ENDEMIC SMALLPOX IN RURAL EAST PAKISTAN II: INTRAVILLAGE TRANSMISSION AND INFECTIOUSNESS1

Abstract

Thomas, D. B. (Johns Hopkins Univ. School of Hygiene and Public Health, Baltimore, Md. 21205), I. Arita, W. M. McCormack, M. M. Khan, M. S. Islam and T. M. Mack. Endemic smallpox in rural East Pakistan. II: Intravillage transmission and infectiousness. Amer J Epidem 93: 373–383, 1971.—Thirty outbreaks of variola major occurred during one year in 27 villages of a rural Bengali study area. Thirteen consisted of a single introductory case; all cases in nine others were confined to one bari (a cluster of houses occupied by patrilineally related families); and transmission between baris occurred in eight. Larger outbreaks consisted of longer chains of transmission, and cases in proportionally more families and baris, than smaller outbreaks. Transmission was not limited by an exhaustion of the supply of susceptible villagers, case isolation, or containment vaccination. Outbreak size did not vary with pre-outbreak vaccination scar prevalence. Baris served as barriers to spread. More cases followed introductions into villages with contiguous than scattered baris. Only within baris having fewer than 20 inhabitants was the family not another barrier to spread, and was vaccination scar prevalence the dominant determinant of the extent of intrabari transmission. Selective vaccination procedures based on these findings can increase the efficiency of outbreak containment. Weaknesses and potential strengths of the official system for reporting smallpox were identified. The infectiousness of cases was directly related to the severity of their clinical syndrome.