How vaccines work

Vaccines contain either a greatly weakened form of the bacterium or virus that causes a disease, or a small part of it. When the body detects the contents of the vaccine its immune system will be primed to make the antibodies (substances that fight off infection and disease) needed to fight off the infection.

If your child comes into contact with the infection, the immune system will recognise it and be ready to protect him or her by producing the right antibodies.

Because vaccines have been used so successfully in the UK, diseases such as diphtheria have almost disappeared from this country.

How do vaccines work?

Watch a short animation that explains how vaccines help the body make the right sort of antibodies to fight a particular disease.

Vaccine safety

Before a vaccine is put into general use it has to be licensed. In order to be granted a licence, the manufacturers have to demonstrate its quality, safety and efficacy in preventing the particular disease for which it’s intended.

Years of extensive laboratory and clinical tests are undertaken during a vaccine’s development. Before clinical trials can be started, the Medicines and Healthcare products Regulatory Agency (MHRA) and Ethics Review Committees have to give their approval.


Once introduced, vaccines are constantly monitored so that any new side effects are quickly noticed and investigated.

The MHRA monitors the safety of vaccines through the Yellow Card Scheme. The information collected through the Yellow Card Scheme is constantly reviewed by independent experts and used to inform national immunisation policy decisions.

Where can I report suspected side effects?

You can report suspected side effects of vaccines and medicines through the Yellow Card Scheme.

This can be done by:

  • visiting the Yellow Card Scheme website
  • phoning the free Yellow Card hotline on 0800 731 6789 (available Monday to Friday, 9am to 5pm)

Vaccine Safety Net Member

Public Health Scotland is a proud member of the Vaccine Safety Net and partners with NHS inform to provide reliable information on vaccine safety.

The Vaccine Safety Net is a global network of websites, evaluated by the World Health Organization, that provides reliable information on vaccine safety.

More about the Vaccine Safety Net

Last updated:
03 June 2024

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