Live attenuated vaccines
These vaccines contain the entire pathogen (disease-causing agent) that has been weakened. The advantage of a live vaccine is that it mimics the natural infection, which produces a strong immune response, but without the disease symptoms. Vaccination with a live vaccine often provides lifelong protection. Live attenuated vaccines are not suitable for people with weakened immune systems, as they can still cause disease in rare cases. These vaccines are also not recommended for pregnant women.
Inactivated (dead) vaccines
These vaccines also contain the full pathogen, but in an inactivated (dead) form. They do not cause disease symptoms and are completely safe for people with weakened immune systems. Because the immune response is usually less strong than with live attenuated vaccines, repeat doses (boosters) are needed to ensure long-term protection.
Subunit vaccines
Instead of the entire pathogen, these vaccines contain only specific parts of it, such as proteins (protein subunits or virus-like particles), sugar derivatives (polysaccharides) or toxins of the pathogen (e.g. anatoxin of tetanus or diphtheria). These vaccines therefore only contain the antigens necessary to induce a protective immune response.
The advantage of this type of vaccine is that they are non-infectious and usually cause few side effects. Because the immune response is lower than with live attenuated vaccines, adjuvants are often added. Multiple doses are required for long-term protection.
Viral vector vaccines
These vaccines use a harmless virus (the viral vector) to introduce genetic material from another virus (from the disease which the vaccine protects against) into the body. In the cells of the vaccinated person, the viral vector produces a protein of the virus against which the person wants to protect him or herself. The viral vector teaches the immune system to recognise the virus and prepares to fight it.
mRNA vaccines
mRNA vaccines contain the genetic code (mRNA) of the antigen, against which an immune reaction is induced. The mRNA is packed in fat globules (lipids) which allow it to enter cells. Once inside the cell, the mRNA is used to make the viral protein, which then triggers an immune response. Defence cells in the body recognise these proteins as foreign to the body, and produce antibodies. The mRNA never reaches the cell nucleus and therefore cannot affect the individual's DNA. One advantage of mRNA vaccines is that they can be developed and produced quickly. Immunity does however appear to be of shorter duration than with the 'conventional' vaccines.