How vaccines work

Sometimes our immune systems need help to fight diseases. Vaccines train the immune system to produce antibodies that protect us from getting sick.

Vaccines train your body to recognise and fight off disease

Diseases can be caused by viruses and bacteria. These are so small that you can’t see them, but they are everywhere. Most are harmless, but some can make you really sick.

Inside your body, your immune system helps fight against diseases caused by viruses and bacteria. Sometimes, though, your immune system needs a little help. Vaccines give your immune system instructions on how to defend itself better so your body will have the right tools for the job, meaning you’re less likely to get sick.

Extra doses of a vaccine remind your body what to do

To be fully protected you usually need more than 1 dose of a vaccine.

When you’re first vaccinated, your body learns what to do if it meets a particular virus or bacteria. Your second (and sometimes third or fourth) dose boost your immune system so you will have stronger, and longer-lasting, protection.

Different vaccines protect you for different lengths of time, which is why you may need a booster vaccination to strengthen your immunity.

Some vaccines protect against more than 1 disease in a single vaccine

Some vaccines provide protection against more than 1 disease in a single vaccine. For example the measles, mumps, and rubella (MMR) vaccine is just 1 injection. This means fewer vaccination appointments and fewer injections.

It’s not always possible to have a different vaccine if you want protection against only one of the diseases. Your immune system is used to dealing with thousands of viruses and bacteria every day – so there are no safety concerns with having multiple vaccines at the same time.

There are 4 different types of vaccine

Live vaccines

These contain just enough of the bacteria or virus to stimulate an immune response. The bacteria or virus have been weakened so that they can't cause disease.

Live vaccines include the:

  • measles, mumps and rubella (MMR) vaccine
  • rotavirus vaccine
  • chickenpox vaccine

Live vaccines have been used in New Zealand for over 50 years, including measles-containing vaccines since 1969.

Inactivated vaccines

These contain bacteria or viruses that have been killed or inactivated. With inactivated vaccines, you usually need more doses to give full protection against disease.

Inactivated vaccines include:

  • the polio vaccine
  • some flu vaccines
  • the hep A vaccine

Subunit vaccines

These contain parts of bacteria or viruses that have been made harmless.

Subunit vaccines include the:

mRNA vaccines

The Pfizer COVID-19 vaccine is a type of mRNA vaccine. It does not contain any virus. Instead, it contains a molecule called mRNA that has instructions for making a protein on the surface of the COVID virus.

The mRNA from the vaccine does not stay in your body, but is broken down shortly after vaccination.

Learn more about different types of vaccines – Health Navigator


Before any vaccine is approved for use, it goes through a long and rigorous testing process by scientists around the world and in New Zealand to ensure its safety and effectiveness.

This process can take several years and compares the health of people who have been immunised with those who haven’t.

Once approved, the safety of the vaccine is also continuously monitored by Medsafe. As part of this process, the Centre for Adverse Reactions Monitoring at Otago University records reactions reported after vaccinations so that scientists can keep track of any reactions that may occur.

Vaccine side effects, reactions, and safety

Video: Dr Owen Sinclair answers your immunisation questions

Why is it important to immunise your tamariki? 

Immunisation basically is a magic therapy. It stimulates your immune system.

It protects against a wide range of illnesses. 

Now, some of those illnesses have gone essentially in our community because immunisation has been so successful.

Why is it important for your child to get all of their immunisations?

It’s really important to get all of the vaccines and not just individual ones.

So if we look at one of the vaccinations, which is called MMR, that is measles, mumps, and rubella. Now, measles is a very severe disease that can kill a large number of children.

Mumps can also kill little children, but it's uncommon.

Rubella is a disease that we haven't seen for such a long period of time. 

Those two, the measles and the rubella have been extremely successful, and if we stop giving those vaccines there's a risk that it can come back.

If a child misses one of their immunisations, can they catch up?

So if you miss one of your child's vaccinations, it's no big deal. Things happen. We all have busy lives. It's really easy to catch up.

The immunisations, again, they're not medicine. There is no overdose that you can have.

So if for whatever reason your child misses the vaccine just present to whoever you want to go to. Typically general practice and say, "Look, my child needs their vaccines," and they can be caught up.

How safe are childhood immunisations?

Childhood immunisations are probably the safest thing that you can ever give your child.

They're much safer than things like paracetamol or ibuprofen, which is something we always give.

The most common side effects you might have from getting an immunisation are things like some pain and swelling around the injection site and sometimes you get a fever.

There are some extremely rare side effects that are very uncommon that we seldom ever see and massively outweighed by the benefits of the immunisations and the protections they give.