In order to prospectively evaluate the relation between serum antibody and protection against smallpox, blood was drawn from 146 contacts of Pakistani smallpox cases before the end of the usual incubation period. Most were well vaccinated, and after follow-up only three cases of secondary smallpox appeared. Hemagglutination inhibition antibody titers proved to have virtually no predictive value for assessing susceptibility against smallpox, since most contacts, despite their protection, had undetectable antibodies. Neutralizing antibodies were present in high titer in most contacts and no contacts with a titer of 1:32 or higher developed smallpox. Two of the cases, however, did have detectable neutralizing antibody at the time of exposure and some persons with no detectable neutralizing antibody were spared. The role of humoral antibody in protection against pox-viruses is still unclear.
Present address: Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02115.
Present address: Middlesex County Hospital, Waltham, Massachusetts.
Present address: Johns Hopkins School of Public Health, Baltimore, Maryland 21205.