Vaccines have to be thoroughly tested for safety before they're made routinely available.

Each vaccine's safety is continually monitored, even after it's been introduced. This is because not all side effects are picked up during the vaccine's development, especially if they're very rare.

Vaccine ingredients

Vaccines need additives to improve the way they work, increase their shelf life and make them as safe and effective as possible. The three main substances added to vaccines are:

• adjuvants or enhancers – to make the vaccine more effective
• stabilisers – to stop the vaccine deteriorating when it is exposed to changes in the environment, such as light and temperature
• preservatives – to increase the vaccine's shelf life

Find more detailed information about vaccine ingredients.

Vaccine side effects

The Medicines and Healthcare products Regulatory Agency (MHRA) is the organisation responsible for checking the safety of vaccines that are in general use.

The MHRA collects information on vaccine safety via the Yellow Card Scheme, where anyone can report a suspected side effect to the MHRA.

Find more information on how to report a vaccine's side effect.

The MHRA also uses other sources of safety information, including medical literature, safety studies done by vaccine makers, databases that track trends in illnesses and other worldwide organisations.

This NHS leaflet tells you about the common side effects of vaccinations in babies and children up to 5 years of age.

Now, read about how vaccines work.