Person-to-person transmission of human monkeypox within households, kasai oriental, zaire, 1996-1997

Monkeypox virus is a zoonotic agent that causes a disease similar to smallpox in humans. During an outbreak in 1996-1997 in Zaire, the proportion of cases that reported contact with another case (73%) was increased compared with past reports (30%). To measure person-to-person transmission, we calculated the incidence of monkeypox per contact episode within household. A monkeypox case was defined as the occurrence since February 1996 of a vesicular, pustular or crusty rash not diagnosed as chickenpox. Cases occurring 7 to 21 days after another case within a household were considered secondary within the household. A contact episode was defined as living in the same household as a case. Persons exposed to more than one case were considered to have had multiple contact episodes. Contact episodes in persons who had developed monkeypox before or within 6 days of onset of the case were excluded from the analysis. The incidence of secondary cases per contact episode within households was obtained by dividing the total number of secondary cases by the total number of contact episodes. There were 368 persons living in 48 households where 92 cases were identified. A total of 37 cases that were secondary within the household occurred following 491 contact episodes (incidence = 7.5 per 100 contact episodes, 95% confidence interval = 5.4-10.2). The incidence of secondary cases by contact episodes within households does not suggest increased person-to-person transmission of monkeypox compared with what was reported in the past among unvaccinated household members. The increased number of susceptible individuals following cessation of vaccinia vaccination may explain that person-to-person transmission accounted for most of the cases during this outbreak.