Mack, T. M. (Harvard Univ. Sch. of Public Health, Boston, Mass. 02115), D. B. Thomas, A. Ali and M. M. Khan. Epidemiology of smallpox in West Pakistan. I. Acquired immunity and the distribution of disease. Am J Epidemiol 95: 157–168, 1972.—Outbreaks resulting from 121 village introductions of variola major within a Punjabi district during a 12-month period were studied in the field. The secondary attack rate among previously unvaccinated compound contacts was over 10 times that among the vaccinated. Virtually all cases appeared in the first indigenous generation. Although previous vaccination was the most important determinant of risk within the family compound, it was an imperfect guide to risk in general, because of variations in exposure between compounds. Among the unvaccinated, age and sex influenced risk, which was greatest in teen-age boys. There was no correlation between the village immunity prevalence and the likelihood of introduction, and degree of spread was not closely related to the overall immunity status of each village. The vaccination state of concurrent cases did not vary with season. It is suggested that routine vaccination of school-age people not in school be given highest priority and that revaccinations be given low priority. Containment measures should be expedited during seasons of decreasing incidence.