The 1972 smallpox outbreak in khulna municipality, Bangladesh: I. Methodology and epidemiologic findings

Between April 28 and June 22, 1972, 1384 smallpox cases were detected in Khulna Municipality, Bangladesh; 1117 occurred in the city at large, and the rest in a non-Bengali refugee camp. The epidemic was mostly limited to slum inhabitants and spread from north to south through the town. The older, more mobile age groups were at slightly greater risk of extra-household exposure and slightly lower risk of intra-household exposure than other members of infected families. The overall risk of disease was independent of both age and sex and related solely to vaccination status. Unvaccinated individuals were at 5 times greater risk of disease and 11 times greater risk of dying from smallpox than those who had been vaccinated. The overall attack rate for members of infected families was 36%, and the secondary attack rate was 16.5%. Eighty-one per cent of adult males in these families were infected outside the home, compared with 50% of the 0- to 4-year-old males and females. The mean number of introductions and cases per infected family was 2.1 and 3.1, respectively. At least 7% of the families experienced late reintroductions after subsidence of their initial outbreak. Case fatality varied from 55.3% of the unvaccinated 0- to 2-year-olds to 3.8% of the vaccinated adults. Almost half the deaths occurred within the second week after onset of rash. Interfamily transmission was three times higher in the crowded refugee camp than the city at large. Intrafamily transmission, however, was the same.