How vaccines work
Vaccines work by teaching the immune system how to fight off a disease in case it ever comes into contact with it. This greatly reduces the risk of becoming seriously ill or spreading a disease to others. Vaccines can protect against one or multiple diseases. Sometimes, multiple vaccines may be given at once to protect against several diseases.
Vaccines can protect against either one or multiple diseases. Sometimes, multiple vaccines may be given at once to protect against several diseases.
Most vaccines contain a weakened or an inactivated form of a virus or bacterium, or a small part of it, called an antigen.
When a person gets vaccinated, their immune system recognises the antigen as foreign. This activates immune cells to produce antibodies and create a memory of the virus or bacterium.
Later, if the person comes into contact with the actual virus or bacterium, their immune system will remember it, and then produce the right antibodies and activate the right immune cells quickly, to kill the virus or bacterium. This protects the person from the disease.
In contrast, people who become immune by getting the actual disease can spread it to others and put themselves at risk of serious complications from the disease.
Protection
Different vaccines bring about different levels of protection. The duration of protection varies depending on the disease. Some vaccines can only protect against a disease for a short period and may need booster doses; for others, immunity can last a lifetime.
Vaccination not only protects people who have received a vaccine. It also indirectly protects unvaccinated people in the community, by decreasing the risk of exposure to infection. This is known as community immunity (also called herd immunity).
Components
In addition to one or more antigens, other components can also be added to help keep the vaccine stable and effective. Regulators make sure that all these components are safe.
These components include:
- stabilisers: to keep vaccine components stable;
- adjuvants: these improve the immune response to the vaccine by making the response stronger, faster and more sustained over time – an example of which is aluminium;
- excipients: these are inactive ingredients, like water, or sodium chloride (salt), as well as preservatives or stabilisers that help the vaccine remain unchanged during storage, keeping it active.
In some vaccines, there may also be trace amounts of other substances used in the manufacturing process, such as ovalbumin (a protein found in eggs) or neomycin (an antibiotic). If these substances might cause a reaction in sensitive or allergic individuals, their presence is declared in the information given to healthcare workers and patients.
Types of vaccines
mRNA vaccines contain a molecule called messenger ribonucleic acid (mRNA) that delivers instructions on how to fight a disease to our cells.
Once the mRNA in the vaccine enters our cells, it provides instructions to make a harmless protein that matches part of the virus the vaccine is designed to protect against.
Because mRNA vaccines deliver instructions to make a small piece of a virus, they cannot cause illness.
After our cells begin making the protein, our immune system recognises it as foreign, activating immune cells and creating antibodies. This process trains our immune system to fight off viruses that contain that protein, meaning that if the body encounters the real virus, it is better able to fight off the infection.
Viral vector vaccines contain a harmless virus that delivers a small section of the genetic code of a disease-causing virus to our cells. While this code cannot make us sick, it is able to trigger an immune response, teaching our system how to fight off a disease.
Viral vector vaccines are able to trigger a strong immune response, meaning they can offer a high level of protection against infection or severe disease.
Inactivated vaccines contain viruses that have been inactivated (killed) in a lab using heat or chemicals.
Inactivated viruses cannot reproduce themselves or cause illness but can still produce an immune response in the body. When a person receives an inactivated vaccine, their immune system identifies the inactive viruses as foreign. This teaches the body to produce antibodies to fight the viruses off.
Live attenuated vaccines contain live viruses or bacteria that have been weakened by changing their DNA or by selecting weakest viruses or bacteria to include in the vaccine.
Weakened viruses and bacteria in live attenuated vaccines cannot cause disease but they can still produce an immune response in the body. When a person receives a live attenuated vaccine, the immune system identifies the weakened bacteria or viruses as foreign. This teaches the body to produce antibodies to fight off the bacteria or viruses.
Live attenuated vaccines produce a strong immune response that can last a long time. This means that fewer doses may be needed than for other types of vaccine.
Common live attenuated vaccines are the measles, mumps and Rubella (MMR) vaccine and the chickenpox vaccine.
The first vaccine ever developed was a live attenuated vaccine. This was a vaccine against smallpox, developed in 1798. Live attenuated vaccine technology is still used today in modern vaccines such as those for measles, chickenpox and yellow fever.
Toxoid vaccines contain toxins that have been inactivated and are therefore no longer toxic.
Toxins are chemicals produced by bacteria and that can cause some diseases, such as tetanus and diphtheria.
Toxoids are made in the lab by deactivating disease-causing toxins, using chemicals or heat.
Inactivated toxins in toxoid vaccines cannot cause illness but they can still produce an immune response in the body. When a person receives a toxoid vaccine, the immune system targets the deactivated toxins and learns how to neutralise them. This teaches the body how to deactivate the toxins and prevent disease in future.
Toxoid vaccines teach the body to combat the toxins produced by the bacteria rather than fight off the bacteria themselves.
Toxoid vaccines often include an adjuvant to make the response stronger.
Common toxoid vaccines are the ones used against tetanus and diphtheria.
Approval of vaccines in the EU
Find out how authorities in Europe test to make sure vaccines are safe and effective before they are approved for use.
Benefits of vaccination
How do vaccines protect us and stop the spread of disease? Find out their benefits for individuals and the community.
COVID-19 medicines
European Medicines Agency - Safety of COVID-19 vaccines