PROBABILITY OF HETEROSEXUAL TRANSMISSION OF HIV - RELATIONSHIP TO THENUMBER OF UNPROTECTED SEXUAL CONTACTS

The objective of this study was to investigate the relationship betwee n the number of unprotected heterosexual contacts with an HIV-infected person and the probability of HIV transmission. Data from a European study involving 563 heterosexual partners of HIV-infected subjects wer e analyzed. The number of unprotected contacts could be estimated for 525 couples (377 with male index case, 148 with female index case) fro m the reported frequency of unprotected contacts and an estimate of th e length of the period during which transmission could have occurred. Nonparametric (isotonic regression) and parametric (Bernoulli model) a nalyses were performed on data at study entry and on follow-up data (1 21 couples). The nonparametric analysis resulted in several exposure g roups, with the proportion of infected partners increasing with the nu mber of contacts. For example, the percentage of female partners infec ted ranged from 10%, among those with <10 unprotected contacts with an infected male, to 23% after 2,000 unprotected contacts. The parametri c estimates of (assumed constant) per-contact infectivity were higher for male-to-female than for female-to-male transmission, but not signi ficantly so. However, in comparison with nonparametric estimates, the model assuming constant infectivity appears to seriously underestimate the risk after very few contacts and to seriously overestimate the ri sk associated with a large number of contacts. Our results suggest tha t the association between the number of unprotected sexual contacts an d the probability of infection is weak and highly inconsistent with co nstant per-contact infectivity. Probable explanations for these findin gs include large variability in infectivity between couples and within individuals over time. Estimates based on partner study data under th e hypothesis of constant infectivity can, therefore, be highly mislead ing at a public health level, particularly when extrapolated to multip le casual contacts.