Epidemiology of variola minor in Brazil based on a study of 33 outbreaks

Surveillance-containment units were established in 1969 in four States of Brazil as part of the national smallpox eradication programme. Their responsibilities included both the investigation of reported cases and the control of outbreaks, and the further extension and development of the reporting network. Altogether, 33 outbreaks of smallpox were investigated in detail, all in endemic areas not yet reached by the attack phase of the eradication programme. Official reports of 27 cases led to the discovery of an additional 1 465 cases, suggesting that reporting was not more than 2% complete in endemic areas. The total of 1 492 cases in 33 outbreaks occurred in 493 households comprising a study population of 3 088 persons: three-quarters of the cases were in children under 15 years of age. Only 5% were in individuals aged 30 years or more. The case-fatality ratio was 0.8%. The overall attack rate among unvaccinated persons (76.2%) was 23 times that among vaccinated persons (3.3%). Age-specific vaccine effectiveness ratios ranged from 94% to 100% with an overall reduction in expected cases among the vaccinated of 95%. There was no evidence to suggest waning of immunity among persons who had been successfully vaccinated. There was an inverse relationship between susceptibility and age since a greater proportion of adults had already had smallpox or a successful vaccination; 97% of pre-school-age children and 82% of school-age children had no history of smallpox and no vaccination scar. In all 33 outbreaks, more than 60% of unvaccinated persons became ill; in 5 outbreaks the attack rate for unvaccinated persons was greater than 90%.