Vaccines are pathogens or part of pathogens injected into the body in order to protect the body from those pathogens. The vaccines work by stimulating the immune system of the body to produce cells and antibodies to fight these injected pathogens so that when the body is infected with such a pathogen in the future, the cells and antibodies that have been produced remember the pathogen and are able to fight it more easily.
You can understand the principle of vaccination by the analogy of training a soldier by taking him to a dummy battlefield. After being trained in the dummy battlefield, when he goes into a real battle, he can better predict the enemy and fight more effectively.
The difference between vaccination and immunization is that vaccination is the process of putting the pathogen into the body of the individual, while immunization is when the body has produced enough immunity to be able to fight the pathogen in the future. Thus, one can be vaccinated without being immunized if enough immunity has not been formed.
There are various types of vaccines available today and they include Whole pathogen vaccines, Subunit vaccines, and Nucleic Acid vaccines.
Whole Pathogen Vaccines
They consist of the full pathogen and can be subdivided into:
- Live attenuated vaccines: These contain a living pathogen that has been weakened so that it can create an immune response but cannot cause disease in the individual. Live vaccines create a very strong immune response and one dose usually confers lifelong immunity to the individual. Examples include Oral Typhoid, Sabin Polio, Varicella (Chickenpox), Smallpox, BCG, Yellow fever, Intranasal Influenza, MMR (Measles, Mumps, Rubella), and Rota Virus.
- Inactivated vaccines: These are vaccines made up of pathogens that have been killed. They usually require several doses to confer effective immunity. Examples include Rabies, Injected Influenza, Salk Polio, Hepatitis A, and Intramuscular Typhoid.
They contain only a part of the pathogen. Because they mount a weak immune response, they usually require several booster doses even years after the first dose. They can be subdivided into:
- Recombinant protein vaccines: A small piece of the virus of bacterium is taken and injected into yeast cells. This combination of a part of the pathogen and yeast cells is then used to make the vaccine. Examples include Hepatitis B, HPV, MenB (Meningococcal Vaccine)
- Toxoid vaccines: These vaccines are composed of toxins that are produced by bacteria. Examples include Diphtheria, Tetanus, and Pertussis.
- Conjugate vaccines: These vaccines are composed of polysaccharides (sugars) from the surface of bacteria attached to something that creates a strong immune response, usually a toxoid protein as described above. The immune system then recognizes the toxoid protein, helping it to mount an immune response to the attached polysaccharide. Examples include Hib (Hemophilus influenza b), MenC, PCV (Pneumococcus), MenACWY, and TCV (Typhoid).
Nucleic Acid Vaccines
They supply the genetic component of the pathogen to the body; the cells of the body produce these pathogens and then an immune response is created towards this pathogen. It can be subdivided into:
- RNA Vaccines: These are composed of RNA of the pathogen. Examples include the Pfizer and Moderna Covid-19 vaccines.
- DNA Vaccines: They are made up of pathogen DNA. There are currently no examples of this but researchers are making progress in developing some.