Immunization

Immunization, in preventive medicine, process of rendering people immune to an infectious organism by inoculating them with a form of the organism that does not cause severe disease but does provoke formation of protective antibodies. The process has also been called vaccination, because the first instance of immunization was the use of vaccinia, or cowpox, virus to produce immunity to variola, or smallpox. Vaccines are the most effective protection against most diseases caused by viruses and related organisms, because few antibiotics work against them. In Western countries vaccines are routinely used in the first years of life to produce immunity to diphtheria, tetanus, poliomyelitis, Haemophilus influenzae type b, and whooping cough.

A vaccine may contain: organisms killed by exposure to heat or chemicals (the first polio vaccine, and one for typhoid fever); an inactivated form of a toxin, produced by the organism and called a toxoid (tetanus and diphtheria vaccines); or a live “attenuated” virus—one grown in such a way that it can no longer cause serious disease (the polio vaccine developed by Albert Sabin, and vaccines against measles and yellow fever).

The first modern use of immunization was by the British doctor Edward Jenner in 1796, when he used cowpox inoculations to produce protection against smallpox. In 1885 the French scientist Louis Pasteur first used an attenuated rabies virus to protect against the natural infection, and in 1897 a vaccine against typhoid fever was developed in England.

The immunizing substance is usually introduced through a scrape in the skin, called inoculation, although the Sabin polio vaccine is taken orally. Protection lasts for varying periods: the plague vaccine for only six months; the yellow fever vaccine for ten years.

A population can be immunized in two ways. In one method, the vaccine is targeted at those most likely to get the disease. In the recent successful campaign to eradicate smallpox worldwide, a form of this strategy was used. Most diseases in Western countries, on the other hand, are controlled through the principle of herd immunity, in which it is held that the transmission of disease will be stopped when an extremely low probability exists that an infected person will come into contact with an unprotected individual. Not every person needs to be immunized, but protection levels of 90 per cent must be reached for some diseases. In some instances a combined strategy is used. For rubella, or German measles, for example, public health workers aim at mass immunization of school-age children as well as of women of childbearing age.

New vaccines are still being developed, such as a safer, less painful vaccine against rabies and vaccines against hepatitis B and pneumonia-causing bacteria. Diseases common in the developing world for which vaccines are being sought include cholera and parasitic infections such as malaria and trypanosomiasis (sleeping sickness). In addition to active immunization (stimulating antibody formation by introducing a form of the infectious organism), protection may also be provided by passive immunization (injecting serum containing antibodies, usually obtained from a person who has recently had the disease). The latter procedure is now seldom used, except in some cases of hepatitis.