Depending on the disease-causing agent, on how it infects the cell and on how the immune system responds to it, scientists decide the best approach to design a vaccine. The following are the main type of options that currently exist.
1. Live-attenuated (weakened) vaccines
These vaccines contain modifed strains of a pathogen (bacteria or viruses) that have been weakened but are able to multiply within the body and remain antigenic enough to induce a strong immune response. The varicella-zoster vaccine, oral poliovirus (OPV) vaccine, or yellow fever virus vaccine are some examples of this type of vaccine.
Heterologous vaccines are a sub-group of live attenuated vaccines produced from strains that are pathogenic in animals but not in humans. The only example known to date is the cowpox virus that protects against smallpox in humans.
2. Killed-inactivated vaccines
To produce this type of vaccines, bacteria or viruses are killed or inactivated by a chemical treatment or heat. This group includes for example the inactivated poliovirus (IPV) vaccine, pertussis vaccine, rabies vaccine, or hepatitis A virus vaccine.
3. Sub-unit vaccines
This type of vaccines contain a very small part of a microorganism (bacterial or viral), selected for its ability to initiate a specifc immune response, which is then isolated and purifed. This method is used for the Haemophilus influenzae type b vaccine and the acellular pertussis vaccine.
Toxoids are an important sub-group of sub-unit vaccines that are made from the micro-organism inactivated toxic compounds. An example of this type of vaccines is the diphtheria toxoid vaccine that contains a chemically modifed bacterial toxin that has retained its immunogenic properties. When the immune system is infected by the pathogen, the formation of antibodies that neutralise the corresponding bacterial toxin is stimulated.