Vaccine Definition

Vaccine ia a substance that initiates an immune response to produce antibodies that prevent infection by the particular pathogen. Vaccines contain attenuated live (weakened) or killed pathogens such as viruses or bacteria.

The antigens of these pathogens activate the body’s immune response, stimulating B-cell lymphocytes to produce antibodies specific to them. Genetic engineering makes it possible to produce large quantities of many vaccines in relatively short order.

There are Four Types of Vaccines

  • Attenuated vaccines contain live but weakened viruses to produce the strongest immune response. Laboratory manipulation of the virus can establish narrow parameters of survival for the virus the vaccine carrier, such as temperature or acidity. These manipulations reduce the risk that the vaccine could cause infection, though such a risk exists. Often an attenuated vaccine requires only a single dose to establish full and long-term immunity.
  • Inactivated vaccines contain killed bacteria or viruses. These pathogens still carry the antigens that will stimulate the immune response to produce antibodies but are incapable of causing infection. Though safer than attenuated vaccines, inactivated vaccines often require multiple doses or provide limited immunity.
  • Acellular vaccines, also called subunit vaccines, contain particles of the virus or bacteria. These particles carry enough antigen to stimulate an immune response but are not complete enough to cause infection.
  • Toxoid vaccines generate antibodies for the toxins certain bacteria generate when they cause infection. Tetanus and diphtheria are illnesses due to such toxins and the vaccines for them provide antibodies for the toxins rather than the bacteria that cause the illness.

Vaccines prevent many infectious diseases that were once major killers. Vaccination has essentially eliminated smallpox worldwide, for example, and is close to eliminating poliomyelitis. Some vaccines, such as for tetanus and pertussis, require multiple doses or periodic booster doses to establish full immunity. Because vaccines are effective for only the specific pathogens they contain, rapidly mutating pathogens such as the influenza virus require a new vaccine for each strain.

List of Vaccines

anthraxchickenpox/shingles (varicella zoster viruses)
cholerayellow fever
diphtheriadiptheria, tetanus, acellular pertussis (DtaP)
Haemophilus influenzae type b (Hib)hepatitis a (hav)
hepatitis B (HBV)influenza
lyme diseasemeasles
meningococcal vaccinemeasles, mumps, rubella (MMR)
plaguepneumococcal vaccine

Vaccines may not be effective in establishing immunity in people who are immunocompromised. Some people have allergies to the ingredients of the vaccine. Vaccines that contain attenuated live viruses sometimes use the preservative thimerosal which contains mercury. Because this heavy metal can cause neurologic damage, the United States has initiated a cooperative effort among vaccine manufacturers to develop thimerosal-free vaccines. Most recommended vaccines are now available without thimerosal or with minimal amounts of thimerosal.

People who travel should receive vaccines appropriate for the regions they intend to visit. The US Centers for Disease Control and Prevention (CDC) maintains a schedule of recommended travelers’ immunizations at its Web site (


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