Herd immunity: How vaccines protect the most vulnerable

Chris Pascoe
11 February 2015

Above: Image © istockphoto.com/johavel

You have probably heard about the dangers associated with falling vaccination rates in North America. But if vaccines are safe and effective for individuals, does it really matter if some people decide not to get themselves or their children vaccinated? If everyone else is vaccinated, why do they need to be?

Not everyone can be vaccinated

Did you know? Mary Mallon, known as Typhoid Mary, was an asymptomatic carrier of typhoid fever who was responsible for transmitting the disease to more than 50 people in New York between 1906 and 1915. The short answer is that not everyone can be vaccinated. So while outbreaks of diseases like measles may not seem like a big deal to the majority of the public who are healthy and have been vaccinated, they can be a huge threat to people who cannot be vaccinated due to weak immune systems.

Immunocompromised groups include people with HIV/AIDS, cancer patients, and transplant recipients. A weakened immune system also makes these same groups more vulnerable to the diseases that the vaccines protect against. Likewise, newborns’ immature immune systems puts them at high risk for infection and makes most vaccines ineffective. Most children receive their first vaccinations when they are about two months old.

Shielding the population from pathogens

The importance of keeping vaccination rates as high as possible is more fully explained by the concept of herd immunity. This refers to the ability of vaccines to protect even those who have not been vaccinated against a specific pathogen. Herd immunity even protects individuals who cannot be vaccinated by decreasing the chance that they come in contact with someone who is infected.

A pathogen is any virus or bacteria that can infect you and make you sick. Over the years, vaccines have been developed to protect against many pathogens, including the measles, smallpox and influenza viruses, as well as the bacteria that causes tetanus.

When no one in a population is immunized against a new pathogen, it can spread throughout the population, leading to an epidemic. An epidemic is defined as the rapid spread of an infection in a population of people in a relatively short time.

Now, if only a few people are immunized, they will be protected against infection, but the rest of the population will remain susceptible. The pathogen still has plenty of people to reproduce in, so it can continue spread. In particular, viruses cannot survive without a host to reproduce in, because viruses need to hijack host cells to be able to make copies of themselves. No host, no viruses.

However, when the majority of the population is immunized, the pathogen has a limited number of potential hosts. This inhibits its ability to spread and provides some protection to those who are not vaccinated.

Keeping vaccination rates high

Did you know? Vaccination rates required to achieve herd immunity can vary depending on the disease. They range from 75% for mumps to 94% for whooping cough. You can think of herd immunity as a group of people carrying shields. The pathogen is looking to attack someone without a shield but has a hard time finding one because there are so many people with shields hiding those who don’t have shields. However, if large numbers of people suddenly decide to stop carrying a shield, it makes it easier for the pathogen to find those who are most vulnerable.

Herd immunity only works if vaccination rates remain high. For example, in the case of measles, herd immunity requires a vaccination rate of between 83% and 94%. A case of herd immunity failing because of decreasing vaccination rates occurred in 2014, where more than 200 people (mostly school children) contracted measles in British Columbia.

When herd immunity fails, people can fall seriously ill from infections that are normally extremely rare or that could be eradicated if vaccination rates remained high. And if you are an asymptomatic carrier—someone infected with a pathogen who doesn’t actually show any symptoms—you could pass the infection on to someone without even knowing it.

If you are vaccinated, the pathogen cannot normally catch a free ride in you to infect others. However, vaccinations are not 100% effective and you may still get sick, usually when the virus that is being vaccinated against comes in multiple strains, as in the case of influenza.

Scientific studies have consistently shown that vaccines are very safe and there is no credible evidence to the contrary. Falling vaccination rates affect everyone, not just people who choose not to have themselves or their children vaccinated. And when herd immunity fails, people with weak or compromised immune system face the greatest risk.

Learn more!

B.C. measles outbreak reveals vulnerability of unvaccinated children (2014)
Janet Davison, CBC News Health

Report on a measles outbreak in British Columbia involving more than 200 cases.

Community Immunity ("Herd Immunity") (2013)
US Department of Health and Human Services

Illustration of the relationship between vaccination rates and herd immunity.

Scientists get a handle on what made Typhoid Mary's infectious microbes tick (2013)
Bruce Goldman, Stanford Medical News Centre

News release on research into how salmonella spreads typhoid fever and food poisoning.

Vaccine safety: Frequently asked questions (2012)
Public Health Agency of Canada

Answers to common questions about vaccine safety.


Chris Pascoe

I recently graduated with my PhD in Experimental Medicine from UBC. My research focused on asthma and airway function. I am currently working as a research associate where I am continuing my research. I love to translate scientific research for everyone to understand.

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