Epidemiology of smallpox in west Pakistan. III. Outbreak detection and interlocality transmission. 1971

During one year, 121 outbreaks of variola major were detected in 99 of the 1717 localities within a rural area of West Pakistan with a total population of approximately 1.2 million. Only 19% of the outbreaks, representing 36% of the 1040 investigated cases, were officially reported, although potential strengths in the government surveillance system were also identified. Persistence of smallpox within the area depended on introductions from the outside, and more than one-half of all outbreaks of known source could be ultimately traced to cities. Within the study area, outbreaks with the largest numbers of cases and those in the larger communities were the ones from which smallpox was most frequently transported. The frequency with which variola was introduced into localities was directly related to population size and to the presence of medical care facilities. Trips between localities by infected individuals were extremely rare events. They were made most often during the late fall and winter, primarily during the incubation period of the disease, and did not differ in purpose, means or destination from journeys unassociated with smallpox. Individuals at relatively high risk of becoming introducers included the unvaccinated (primarily children under five), the unschooled and those not native to the area. Vaccination priorities based on these findings could increase the efficiency of smallpox eradication efforts.