Vaccines are biological substances that contain attenuated microorganisms and/or antigens to a particular infectious disease, and which are used to protect an individual against the disease in future. They are usually suspensions of weakened, killed or fragmented microorganisms or toxins administered primarily to prevent diseases in living systems including man and animals. The medical procedure of administering these vaccines to humans or animals is generally known as vaccination. Vaccination is simply defined as the administration of vaccines to a living host. It is the most effective way to prevent killer diseases especially amongst children between the ages of 0-12 years old. The term vaccination is usually used synonymously or interchangeably with immunization. However, immunization is generally the procedure of making a person immune to a particular infectious disease. And it can be achieved through several processes which include but not limited to:
- Passive immunization – which a newborn usually acquires from the mother on birth before the baby starts developing or building its own active immunity.
- Injecting an antiserum or vaccine into the body.
- Oral administration of vaccines in tablet forms.
- Direct inoculation of a weakened or live organism into the body (this method is archaic, and it is not practiced again).
Generally, vaccines are pathogen-imposters – since they act as the carbon copy of the actual pathogen in vivo, thereby stimulating the host’s immune system to produce potent antibodies and other immune system molecules against the invading microbe in advance. Vaccines prepare and stimulate the body to fight against invading infectious disease agents in advance. According to the World Health Organization (WHO), the widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles and tetanus from most parts of the world. Vaccination therefore has significantly contributed to the prevention and control of many human infections and diseases.
It is noteworthy that the concept of using microbes (as vaccines) especially in their attenuated/weakened forms to treat microbial diseases in humans was first established and conceptualized by Louis Pasteur. Louis Pasteur was the first to develop a potent human vaccine (rabies vaccine in particular) in 1881. Pasteur’s work showed that pathogens can be isolated, inactivated and administered in their weakened or live forms to humans to prevent them from acquiring a particular infectious disease. Though the field of vaccinology as we know it today was first introduced into clinical medicine by Edward Jenner in 1796 – when he inoculated a 13 year old boy with vaccinia virus obtained from a woman accidentally infected with cowpox. Jenner’s method of vaccination is generally known as inoculation – since it involved the direct introduction of live organisms into the body.
However, this practice has become obsolete and is no longer practiced in clinical medicine. It was later discovered by Jenner that the young lad developed immunity to smallpox infection – which caused the death of millions of people worldwide. The immunity developed in the young man against smallpox disease was due to a challenge with variola (vaccinia) virus which Edward Jenner administered into the boy. Through vaccination and/or immunization, a large population of humans can be protected from becoming infected by an infectious disease. This is because, the vaccination of a large human population against a given infection can develop herd immunity in the population – since contacts amongst people in that population will be more with protected and vaccinated individuals instead of with sick and infected persons. Herd immunity helps to reduce the spread and circulation of infectious agents within a particular human population. Most of the vaccine-preventable diseases including polio have been significantly reduced and contained in the developing parts of the world.
All the vaccines used today for the vaccination and/or immunization of humans are based o the use of killed or live forms of microorganisms or their purified subunits. Purified antigens of pathogens, toxins and their conjugated protein or polysaccharide molecules can also serve the purpose of vaccination in humans depending on the type of infectious disease they are used to prevent. For human vaccines to be available on a global scale, complex production methods, meticulous quality control measures and reliable distribution channels are needed to ensure that the products are safe, potent and effective for human use. A vaccine can confer active immunity against a specific harmful agent by stimulating the immune system to attack the agent. Once stimulated by a vaccine, the antibody producing cells known as B- lymphocytes remain sensitized and ready to respond should it ever gain enter into the body the body of the vaccinated human host. Even though no vaccine is entirely safe or completely effective, their use is strongly supported by their benefit to risk ratio. That is, the benefit of using them far outweighs any risk associated to the application in humans.
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