Vaccines are first tested on animals. Then, if a vaccine is found to be safe in animal trials, it is evaluated in humans in three phases of clinical trials:

• Phase 1 trials: The new vaccine is given to a small number (25-50) of healthy adults with the primary aim of assessing safety.
• Phase 2 trials: If the new vaccine is found to be safe in Phase 1, it is then given to hundreds of people to determine how effectively it stimulates immune responses; how much or how many doses need to be given in order to protect against the target disease; and whether there are any side effects.
• Phase 3 trials: If the vaccine is found to be effective and safe, it is then given to many thousands of people to test whether it protects large populations from the target disease and check if there are any uncommon or serious side effects.

In Australia, every vaccine must pass all of those phases before it is registered for use by the Therapeutic Goods Administration (TGA).

Some side effects are so rare they can’t be detected even after studying the effects of a vaccine on tens of thousands of people. For that reason, the safety of vaccines continues to be monitored even after they are in routine use.

Healthcare providers and parents are encouraged to report possible vaccine side effects to the vaccine safety service in their state. If needed and where possible, people who have had a reaction to a vaccine will be offered clinical review at a specialist immunisation clinic or with an immunisation expert. Reports of possible vaccine side effects are reviewed carefully and may signal the need for further investigation and studies at different times.

In addition, large populations of people who receive vaccines are often studied to identify any possible links between vaccination and rare or serious health conditions.

Led by the National Centre for Immunisation Research and Surveillance (NCIRS) and funded by the Australian Department of Health and Aged Care (DHAC), AusVaxSafety is a national, collaborative, active vaccine safety surveillance initiative. AusVaxSafety monitors all adverse events following vaccination in Australia, and investigates those that are unusual or unexpected, to ensure continuing vaccine safety.

Any suspected problem with a vaccine is thoroughly investigated by the Therapeutic Goods Administration (TGA). These investigations involve other health authorities including state and federal governments and medical experts. Investigations usually involve more tests of vaccine quality and manufacturing processes and studies of vaccinated and unvaccinated people who have and who have not had reactions. 

If a suspected problem could be serious, authorities will consider a range of actions including suspending use of the vaccine during the investigation.