Pattern of intrafamilial transmission of smallpox in Calcutta, India.

The pattern of intrafamilial transmission of smallpox in Calcutta was studied in 43 index cases, 3 of which were haemorrhagic, 14 confluent, and 26 discrete. They had 741 contacts. The attack rate in vaccinated contacts was significantly less than in unvaccinated contacts, but there was no such difference in the case rates caused by severe and mild index cases. Females had higher attack rates than males, the difference being more marked among the vaccinated. The vaccination status of the index cases seemed to affect their secondary case rates. The incidence of secondary cases among contacts living in the same room as a patient and in other rooms in the same compound was practically equal.