Introduction

The probability with which the human immunodeficiency virus (HIV) is transmitted from an infected to a susceptible individual during the course of one or more unprotected sexual contacts plays an important role in models of the AIDS epidemic. Several studies found no association between the number of contacts with a given partner and transmission, and some modellers have therefore preferred to use transmission rates per partnership rather than per contact. However, an analysis of data from the California Partners' Study (Padian et al. 1990) indicated the presence of an association, although not consistent with a constant probability of transmission per contact. Similar data from a European study have been analyzed to investigate further the relationship between the number of unprotected sexual contacts and the probability of transmission of HIV.

Methods

Data on 563 HIV-infected subjects (index cases) and their stable heterosexual partners were collected at study entry (between March 1987 and March 1991) and at 6-monthly intervals thereafter (European Study Group on Heterosexual Transmission of HIV 1992). For each couple, the number of unprotected sexual contacts was estimated using the reported frequency of contacts and of condom use, both before and after any reported change in behaviour, together with an estimate of the length of the period during which the partner was at risk. This latter was determined as the duration of the relationship prior to the date of HIV test of the partner and from either the date of infection of the index case, when known (rarely the case), or January 1982 (or one of several alternative dates).