Between Jan 20 and March 5, 1970, 47 cases of measles occurred in residents of Scott City, Kan, which had not reported a case of measles since 1966. Spread of measles was demonstrated through four generations, involving 35 elementary-school children, 10 preschoolers, and two high-school students. Fifteen of the 47 patients had a documented history of previous measles vaccination. Based on a measles immunity survey, attack rates by school and vaccination status were calculated; the attack rates in the elementaryschool population, which generated the epidemic, were 2.6 and 30.3 for vaccinated immune and unvaccinated susceptible students, respectively. Based on these rates, measles vaccine was 91.4% effective. The immunity survey also demonstrated that a "herd immunity" level of 89% in the elementary-school population did not prevent propagation of the epidemic.
Wyll SA, Witte JJ. Measles in Previously Vaccinated Children: An Epidemiological Study. JAMA. 1971;216(8):1306–1310. doi:10.1001/jama.1971.03180340026006
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