How Vaccines Work

What happens

• The first time your immune system meets a new germ, it can take a week or two to build its defences, while the germ multiplies.
• Antibodies lock onto germs, and memory B and T cells can remember a germ for years, sometimes for life.
• A vaccine works like a wanted poster: a weakened or inactivated germ, a piece of it, or mRNA instructions to make that piece — without the disease.
• A sore arm, tiredness or mild fever are signs of the immune system in training and usually pass in a day or two; serious reactions are rare and monitored.
• When most of a community is immune, germs struggle to spread, protecting newborns and people with weakened immune systems.

What you can do

• Ask your doctor, nurse or pharmacist which vaccines are right for you and your family.
• Keep a vaccine record so you know which vaccines you've had and when the next dose is due.
• Get boosters when they're recommended; they refresh your immune memory.
• Stay at the clinic for a short time after a vaccine if you're asked to, so staff can help right away if needed.
• Being vaccinated also helps protect people who can't be, such as newborns and people having cancer treatment.

Common questions

Are vaccine side effects normal?

Yes. A sore arm, feeling tired or a mild fever are common signs that your immune system is responding, and they usually pass in a day or two. Serious reactions are rare: a severe allergic reaction happens about once in a million doses, and clinic staff are trained to treat it right away.

Can mRNA vaccines change your DNA?

No. mRNA vaccines carry instructions that let your cells make one harmless piece of a germ for a short time. The mRNA stays outside the cell's nucleus, where DNA is kept, and it is broken down within days.

Do vaccines cause autism?

No link has been found. The 1998 study that started the claim was retracted, and large studies following more than a million children, including a Danish study of over 650,000 children, found no link between vaccines and autism.

Transcript

Every day, your body meets countless germs. Most never make you sick, thanks in part to a defence force that can learn: your immune system.

It can learn from a real infection, but that lesson can be costly. A vaccine teaches the same lesson safely, before the germ arrives. Let's see how vaccines work.

Every germ carries its own markers on its surface, a bit like a face. Your immune system learns to recognise these markers. They're called antigens.

The first time a new germ gets in, your immune system has never seen its face. It has to find the right defenders and build up their numbers, and that can take a week or two.

Meanwhile, the germ keeps multiplying. That's why a first infection can make you quite sick, and with some germs, dangerously so.

In time, cells called B cells find the right match and make antibodies: Y-shaped proteins that lock onto the germ and mark it for destruction.

T cells join in. Some direct the response. Others find and destroy the body's own cells that the germ has infected.

Then comes the important part. Once the infection clears, some of those defenders stay on as memory cells, for years, and sometimes for life.

If the same germ comes back, memory cells recognise it straight away. The response is faster and stronger, often stopping the germ before you feel ill.

So what if your immune system could build that memory without getting sick first? That's exactly what a vaccine does.

Think of a vaccine as a wanted poster. It shows your immune system what the germ looks like, without you having to get the disease.

Some vaccines use a weakened or inactivated germ. Others use just a piece of it, such as one of its surface proteins.

And some, like mRNA vaccines, carry instructions that let your cells make that one harmless piece for a short time. The instructions are soon broken down, and they never change your DNA.

Your immune system studies the poster, makes antibodies, and builds memory cells. Protection usually builds over the next couple of weeks.

Some vaccines need more than one dose, or a booster later on. A booster is a reminder that refreshes the memory and keeps protection strong.

After a vaccine, you might have a sore arm, feel tired, or run a mild fever. These are signs of your immune system in training, and they usually pass in a day or two.

Serious reactions are rare. A severe allergic reaction happens about once in a million doses, and clinic staff are trained to treat it right away.

Safety is watched after approval, too. Systems in Canada, the United States and around the world track reported problems and look into them.

Vaccines also protect the people around you. When most of a community is immune, a germ struggles to spread.

That shields people who can't be vaccinated, or who respond less well: newborn babies, and people with weakened immune systems, such as during cancer treatment.

The results have been remarkable. Smallpox, which killed hundreds of millions of people, was declared eradicated in nineteen-eighty, thanks to vaccination.

And the World Health Organization estimates that measles vaccination prevented nearly fifty-nine million deaths between the year two thousand and twenty twenty-four.

You may have heard that vaccines cause autism. It's an understandable worry, so it has been studied very carefully.

The study that started the claim was retracted. And large studies, following more than a million children, have found no link between vaccines and autism.

If you have questions about a vaccine, talk with your doctor, nurse or pharmacist. And keep your vaccine record handy, so you know when you're due.

So, to recap. Germs carry markers. Your immune system learns them and remembers. A vaccine teaches that lesson safely. And boosters keep the memory fresh.

Now you know how vaccines work. They train your immune system, so it's ready before the real germ ever arrives.

Sources

1. WHO — How do vaccines work?
2. CDC — Explaining How Vaccines Work
3. WHO — Measles fact sheet
4. WHO — WHA33.3 Declaration of global eradication of smallpox (1980)
5. Hviid A et al. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Ann Intern Med 2019;170:513–520
6. McNeil MM et al. Risk of anaphylaxis after vaccination in children and adults. J Allergy Clin Immunol 2016;137:868–878
7. Public Health Agency of Canada — Canadian Immunization Guide: Vaccine safety and pharmacovigilance
8. Amanna IJ et al. Duration of humoral immunity to common viral and vaccine antigens. N Engl J Med 2007;357:1903–1915