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. All vaccines in general use are continually and carefully monitored, so that any side effects are identified.

Vaccine ingredients

Vaccines need additives to improve the way they work and increase their shelf life (how long they can be kept for). The three main substances added to vaccines are:

• adjuvants or enhancers – to make the vaccine more effective by stimulating your immune system, making it respond quicker and more strongly to the vaccine
• 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 medicines (including vaccines) that have been made available to the public.

The MHRA collects information on vaccine safety via the Yellow Card Scheme, where anyone can report a suspected side effect to the MHRA. For more information, read how to report a vaccine's side effect.

As well as the Yellow Card Scheme, the MHRA uses a variety of other sources of safety information, including medical literature, safety studies done by vaccine makers, databases that track trends in illnesses and other worldwide organisations.

NHS Choices website was assessed by the World Health Organization (WHO) and passed their "credibility and content good information practices criteria". It is included in the WHO's list of vaccine safety websites.

Now, read about how vaccines work.