Guidance

Vaccine safety – patient factsheet

Published 2 December 2025

Understanding vaccines 

• A vaccine is a medicine which is administered to protect against a specific disease. 

• Vaccines are mainly given through an injection but can also be given orally (by mouth) or sprayed into the nose.  

Uses of vaccines 

• Vaccines are a simple and effective way to help protect you against harmful diseases caused by bacteria or viruses such as flu, measles, and COVID-19. 

What is in vaccines 

• Vaccines contain a safe version or part of a virus or bacteria. 

• This could be DNA, mRNA or a protein, or a weakened or inactivated form of the virus or bacteria that cannot cause the disease itself. 

• Some vaccines contain substances called adjuvants that boost the body’s immune response to the vaccine. A commonly used vaccine adjuvant is aluminium.  

• A recent large-scale study did not find evidence to support an increased risk for neurodevelopmental disorders including autism, with early childhood exposure to aluminium containing vaccines. 

• Some vaccines contain a preservative called thiomersal. There is no evidence to suggest it causes harm or is linked to neurodevelopment disorders, including autism. 

• In the UK, as a precaution thiomersal is not used in any vaccines within the current routine immunisation schedule for adults, children, or during pregnancy, including for children in specific clinical risk groups. 

Types of vaccines 

• Inactivated vaccines contain viruses or bacteria that have been killed and cannot cause disease.   
• Live-attenuated vaccines contain a weakened form of the virus or bacteria that cannot cause serious disease.    
• Subunit, recombinant, or conjugate vaccines contain only fragments of the virus or bacteria, such as proteins that trigger an immune response. 
• Toxoid vaccines contain inactivated toxins that are produced by a pathogen. These vaccines are important as some diseases are caused by the toxins the pathogen produces, and not the pathogen itself.  
• Viral vector vaccines contain a modified, harmless virus (called a vector) that delivers genetic instructions to your cells to make proteins, triggering an immune response.  
• Messenger RNA (mRNA) vaccines contain a piece of mRNA that give your cells instructions on how to make a small, harmless part of a protein found in a virus or bacteria. Your immune system recognises these proteins and produces antibodies to fight them off.    

How vaccines work 

• Vaccines work by training your body’s immune system to recognise specific infections. 
• When a vaccine is given, a small, harmless part of the virus or bacteria is introduced into the body.  
• Your immune system recognises the harmless component and responds by producing protective proteins, called antibodies, that learn how to fight off the infection if encountered in the future. 
• This can protect against diseases altogether or make their symptoms much milder. 

Why vaccines are important 

• Vaccines are proven to protect you against serious illnesses. 
• Getting vaccinated is one of the most effective ways to protect yourself and others from harmful diseases. 
• Vaccines help to prevent the spread of harmful bacteria and viruses which can make you seriously unwell. 

How vaccines are made 

• Before any vaccine is made available for public use, it goes through a rigorous development process to test its safety, quality and efficacy. 
• This includes laboratory research, safety testing, and clinical trials. 
• Vaccine clinical trials are controlled scientific studies that test for the safety and efficacy of a vaccine in protecting against diseases before it is approved for public use. 
• Clinical trials typically involve multiple testing phases involving people of different ages, genders and ethnic backgrounds to ensure the vaccine is safe and effective for everyone. 
• Vaccines are batch tested to ensure that each batch of products meets the quality standard defined by their marketing authorisations (product licences). 
• All vaccines used in the UK go through robust monitoring and surveillance. 
• Regulatory bodies like the MHRA regularly track and review adverse drug reaction reports through the Yellow Card scheme to ensure vaccines remain safe throughout their use. 

Vaccine side effects 

• As with every medicine, it is very important to familiarise yourself with the information provided in the product information sheet. 
• Vaccination symptoms are normally mild and last less than a week. 
• Any medicine, including vaccines, can cause an unwanted side effect. 
• Some very common side effects of vaccination include: 
  • general aches, or mild flu-like symptoms 
  • pain and tenderness in the arm where you had your injection which could last for 2 to 3 days 
  • headaches 
  • feeling tired 
• Vaccines are regularly assessed to ensure the benefits of the vaccine in preventing disease far outweigh the risks of known side effects in patients who receive them. 

Report side effects to the Yellow Card scheme 

• You should report any suspected side effects of vaccination to the MHRA via a Yellow Card to: 
  • the Yellow Card website 
  • the Yellow Card app; download from the Apple App Store  or  Google Play Store 
• Reports help us gain a better understanding of vaccine interactions and safeguard patients through vigilant monitoring. 

Vaccines are one of the safest and most effective ways to protect yourself and others from harmful diseases. Vaccines are rigorously tested before being approved for use and are continuously monitored to ensure their safety and effectiveness. By getting vaccinated, you help to reduce the spread of infections and protect yourself and others from serious diseases. 

Glossary of terms: 

Antibody: protective proteins produced by the immune system in response to foreign infectious substances called antigens, such as bacteria or viruses. Antibodies recognise and attach to antigens in order to remove them from the body. 

Antigen: a foreign or harmful substance that can trigger an immune response in the body. 

Immune system: the body’s natural defence system that helps to fight off infections and diseases. 

Messenger RNA (mRNA): a type of genetic material that gives cells instructions on how to make proteins. 

Pathogen: a microorganism that causes a disease such as viruses, bacteria, fungi or protists.