Smallpox epidemic in a Brazilian community.

The most severe outbreak of smallpox in Brazil in 1969 took place in the municipio of Utinga, Bahia. Of 507 cases, 246 occurred in residents of the urban area of the municipio. This paper describes the findings of the detailed study of the urban area outbreak. An inverse relationship between age and susceptibility was evident; fully 91% of the children less than 5 years of age as well as 75% of the school-age children had no history of smallpox and had never been vaccinated. The overall attack rate was 11% with nearly three-fourths of the cases in children under 15 years of age. Only 4 of the 246 patients had a vaccination scar, and the estimate of vaccine efficacy was 94%. The continued occurrence of cases after the initial vaccination containment measures prompted detailed studies during 2 follow-up visits to the community which demonstrated that 1) transmission of smallpox may continue in a pocket of susceptibles although the overall immune status of a community is high following containment measures, and 2) clinical illness may have been aborted in those affected individuals who were only in their second to fourth day of incubation at the time of vaccination.