Vaccination (Latin: vacca-cow) is so named because the first vaccine was derived from a virus affecting cows, the cowpox virus. This virus in its weakened form provides a degree of immunity to small pox, which is a contagious disease and is deadly to humans.
A vaccine is a preparation used to produce active immunity to a disease, in order to prevent infection by any organism. The term vaccine has been derived from vaccinia, the infectious viral agent of cowpox, which, when administered to humans, provided immunity against small pox.
The process of introducing vaccines is called vaccination. The vaccine can be introduced in the body of a person mostly by injection and sometimes orally. When vaccine enters the body, it stimulates white blood cells (WBCs) in the body to produce antibodies against the disease-causing germ. This practice is known as prophylaxis.
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Kinds of Vaccines:
On the basis of how they are produced, vaccines are of many kinds.
1. Vaccines Produced by Killing the Germs:
The vaccine is made from the entire organism, killed to make it harmless. For example TAB vaccine for typhoid, Salk’s vaccine for Poliomyelitis.
2. Vaccines Produced by Living Weakened Germs:
The organism is cultured so as to reduce its pathogenicity, but it still retains some of the antigens of the virulent form. The Bacillus of Calmette- Guerin (BCG) is a weakened version of the bacterium that causes tuberculosis in cows. BCG is used as a vaccine against tuberculosis.
3. Vaccines Prepared From Living Virulent Germs:
In this a fully living virulent virus is introduced in a person which provides resistance to a disease. For example, vaccine against small pox.
4. Vaccines Prepared From Toxoids:
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In some diseases, such as diphtheria and tetanus, it is not the growth of the bacterium that is dangerous, but the protein toxin that is liberated by it. Toxoids are the extracts of toxins secreted by bacteria. These formulations are made harmless by treating with formaldehyde. Treating the toxin with formaldehyde denatures the protein so that it is no longer dangerous, but retains some capacity to produce antibodies.
5. Vaccines Produced by 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 (MMR) vaccines are examples of attenuated virus. The oral polio vaccine (OPV) is another example of such vaccines.