What is a vaccine?

Vaccination prepares the immune system to fight against a future infection. Most vaccines contain tiny amounts of dead or weakened viruses or bacteria, called antigens. Your immune system responds to these antigens by training itself to fight disease without you getting sick. This means that your body is better prepared to fight the disease if exposed to it in the future.

Most vaccines are given by injection but some are given orally (by mouth).

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What is in vaccines?

Some vaccines contain a very small dose of a live but weakened form of a virus. Some vaccines contain a very small dose of dead bacteria or small parts of bacteria. Other vaccines contain a small dose of a modified toxin produced by bacteria. Newer vaccines, such as most vaccines against COVID-19, contain instructions that ‘teach’ your immune system how to fight a virus.

Vaccines may also contain either a small amount of preservative or a small amount of an antibiotic to preserve the vaccine. Some vaccines also contain a small amount of an aluminium salt, which helps produce a better immune response.

What is the difference between immunisation and vaccination?

The terms ‘vaccination’ and ‘immunisation’ are similar, but don’t exactly mean the same thing. Vaccination is the term used for getting a vaccine — that is, actually having the injection or taking an oral vaccine dose. Immunisation is the process of both getting the vaccine and becoming immune to the disease after vaccination.

How does immunisation work?

All forms of immunisation work in the same way. When someone receives a vaccine, their body produces an immune response in the same way it would after exposure to a disease, but without the person getting the disease. Your immune system’s familiarity with the disease allows your body to make a faster immune response if you are ever exposed to the disease naturally.

Often, your immune system responds fast enough to prevent you from developing any symptoms of the disease. In other cases, it may not occur fast enough to prevent you from developing symptoms, but vaccination will reduce your chance of becoming seriously ill.

What’s the difference between a booster dose and a primary vaccine course?

A primary vaccine course involves the vaccine doses you need for very good protection against a disease.

A booster dose refers to an extra dose of a vaccine that is given after you’ve completed the primary vaccine course. It gives your immune system a 'boost' and helps reach a higher level of protection from the disease.

Booster doses are common with diseases such as COVID-19, tetanus and whooping cough (pertussis).

How long do immunisations take to work?

It takes time for your immune system to respond to a vaccine, usually between 1 and 3 weeks. The time it takes will depend on the vaccine and your age and general health. This means that immunisation will not usually provide protection from infection straight away.

How many doses of vaccine do I need to be protected?

Most immunisations need to be given several times to build long-lasting protection. For example, a child who has had only 1 or 2 doses of the DTPa vaccine is only partly protected against diphtheria, tetanus and pertussis (whooping cough). Until they receive all the doses they need, they are still at risk of getting sick if they come into contact with these diseases.

There are some newer vaccines, such as one type of meningococcal ACWY vaccine, that provide long-lasting immunity after only one dose.

How long do immunisations last?

The protective effect of immunisations is not always lifelong. Some, like tetanus vaccine, can last up to 10 years (depending on your age), after which time you will need a booster dose to restore your immunity.

Some immunisations, such as the whooping cough vaccine, give protection for about 5 years after a full course. You need influenza immunisation every year due to the changes to the type of flu virus that are most common in the community from year to year.

Is everyone protected from disease by immunisation?

Even when all the doses of a vaccine have been given, not everyone is protected against the disease. Most childhood vaccines will give protective immunity to about 9 out of 10 children who have received all the recommended doses.

When vaccination rates in the community are high, the risk of catching vaccine-preventable diseases is low for everyone. This is called herd immunity. This is because immunisation makes these diseases less common in the community, so even someone without immunity is less likely to catch them.

Some vaccines, such as for COVID-19, won’t necessarily prevent you from catching the disease, but can reduce your risk of serious illness.

Other vaccines, such as for whooping cough, prevent disease in most people (about 8 in 10). These vaccines are still important for everyone, as they reduce the chance of you becoming seriously unwell, in the event that you do catch the disease.

How are vaccines approved in Australia?

Before a vaccine becomes available in Australia, it must pass the strict approval processes of the Therapeutic Goods Administration (TGA). This includes assessing every ingredient in the vaccine for safety, quality and effectiveness.

A clinical trial is a scientific study conducted by the makers of a vaccine. Clinical trials of medicines are done in phases.

The TGA carefully assesses the results of clinical trials, as well as the way the trials were run.

The TGA ensures that vaccine manufacturers meet manufacturing quality standards. TGA laboratories check the quality of every batch of a vaccine before it can be used in Australia.

Sometimes a ‘provisional approval pathway’ is needed for the temporary registration of promising new medicines and vaccines. This is only done where the need for early access is greater than any possible risks.

Resources and support

In Australia, vaccines are funded by the National Immunisation Program and protect millions of Australians from vaccine-preventable diseases.

If you have any questions, you can speak to your doctor or call healthdirect on 1800 022 222 (known as NURSE-ON-CALL in Victoria).

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