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Herd Immunity: How Vaccines Protect the Most Vulnerable

Icon showing a syringe on a shield

Icon showing a syringe on a shield (johavel, iStockphoto)

Icon showing a syringe on a shield

Icon showing a syringe on a shield (johavel, iStockphoto)

Chris Pascoe

How does this align with my curriculum?

What is herd immunity and how does it protect some of the most vulnerable people in our communities?

You’ve been told you need to get a vaccination. But if everyone else is getting one, why do you need to get it too? This article answers that question - and more!

Vaccines and Vaccination

Over the years, many vaccines have been developed to protect people against pathogens. A pathogen is a microorganism that can infect you and make you sick. Pathogens include bacteria, viruses, protozoa and fungi. Pathogens cause many infectious diseases like measles, smallpox, influenza, pneumonia and tuberculosis. 


Variety of pathogens including examples of bacteria, viruses, protozoa and fungi
Variety of pathogens including examples of bacteria, viruses, protozoa and fungi (Let’s Talk Science using images by mariaflaya via iStockphoto and lerik78 via iStockphoto).


A vaccine helps activate your body’s immune system to recognize and fight a certain pathogen or group of pathogens. People who receive a vaccination will be protected against the pathogen it was created to fight. People who do not receive a vaccination will not be protected. 

Misconception Alert!

Vaccination means getting a vaccine. Immunization is the process of becoming immune to a disease after a vaccination.

People who do not receive a vaccine can become hosts for pathogens. Some pathogens, like viruses, can’t spread on their own. They need to hijack host cells, like the ones in your body. This is the only way they can make copies of themselves. If there is no unprotected host, there is no virus. So, if most people in a population are immunized, there are fewer hosts for the pathogen. This means it’s more difficult for the pathogen to spread. 

But what if there is no vaccine? This can happen when a new type of pathogen appears. Because it’s so new, scientists have not had time to develop a vaccine for it. In this case, the pathogen, and the infectious disease it causes, can spread through a population

When an infectious disease spreads through a community at a specific time, it’s called an epidemic. When a new disease spreads worldwide it’s called a pandemic. One example of a pandemic is the spread of the infection caused by the COVID-19 virus in 2020.

Did you know?

The COVID-19 coronavirus can survive up to four hours on copper, up to 24 hours on cardboard, and up to two to three days on plastic and stainless steel.

Why doesn’t everyone get vaccinated?

There are many reasons why some people do not get vaccinated. Certain people should not receive vaccinations for medical reasons. And some people choose not to get vaccinated because of their personal beliefs. 

People with weak immune systems do not usually get vaccinated. If someone’s immune system does not work properly, they are called immune deficient or immunocompromised. People who are immunocompromised include people with HIV or AIDS, cancer patients, and people who have received organ transplants

If you are immunocompromised, vaccines will not activate your immune system to respond properly. So, you do not become immune to the diseases that vaccines protect against. Also, live vaccines can be dangerous for immunocompromised people. These vaccines rely on a healthy immune system to combat the weakened virus in the vaccine. If the person receiving the vaccine has a compromised immune system, the weakened virus could replicate and spread, making the person sick.

Newborn babies are too young to be vaccinated. Their immune systems are not fully mature. Most vaccines do not work on them, so they are at high risk for infection. This is why doctors have a specific vaccine schedule for children as they grow up. 

Did you know? 

Some vaccines provide immunity for your whole life. Others need to be repeated over time to work properly.

People who are allergic to ingredients in some vaccines do not receive vaccines with those components. They are not allergic to the active, immunizing agent of the vaccine. But they are allergic to other ingredients, like egg protein or gelatin. These may be used to grow the vaccine, or keep it stable. 

Adjuvants are ingredients that may be added to some vaccines to improve your body's immune response. There is controversy surrounding the use of some adjuvants, like aluminum. But the scientific evidence suggests adjuvants are safe. This is because vaccines contain much less aluminum than we usually ingest every day. 

Some people do not have medical reasons for avoiding vaccines. They may choose not to get vaccinated because of their religious beliefs. Or they may fear that vaccines are not safe. But scientific studies have consistently shown that vaccines are very safe. There is no good evidence against this. 

Herd Immunity

There is a way to protect people who have not received vaccines. It’s called Herd Immunity. This protects unvaccinated individuals by reducing the chance that they will come in contact with people who are infected. 

Did you know? 

Mary Mallon was known as Typhoid Mary. Scientists believe she was responsible for transmitting typhoid fever to more than 50 people in New York between 1906 and 1915.

You can think of herd immunity like a group of people carrying shields. The pathogen is looking to attack someone without a shield. Luckily, a lot of people with shields are protecting those who don’t have them. So, the pathogen has a hard time finding a target. But what if large numbers of people decide to stop carrying a shield? Then it will be a lot easier for the pathogen to find those who are most vulnerable.


Herd immunity is like a group of people carrying shields protecting people without shields
Herd immunity is like a group of people carrying shields protecting people without shields (Let’s Talk Science using an image by alashi via iStockphoto).


Did you know? 

Different vaccination rates are needed to reach herd immunity depending on the disease. They range from 75% for mumps to 94% for whooping cough.

Herd immunity only works if a lot of people in a population get vaccinated. For example, to have herd immunity against measles, 92% to 94% of the population must be vaccinated. 

In British Columbia, in 2014, herd immunity failed. This happened because the vaccination rate fell too low. 343 people contracted measles in British Columbia in 2014. Most of them were school children.

When herd immunity fails, people can get seriously ill from infections that are normally rare. If you are an asymptomatic carrier, you could be infected with a pathogen, but not show any signs of illness. But you could pass the infection on to someone else without even knowing it.

If you are vaccinated, the pathogen cannot normally catch a free ride in your body and infect others. But vaccinations are not 100% effective. You might still get sick. This usually happens when a virus comes in multiple strains. Like influenza for example. The flu vaccine may protect you against some strains, but not all of them.

Repeated scientific studies have shown that vaccines are very safe. There is no credible evidence against this. When vaccination rates fall, everyone is at risk. Not just people who are not vaccinated. 

So, the next time it’s your turn for a vaccination, remember that you’re one of the herd. The more people who get vaccinated, the better herd immunity works. When herd immunity fails, people can get very sick. The most vulnerable people in our communities face the most risk to their lives and their health. So take one for the herd!


<p>What is herd immunity? (2020) by The Royal College of Pathologists (1:19 min.)</p>

Herd Immunity (Herd Protection) (2019)

Explanation of herd immunity, with video (2:46 min.), from the University of Oxford's Vaccine Knowledge Project.

B.C. measles outbreak reveals vulnerability of unvaccinated children (2014) 

Report by Janet Davidson for CBC on a measles outbreak in British Columbia involving more than 200 cases.

Vaccine Safety (2019)

Explanation from Immunize Canada of vaccine safety in Canada, with video (1:52 min.) and links to more sources.

How Did We Get Here? 7 Things to Know About Measles (2019)

Article by Jonathan Lambert for NPR explaining measles, recent outbreaks, vaccine rates and herd immunity.

Scientists get a handle on what made Typhoid Mary's infectious microbes tick (2013) 

News release from Bruce Goldeman from Stanford Medical News Center on research into how salmonella spreads typhoid fever and food poisoning.



BC Centre for Disease Control. (2016). Measles in 2014 in British Columbia, Canada. 

Boskey, E. (2020). What it means to be immunocompromised. Verywell Health.

Centres for Disease Control and Prevention. (2018) What is an adjuvant and why is it added to a vaccine? 

Centres for Disease Control and Prevention. (2012). Principles of epidemiology in public health practice, lesson 1

Children’s Hospital of Philadelphia. (2018). Vaccine ingredients - Aluminum.

Connolly, K. (2020). How Typhoid Mary left a trail of scandal and death. BBC News.

Klappenbach, L. (2018). The basics of population biology. ThoughtCo.