What is a vaccine?
Vaccination prepares the immune system to fight against a future infection. Vaccines often contain tiny amounts of dead or weakened viruses or bacteria, called antigens. The immune system responds to these antigens without you getting sick, effectively training the immune system to fight the disease if exposed to it in the future.
Some vaccines need to be given more than once — known as ‘booster’ vaccinations. Some vaccines, such as the seasonal influenza (flu) vaccine, only work for a short period of time. This is because the virus itself can change over time.
Vaccines are usually given with an injection.
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 killed bacteria or small parts of bacteria, and other vaccines contain a small dose of a modified toxin produced by bacteria.
Vaccines may also contain either a small amount of preservative or a small amount of an antibiotic to preserve the vaccine. Some vaccines may also contain a small amount of an aluminium salt, which helps produce a better immune response.
How does immunisation work?
The terms ‘vaccination’ and ‘immunisation’ don’t mean quite the same thing. Vaccination is the term used for getting a vaccine — that is, actually getting the injection or taking an oral vaccine dose. Immunisation refers to the process of both getting the vaccine and becoming immune to the disease following vaccination.
All forms of immunisation work in the same way. When someone is injected with a vaccine, their body produces an immune response in the same way it would following exposure to a disease but without the person getting the disease. If the person comes in contact with the disease in the future, the body is able to make an immune response fast enough to prevent the person developing the disease or developing a severe case of the disease.
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 provide 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?
In general, the normal immune response takes approximately 2 weeks to work. This means protection from an infection will not occur immediately after immunisation. Most immunisations need to be given several times to build long-lasting protection.
A child who has been given only 1 or 2 doses of the DTPa vaccine is only partially protected against diphtheria, tetanus and pertussis (whooping cough) and may become sick if exposed to these diseases until they have all the doses they need. However, some of the new vaccines, such as the meningococcal ACWY vaccine, 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 a booster dose may be given. Some immunisations, such as whooping cough vaccine, give protection for about 5 years after a full course. Influenza immunisation is needed every year due to frequent changes to the type of flu virus in the community.
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. Measles, mumps, rubella, tetanus, polio, hepatitis B and haemophilus influenzae type b (Hib) vaccines protect more than 95% of children who have completed the course. One dose of meningococcal ACWY vaccine at 12 months protects over 90% of children.
Three doses of whooping cough vaccine protect about 85% of children who have been immunised, and will reduce the severity of the disease in the other 15% if they do catch whooping cough. Booster doses are needed because immunity decreases over time.
Some vaccinations, like for COVID-19, won’t necessarily prevent you from catching the disease, but can reduce the risk of serious illness.
How are vaccines approved in Australia?
Before a vaccine becomes available in Australia, it must pass the rigorous 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 in which the trials were run. The TGA also checks that the trials involved enough human participants that represented the people for whom the vaccine is intended.
The TGA ensures that vaccine manufacturers meet manufacturing quality standards. TGA laboratories assess the quality of every batch of a vaccine before it can be supplied in Australia.
Sometimes a ‘provisional approval pathway’ is needed for the temporary registration of promising new medicines and vaccines — where the need for early access outweighs any 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).
Learn more here about the development and quality assurance of healthdirect content.
Last reviewed: May 2021