The terms vaccination and vaccine derive from the work of Edward Jenner who, over 200 years ago, showed that inoculating people with material from skin lesions caused by cowpox (L. vaccinus, of cows) protected them from the highly contagious and frequently fatal disease smallpox.
Since Jenner's time, the term has been retained for any preparation of dead or weakened pathogens, or their products, that when introduced into the body, stimulates the production of protective antibodies or T cells without causing the disease. In molecular terms, the goal is to introduce harmless antigen(s) with epitopes that are also found on the pathogen.
Vaccination is also called active immunization because the immune system is stimulated to develop its own immunity against the pathogen. Passive immunity, in contrast, results from the injection of antibodies formed by another animal (e.g., horse, human) which provide immediate, but temporary, protection for the recipient. [Link to discussion of passive immunity]
Kinds of Vaccines
1. Killed whole organisms
In this relatively crude approach, the vaccine is made from the entire organism, killed to make it harmless. The typhoid vaccine is an example.2. Attenuated organisms
Here, the organism has been cultured so as to reduce its pathogenicity, but still retain some of the antigens of the virulent form. The Bacillus Calmette-Guérin (BCG) is a weakened version of the bacterium that causes tuberculosis in cows. BCG is used as a vaccine against tuberculosis in many European countries but is rarely used in the U. S.3. Toxoids
In some diseases, diphtheria and tetanus are notorious examples, it is not the growth of the bacterium that is dangerous, but the protein toxin that is liberated by it. Treating the toxin with, for example, formaldehyde, denatures the protein so that it is no longer dangerous, but retains some epitopes on the molecule that will elicit protective antibodies.4. Surface molecules
Antibodies are most likely to be protective if they bind to the surface of the invading pathogen triggering its destruction. Several vaccines employ purified surface molecules:- Influenza vaccine contains purified hemagglutinins from the viruses currently in circulation around the world. [more]
- The gene encoding a protein expressed on the surface of the hepatitis B virus, called hepatitis B surface antigen or HBsAg, can now be expressed in E. coli cells and provides the material for an effective vaccine. Hepatitis B infection is strongly associated with the development of liver cancer. Here then is a vaccine against a cancer.
- The genes encoding the capsid proteins of 4 strains of human papilloma virus (HPV) can be expressed in yeast and the resulting recombinant proteins are incorporated in a vaccine (Gardasil®). Because infection with some of these strains of HPV can lead to cervical cancer, here is another vaccine against cancer.
- Some 80 different strains of Streptococcus pneumoniae cause pneumonia in humans. They differ in the chemistry of the polysaccharide capsule that surrounds them (and makes it difficult for phagocytes to engulf them by endocytosis). The current vaccine consists of tiny amounts of the purified capsular polysaccharides of the 23 most common and/or dangerous strains.
5. Inactivated virus
Like killed bacterial vaccines, these vaccines contain whole virus particles that have been treated (again, often with formaldehyde) so that they cannot infect the host's cells but still retain some unaltered epitopes. The Salk vaccine for polio (IPV) is an example.6. Attenuated virus
In these vaccines, the virus can still infect but has been so weakened that it is no longer dangerous. The measles, mumps, and rubella ("German measles") vaccines are examples. The Sabin oral polio vaccine (OPV) is another example. It has advantages over the Salk vaccine in that- it is given by mouth rather than by injection;
- the viruses it contains can spread to the other members of the vaccinee's family thus immunizing them as well.
It has the disadvantage that — on rare occasions — one of the strains in the vaccine regains full virulence and causes the disease. For this reason, the Salk vaccine has once again become the preferred vaccine in the U. S.
