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How vaccination works

By Patricia Nicholson

We now have vaccines that protect us against conditions ranging from the flu to cervical cancer. According to the World Health Organization, vaccines prevent millions of cases of illness, disability and death every year. But many people aren’t sure just how vaccines work, or why they are so important.

A vaccine primes the immune system to make it possible for the body to recognize and fight off an infection later, making someone immune to that infection (that’s why it’s called immunization).

“A vaccination is a preventive health-care intervention. What it does is exposes the body to a very tiny amount of a synthetic or killed version of an infectious disease so that the body mounts an immune response,” says Women’s College Hospital family physician Dr. Danielle Martin. “If the person is ever exposed to the real disease, they will be able to respond to it and not get sick or die from that disease.”

Dr. Martin stresses that in most cases you can’t get the illness that you’re being vaccinated against from the vaccine, which is a common misconception. A vaccine resembles the infectious agent closely enough for the body’s immune system to react to it.

“Your body produces immune system memory cells that make antibodies which then lie in wait, and if the real thing ever comes along then they are marshalled into action,” Dr. Martin explains.

Some people do experience side-effects from vaccines, but most of these are minor, such as tenderness or swelling at the injection site, or mild fever. Serious side-effects from vaccines are very rare.

“The risks of most commonly used vaccines are pretty minimal, and the number of lives saved is enormous,” says Dr. Martin.

However, for vaccines to be most effective, the whole population must be vaccinated.

“It’s critical that everyone be vaccinated,” Dr. Martin says, using the example of smallpox, which has now been eradicated around the globe. “If you vaccinate everybody, over time the disease disappears because it stops propagating itself through the population. Because war was declared on smallpox and we vaccinated the world against it, we now no longer need to use immunization against smallpox because it doesn’t exist any longer. But that only occurs if the entire population is vaccinated.”

Vaccinating entire populations leads to what’s called herd immunity, which protects the occasional person who may not be immune.

“Because everyone around him or her has been immunized, the disease is not circulating and so they’re not likely to be exposed to it,” Dr. Martin explains. However, when vaccination rates drop, herd immunity is lost. “What we’re now seeing in Ontario, for example, is the re-emergence of some diseases that we thought we were essentially rid of, because a growing number of people are choosing not to immunize their children. That herd immunity protection is waning, which is extremely dangerous.”

Unfortunately there are many misconceptions about vaccines, including beliefs that they don’t work, aren’t safe, or that they cause other types of illness. There is no medical evidence to support these beliefs.

“In fact, vaccinations are probably one of the most important contributing factors in health care to improvements in people’s length of life and quality of life in the last several hundred years,” Dr. Martin says.

While most immunizations are completed in childhood, some, such as tetanus, require boosters in adulthood. Flu vaccinations are annual, and must be administered every year in order to be effective.

Dr. Martin stresses the importance of flu shots for women in particular, even if they aren’t necessarily part of a vulnerable group.

“If a middle-aged healthy woman with no other medical problems gets influenza, she’ll be miserable for a couple of weeks, but she probably won’t die from it,” she says. However, many women are caregivers of some kind or another and the people who they take care of can be extremely vulnerable: infants, pregnant women, the elderly, or people with multiple illnesses can die of the flu.

“So the reason to get immunized is not only to protect yourself but also to protect the people around you,” Dr. Martin says. “If you think you are going to be in contact with a vulnerable person then it’s really important to get your flu shot every year. That’s why all health-care providers are required to get flu shots annually so that we don’t pass the flu to vulnerable patients even though we ourselves might be in good health.”



This information is provided by Women’s College Hospital and is not intended to replace the medical advice of your doctor or healthcare provider. Please consult your healthcare provider for advice about a specific medical condition. This document was last reviewed on: Oct. 5, 2012

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