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Vaccines and the Diseases they Prevent
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Why Immunize?

How Vaccines Work

Every day, the body is bombarded with bacteria, viruses and other germs. When a person is infected with a disease-causing germ, the immune system mounts a defense against it. In the process, the body produces substances known as antibodies against that specific germ. The antibodies eliminate the germ from the body. The next time the person encounters the germ, the circulating antibodies quickly recognize it and eliminate it before signs of disease develop.

This is why a child who has had chickenpox will only rarely develop the disease again. The immune system has memory. The next time the child encounters the virus that causes chickenpox, the antibodies destroy the virus before disease causes sickness. Medical experts estimate that the immune system can recognize and effectively combat hundreds of thousands, if not millions, of different organisms, or more.

A vaccine works in a similar way. However, instead of one natural infection, for immunity to develop after a vaccine it usually takes several doses over several months or years. The vaccine contains an inactivated (killed), weakened form of the germ, or a germ component. When introduced into the body, the dead or harmless germ causes an immune response without causing the disease. The immune system develops antibodies that will effectively kill or neutralize the germ if exposed to it in the future. The antibodies circulate in the bloodstream. Vaccination protects a child against infection with a germ without the child ever suffering through the disease.

Types of Vaccines

Vaccines can be developed in four different ways by using:

  • Live bacteria or viruses that have been altered so that they cannot cause disease
  • Killed bacteria or inactivated viruses
  • Toxoids (bacterial toxins that have been made harmless)
  • Parts of bacteria or viruses

Because the immune response may decrease over time, vaccines known as "boosters" are sometimes given to restore the immune response against that particular germ. Protective immunity lasts longer when boosters are given.

Live attenuated vaccines are usually derived from the naturally occurring germ. They can infect people, but not cause serious disease. Live attenuated vaccines are made by passing the virus through cell cultures over time until its disease-causing ability has deteriorated.

Live attenuated vaccines include:

Inactivated (killed) vaccines cannot cause an infection, but still stimulate antibody production. Viruses are inactivated with chemicals such as formaldehyde. The inactivated polio vaccine (IPV) is made this way.

Toxoid vaccines are made by treating toxins with heat or chemicals, such as formalin, to destroy toxicity. Toxoids do not cause illness but stimulate the body to produce antibodies. The diphtheria and tetanus vaccines are toxoid vaccines.

Vaccines are also made by using only part of the virus or bacteria, which cannot cause disease. The immune system can mount a response against the partial virus or bacteria. Four of the newest vaccines are made this way:

Community immunity or "herd immunity" is an important part of protecting the community against disease. Because vaccinated people have antibodies that neutralize a germ, they are much less likely to transmit that germ to other people. Thus, even people who have not been vaccinated (and those whose vaccinations have become weakened or whose vaccines aren't fully effective) often can be shielded by the herd immunity because vaccinated people around them are not getting sick. Herd immunity is more effective as the percentage of people vaccinated increases. It is thought that approximately 95% of the people in the community must be protected by a vaccine to achieve herd immunity. People who are not immunized increase the chance that they and others will get the disease.

For some diseases, however, herd immunity offers no protection. For example, tetanus is not contagious. It is contracted when a wound comes in contact with soil contaminated with the tetanus bacterium.

It is important to keep in mind that a few people may not be protected from the disease even though they have been vaccinated. About 1 or 2 of every 20 people immunized will not have an adequate immune response to a vaccine. But if 95% of the population is immunized, then the unprotected people are not as likely to be exposed to the germ at all, so they have a smaller chance of becoming infected.

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