Vaccines: what they are and how they work
A vaccine is a biological preparation developed from killed or weakened forms of a microorganism, its antigens, substances produced by the microorganism and rendered harmless (e.g. tetanus toxoid, obtained by treating tetanus toxin) or genetically-engineered proteins. Vaccines usually contain also sterile water (or a saline solution) and some can include small amounts of: an adjuvant, to enhance the immune response; a preservative (or an antibiotic), to prevent bacterial contamination of the vaccine; stabilizers, to maintain the properties of the vaccine during storage.
There are different types of vaccines:
- Attenuated live vaccines (e.g. measles, rubella, mumps, chickenpox, yellow fever and tuberculosis vaccines): they are developed from infectious agents rendered non-pathogenic
- Inactivated vaccines (e.g. hepatitis A, poliomyelitis and split influenza vaccines): they are developed from viruses or bacteria killed with heat or chemicals
- Purified antigen vaccines (e.g. acellular pertussis, meningococcal and subunit influenza vaccines): for their development, sophisticated techniques are used to purify bacterial or viral components
- Toxoid vaccines (e.g. tetanus and diphtheria vaccines): developed using molecules from the infectious agent that, while unable to cause the disease, can still trigger an immune response
- Recombinant protein vaccines (e.g. hepatitis B and meningococcal B vaccines): developed using recombinant DNA technology, which involves the insertion of genetic material coding for the antigen (a protein or part of it) into microorganisms, which will then produce the antigen, allowing it to be collected and purified.
Read also the dedicated section on COVID-19 vaccines.
How vaccines work
Once administered, a vaccine simulates the first encounter with the infectious agent and elicits an immune response (humoral and cellular immunity) similar to that seen during natural infection, without causing the disease and its complications. This mechanism is based on the principle of immunological memory, i.e. the ability of the immune system to recognize and quickly respond to microorganisms that the body has previously encountered (small children do not have immunological memory and are, therefore, more prone to infectious diseases than adults). Without vaccination, it may take up to two weeks for the body to produce enough antibodies to fight the infectious agent, during which time the microorganism can continue causing damage.
Some vaccines require boosters, that is repeated administrations at specific time intervals.
Vaccination is, by definition, a preventive measure that should be taken before exposure to the infectious agent. In some cases, however, vaccines can be administered after exposure to the agent, a practice known as “post-exposure prophylaxis”. The rabies vaccine, for example, can be administered to individuals who have been bitten or come into contact with a rabid animal: this is because the rabies virus takes some time to reach the nervous system and cause the symptoms of the disease, and during this time the vaccine could trigger an immune response to eliminate the virus before the onset of disease. Measles and chickenpox vaccines can also be effective when used as post-exposure prophylaxis. If given within 72 hours of exposure, the measles vaccine may help prevent the disease or reduce the severity of its symptoms. The studies conducted to date suggest that, if given up to 5 days after exposure, the chickenpox vaccine is also useful to prevent the disease or reduce its severity.
The influenza virus requires separate consideration, as it changes every year, and the World Health Organization (WHO) makes yearly recommendations for the composition of the influenza vaccine, based on the strains that are expected to circulate during the winter months.