Chinese physicians discovered about a thousand years ago how to reduce the risk of dying from smallpox by scarifying the skin of susceptible persons with secretions from a healing smallpox bleb, thus inducing a mild attack of smallpox. This procedure, called variolation, was not without risk–about one person in a hundred sustained a severe and sometimes fatal attack of smallpox. Nonetheless it was widely used to protect the children of educated well-to-do people in China, and the procedure spread westward along the silk route. Lady Mary Wortley Montagu, wife of the British ambassador in Constantinople, described it in a 1717 letter to a friend in England and introduced it in England when she returned home.
Edward Jenner, a naturalist and family doctor in the village of Berkeley, Gloucestershire, knew about variolation, and knew that milkmaids who had been infected with cowpox, a common disease of cattle in that area, never got smallpox. He reasoned that it might be possible to inoculate cowpox serum into the skin in the same way as the more risky smallpox secretions. During an EPIDEMIC of smallpox in 1796, Jenner inoculated a nine-year-old boy, James Phipps, with fluid from a cowpox lesion, and over the following months he inoculated a total of twenty-three people, mostly children, in the same way. All survived unharmed and none got smallpox. Jenner's experiment would not withstand the rigorous ethical scrutiny required for modern human experimentation, but its lasting benefits for humankind have been enormous. Jenner reported his results in An Inquiry into the Causes and Effects of the Variolae Vaccinae (1798). Vaccination is derived from vaccinae, the Latin for the possessive of vacca, or cow. Vaccine is the fluid containing weakened or dead pathogens, which stimulate immune responses that protect against CONTAGIOUS DISEASE. First applied to protection from smallpox, vaccination and vaccine broadened in meaning to include all such immunizing procedures as these developed.
Before vaccination, smallpox epidemics often afflicted virtually all exposed susceptible persons in the population, in other words, all who were not immune because they had survived previous epidemics. Children were the main victims. Depending on the virulence of the strain of smallpox virus, about one child in every eight to twenty would die, and many who survived were left with unsightly scars after the infected blebs on the skin had healed. If the eyes were affected, the result was blindness.
Despite fierce opposition from antivaccination critics, vaccination programs against smallpox began in Europe and the United States in the early nineteenth century, gathering momentum whenever smallpox epidemics occurred, as they continued to do, albeit with declining ferocity, throughout
the nineteenth and early twentieth centuries. Vaccination against smallpox was not risk-free. It induced fever, painful swelling, and frequently an unsightly scar at the vaccination site. Adverse effects increased in severity with age, strengthening the case for vaccination in childhood. In an epidemic in 1946, the public health authorities in New York City vaccinated about 5 million people over a six-week period–a considerable logistic feat. The human costs included forty-five cases of vaccine-induced encephalitis (severe brain inflammation) and four deaths, along with many thousands of the milder adverse reactions described above. One of the worst possible adverse reactions was a fatal generalized vaccinia infection of the unborn fetus if a pregnant woman was vaccinated.
In 1949 Donald Soper, an American epidemiologist, developed the containment strategy, which consisted of vaccinating all known contacts of every diagnosed case of smallpox rather than indiscriminately vaccinating the entire population. Containment stopped transmission by removing the possibility of the smallpox virus passing from an infected person to others who would have been susceptible in the absence of vaccination, thus reducing the numbers exposed to the risk of adverse reactions. However, in 1965, when the World Health Organization began a vaccination campaign aimed at worldwide eradication of smallpox, total coverage of the entire population in affected countries was the aim. The containment strategy was used later, when the risk of epidemics had declined and the risk of contagion arose mainly from sporadic cases. The last known case of naturally occurring smallpox was a teenage girl in Somalia in 1977. In 1980, the World Health Assembly at its annual meeting declared
that the vaccination campaign had succeeded and that smallpox had been eradicated from the world.
Vaccination against smallpox was the only immunological method of preventing any kind of contagious disease until the rise of scientific bacteriology almost a hundred years after Jenner's experiment. In 1885 Louis Pasteur used an attenuated rabies vaccine to protect a teenage boy, Joseph Meister, after the boy had been bitten by a rabid dog. Before Pasteur's anti-rabies vaccine, rabies was always fatal. By applying Pasteur's methods of developing attenuated strains of pathogens to create innocuous cultures suitable for vaccination, vaccines were soon developed to provide protection against several other previously dangerous diseases of children: diphtheria, tetanus, whooping cough, tuberculosis, and then, with advances in virology and immunology in the middle third of the twentieth century, measles, mumps, poliomyelitis, and others. By the end of the twentieth century, vaccines were available to protect against many diseases that were once a danger to the health and life of infants and children (see Table 1). A high priority for public health science is to develop vaccines for contagious diseases against which so far this preventive method has not been available.
The efficacy of vaccines as a way to protect populations depends on factors that influence herd immunity. For example, diphtheria was a terrible and much feared disease that killed by causing inflammation of the windpipe, so that children who got it often choked to death. After about 50 percent of a population has been immunized (vaccinated) against diphtheria, the probability of transmission to susceptible persons declines sharply. This is called the epidemic threshold.
The epidemic threshold of vaccine-preventable contagious diseases varies according to the infectivity of the pathogen, its mode of spread, and a very large number of
other variables. Measles used to kill from one in ten to one in a thousand children, depending on their prior state of health, nutrition, and resistance to infection. It remains an epidemic risk until over 95 percent of the susceptible population has been vaccinated. Thus it is important to achieve complete coverage of the susceptible population in measles vaccination campaigns, because those who remain vulnerable can be struck down by dangerous complications such as measles encephalitis (which causes permanent brain damage) as well as by bronchopneumonia, a more common and also dangerous complication. Similarly, vaccination against rubella must reach a very high proportion of women in order to protect all against the small risk that a pregnant woman will get rubella and infect a developing fetus with congenital rubella.
Most vaccines are imperfect; although it is rare, they are sometimes contaminated and tragedies occur. Adverse reactions of varying severity can also occur. The frequency of these reactions has been measured in several large-scale campaigns under the auspices of the World Health Organization (WHO) and the United Nations Children's Fund (UNICEF) (see Tables 2 and 3). However, death and serious disease is a far higher risk without than with vaccination against all the common childhood contagious diseases. Despite the fact that the benefits of vaccination far exceed the risks, resistance continues, occasionally jeopardizing the success of communitywide vaccination programs. For example, vaccination against whooping cough was disrupted in Britain in the late twentieth century when a pediatrician made widely publicized but inaccurate statements about the risks of fatal outcome. It is a challenge for public health authorities, pediatricians, and family doctors to allay the understandable anxiety of parents that their children will suffer harm from vaccination against diseases that have been eliminated from affluent modern societies.
Gruenberg, E. M., ed. 1986. Vaccinating against Brain Syndromes: The Campaign against Measles and Rubella. New York: Oxford University Press.
Henderson, D. A. 1980. "The Eradication of Smallpox." In Public Health and Preventive Medicine, 11th edition, ed. J. M. Last. New York: Appleton-Century-Crofts.
Wilkinson, Lise. 2001. "Vaccination." In Oxford Illustrated Companion to Medicine, 3rd edition, ed. S. P. Lock, J. M. Last, and G. Dunea: New York: Oxford University Press.
JOHN M. LAST