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To understand how vaccines work, it helps to first look at how the immune system works.
The immune system
The immune system is a network of organs, cells, and tissues that work together to protect the body against disease.
When a disease-causing germ, like a virus or bacteria, enters the body, the immune system responds by making proteins called antibodies. These antibodies work with the rest of the immune system to destroy the germ.
The first time the body gets exposed to a germ, it takes time for the immune system to respond and produce antibodies. In the meantime, a person could become sick. But after the first encounter, the immune system usually remembers the germ and how to fight it. Then, if the body is exposed to the same germ again, the immune system can quickly respond and destroy the germ before it makes a person sick.
How vaccines work
Vaccines teach the immune system how to recognize and fight off specific disease-causing germs. They help protect you against disease without the risk of getting sick first.
Vaccines contain killed, weakened, or synthetically manufactured versions of the disease-causing germ or parts of the germ called antigens. Some newer vaccines (e.g., COVID-19 mRNA vaccines) contain instructions for producing antigens rather than the antigen itself.
When a person gets a vaccine, the immune system responds to the antigen as if it were exposed to the actual germ (it makes antibodies and remembers how to defeat it). Then, if the body gets exposed to the actual germ, the immune system can recognize it right away and quickly fight it off to prevent disease.
While most vaccines are highly effective in preventing disease, no vaccine is 100% effective. Not all people get full protection after immunization. But those who get a disease after being immunized are less likely to become seriously ill.
It's much safer to get a vaccine than to get the disease it prevents.
How vaccines work
Video courtesy of Health Canada. All contents may not be reproduced without permission and are copyright of Her Majesty the Queen in Right of Canada, Represented by the Minister of Health, 2012.