someone with measles will catch it unless they’re immune. The critical immune threshold (or percentage of immune people in a community) for preventing measles outbreaks is around 90-95%, and this number is high because of how easily measles is transmitted from one person to another. So, for every 10 people in a community, nine of them need to be immune to measles to prevent an outbreak. We are lucky that there is an excellent measles vaccine available, as I’ve discussed before, but being vaccinated doesn’t necessarily mean you will be immune. The vaccine protects around 97% of vaccinated people after two doses, which means that even in a community where everyone is vaccinated, there will be about 3% of people who are not immune. On top of this, there are people either too young to be vaccinated or who for medical reasons cannot be vaccinated, or who are not immune because of a suppressed immune system. This means that maintaining herd immunity to measles really needs everyone who can be vaccinated to be vaccinated, otherwise that critical threshold of immune people cannot be maintained. Because our social and community interactions are complex, it is possible that even in a country or region with a high vaccination rate, there may be clusters and smaller communities where the vaccination rate is much lower. Communities where the overall vaccination rate drops below the 90% threshold are at risk of measles outbreaks, and it is these pockets where recent measles outbreaks have occurred in Australia, Europe and the US.

Now let’s look at another very infectious disease, whooping cough, or pertussis. The number of people in a non-immune population who will catch whooping cough from an infected person is similar to measles – between 12 and 18 people, and the critical immune threshold is also similar to measles – around 90-95% of the population need to be immune to prevent the spread of pertussis. As I mentioned in my post on whooping cough, the whooping cough vaccine is less effective than the measles vaccine, with only around 80-85% of vaccinated people fully immune to whooping cough, and whooping cough immunity (either from vaccination or from natural infection) wanes over time. So even in a fully vaccinated population, around 20% of people will not be fully immune to whooping cough. In this case, until we get a better vaccine, vaccination is never going to be able to completely stop the transmission of whooping cough – the 90-95% threshold of immune individuals will never be reached. It is possible, however, to maximise the chance that vaccination will provide protection, and this is through getting regular boosters (around every 10 years), especially when either planning pregnancy or in the third trimester of pregnancy or when you know that you are going to come into close contact with babies (for example, non-gestational parents, childcare workers, health professionals, grandparents etc). Because whooping cough is so difficult to get rid of, we need to take particular care to make sure that babies, the most vulnerable victims of whooping cough infection, are protected by a bubble of immune people around them.

Herd immunity is a wonderful by-product of successful vaccination campaigns. Even though an individual can only be 100% protected either by vaccination or prior disease, herd immunity can protect young babies and other people in our communities who are unable to be vaccinated. In the case of highly infectious diseases like measles and whooping cough, we really need everyone who can be vaccinated to be vaccinated to maximise the development of herd immunity. It is a community service that pretty much everyone can contribute to.

Some additional links:

A very clear argument for 100% measles vaccination coverage: http://theconversation.com/herd-immunity-and-measles-why-we-should-aim-for-100-vaccination-coverage-36868

A nice explainer about herd immunity http://theconversation.com/braving-the-jab-for-community-immunity-330

An accessible piece on herd immunity, including a video animation http://www.ovg.ox.ac.uk/herd-immunity

Another herd immunity animation http://www.historyofvaccines.org/content/herd-immunity-0