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A vaccine (named after vaccinia, the infectious agent of cowpox, which, when innoculated, provides protection against smallpox) is used to prepare a human or animal's immune system to defend the body against a specific pathogen, usually a bacterium, a virus or a toxin. Depending on the infectious agent to prepare against, the vaccine can be a weakened bacterium or virus that lost its virulence, or a toxoid (a modified, weakened toxin or particle from the infectious agent).

The immune system recognizes the vaccine particles as foreign, destroys them and "remembers" them. When the virulent version of the agent comes along, the immune system is prepared for a fast strike, neutralizing the agent before it can spread and multiply to vast numbers.

Live but weakened vaccines are used against tuberculosis, rabies, and smallpox; killed agents are used against cholera and typhoid; toxoids against diphtheria and tetanus.

Vaccines, though they are by far not as virulent as the "real" agent, can have unpleasant side effects, and have to be renewed every few years. A new attempt to avoid these obstacles of "classic" vaccination is DNA vaccination. The DNA coding for a part of a virus or a bacterium that is recognizable by the immune system is inserted and expressed in human/animal cells. These cells now produce the toxoid for the infectious agent, without the effects other parts of a weakened agent might have. As of 2003, DNA vaccination is still experimental, but shows some promising results.

Many diseases such as polio have been largely controlled in developed countries through mass use of vaccines (indeed smallpox appears to have been completely eliminated in the wild).

As long as the vast majority of people are vaccinated it is difficult for an outbreak of disease to spread. This effect is called herd immunity. Some people, particularly those who practice alternative medicine, refuse to immunize themselves or their children, as they believe vaccines' side effects outweigh their benefits. Thus, they reap some of the benefits of vaccines, through herd immunity, without assuming any of the risks. Advocates of routine vaccination argue that side effects of approved vaccines, whilst real, are either far, far less serious than actually catching the disease, or are very rare, and argue that the calculus of risk/benefit ratio should be based on benefit to humanity rather than simply on the benefit to the immunized individual. The main risk of rubella, for example, is to the fetuses of pregnant women, but this risk can be effectively reduced by the immunization of children to prvent transmission to pregnant women.

Vaccination against smallpox seems especially contentious. Dissenters falsely suggest that modern vaccines might result in outbreaks of smallpox (in fact, they contain no smallpox virus: they contain live vaccinia virus), and suggest that infections in the past originated (as in the small vaccinia outbreak in England in 1961), spread in greater proportion, and/or were more lethal amongst the vaccinated. They contend that improved sanitation and nutrition, not vaccine, account for most of the reduction in smallpox. Inoculation proponents dispute these allegations, and recognize that the current vaccine has risks. As the risk of the disease (barring germ warfare) is effectively zero, routine smallpox vaccination has not been recommended for many years.

The threat from smallpox as a biological weapon has however, caused a reconsideration, though not a reversal, of this position. A 2003 US vaccination effort was unpopular. There was poor turnout among hospital staffs who were designated to be the first to receive the vaccine. In addition to the known risks of the vaccine, several deaths were reported from cardiac causes among the immunized, suggesting that people with coronary artery disease might be at risk. Development of newer formulations that might avoid the newly discovered risks is underway. Illinois and New York suspended the program pending investigation of the apparent vaccine deaths of two healthcare workers (the main focus of the program). The California Nurses Association opposed the effort from the start.class="external">[1

The safety of inoculant additives is being questioned. Some suspect the mercury-containing preservative Thimerosal to be a contributor to autism. Mercury has been long known to have deleterious neurological effects, and is tightly regulated by the EPA, but the CDC and pharmaceutical companies deny there is any connection to autism, whose cause remains unknown. [1]. For those who remain concerned despite the lack of evidence, thimerosal-free formulations of DTP (diphtheria, tetanus, and pertussis) vaccine, pneumococcal conjugate vaccine, polio vaccine, MMR (measles, mumps, and rubella), hepatitis B, Haemophilus influenzae type b (HIB) vaccines and Hib/Hepatitis B combination vaccines are available.

See also : vaccination -- Edward Jenner -- immunology -- immunization -- medicine -- genetics -- AIDS vaccine -- Timeline of vaccines

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