A STUDY OF INAPPARENT INFECTION IN SMALLPOX

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Abstract
Heiner, G. G. (Institute of International Medicine, Univ. of Maryland School of Medicine, Baltimore, Md. 21201), N. Fatima, R. W. Daniel, J. L. Cole, R. L. Anthony and F. R. McCrumb, Jr. A study of inapparent infection in smallpox. Amer J Epidem 94: 252–268, 1971.—A retrospective serologic study was conducted in villages and households in which recent smallpox outbreaks had occurred but in which vaccination by public health authorities had not been carried out. Sera were collected from 143 household and compound contacts of smallpox cases in 20 villages and from 62 controls in five of the same villages; all had been vaccinated in the past. Serum antibody was assayed by the techniques of complement-fixation, hemagglutination-inhibition, passive hemagglutination, neutralization and immunodiffusion. Among the contacts, 54.5% showed elevated complement-fixing antibody levels, compared to 6.5% of controls. Similar proportions were found employing the other serologic tests. Analysis of the 56 households from which the study subjects were drawn showed a clinically-overt secondary attack rate of 14% and a 27.3% incidence of serologic response suggestive of inapparent infection. Since 35% of the contacts were not tested serologically, the true incidence of inapparent infection may have been considerably higher.

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