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Chapter 4 : From Milkmaids to Vaccines
Farmers of Gloucestershire in England, who come into contact with cowpox-infected cows, develop a mild reaction with the eruption of a few blisters on the hands or lower arms. Once exposed to cowpox, neither cows nor humans develop any further symptoms. Early on it was noted that hardly any milkmaids or farmers who had contracted cowpox showed any of the disfiguring scars of smallpox, and most milkmaids were reputed to have fair and almost perfect skin. The exact relationship between the virus used in vaccines today and Edward Jenner's cowpox virus is obscure. The two viruses, Variola vaccinae and V. major, are 95% identical genetically and differ by no more than a dozen genes. For human protection to occur, there must be cross-reactivity between the two viruses such that antibodies are produced in the vaccinated individual that can neutralize the variola major virus, should the need arise. Fortunately, this does happen. Recovery from a smallpox attack gives lifelong immunity, but vaccination ordinarily does not. The advantages of vaccination over variolation are twofold. First, the recipient of a vaccination is not infectious to others; second, death is very rare. This chapter presents a list of successful protective vaccines, including live attenuated microbe, killed microbe, and purified protein or polysaccharide from microbe.