The overwhelming majority of new HIV infections amongst gay men in Amsterdam are happening within the context of steady relationships, according to a mathematical model which was constructed with reference to on-going surveys of young gay men’s sexual practices and published in the May 2nd 2003 edition of AIDS. The model also predicts that any reduction in the infectiousness of HIV due to HAART could be cancelled out by increased rates of unprotected sex, particularly in relationships.
Every six months since 1995 members of the Amsterdam Cohort Study were asked to complete a questionnaire about their sexual behaviour. The cohort is made up of gay men aged under 30 who live in the Amsterdam area. Investigators constructed a mathematical model to predict the prevalence of unprotected anal sex within steady partnerships and in casual encounters. The model also indicated how HIV incidence would be affected by increased rates of unprotected anal sex with regular and casual partners; different levels of HIV testing; and the use of HAART.
Investigators calculated that at the 1995 pre-HAART baseline there were 0.67 new HIV infections per 100 person years of follow-up and that there was an overall HIV prevalence rate of 8.84% in the cohort. Between 1995 and 2000 HIV incidence fluctuated between 0.34 and 1.93 per 100 person years.
Far more unprotected anal sex took place between regular partners (30 instances per year) than causal partners (1.5 instances per year), and the investigators estimated that 86% of new HIV infections were occurring within the context of relationships. As less than 50% of gay men in Amsterdam had tested for HIV, gay men were often having unprotected sex without a reliable knowledge of their own or steady partner’s HIV status. Further, an estimated 10% of gay men in steady relationships who were having unprotected sex with their main partner did not have a “negotiated safety” agreement to help prevent HIV being introduced into the relationship from unprotected sex with casual partners. In addition, an estimated 12.5% of gay men who did have such agreements broke them by having unprotected sex with casual partners.
Increased amounts of unprotected sex between regular partners had the potential to cancel out the reduced infectiousness of HIV from HAART. The investigators calculated that a HAART-related reduction of between 75% and 99% in HIV’s infectiousness would be cancelled out if the amount of unprotected anal sex between regular partners increased by 50%. Because so little HIV transmission was calculated to occur during casual sexual encounters, the investigators predicted that HAART would still reduce HIV incidence even if the amount of unprotected anal intercourse between casual partners were to double.
At the start of the study, approximately 42% of gay men in Amsterdam had tested for HIV. According to the investigator’s model, if HIV testing increased from the baseline level to 80% and use of HAART by HIV-infected men increased from 80% to 85%, the incidence of new HIV infections could be further reduced.
An increase in HIV testing to 70% would require a 30% increase in risk behaviour between regular partners and over a 100% increase in unprotected anal sex between casual partners to cancel out the beneficial effects of more gay men knowing their HIV status. HAART would only fail to limit new infections when unprotected anal intercourse between regular partners had increased by 70%, and HAART would continue to reduce the incidence of new infections even if the level of unprotected anal intercourse between casual partners doubled.
The investigators note that the mathematical model used in their study “suggests that the majority of new infections among young homosexual men in Amsterdam can be attributed to steady partners. Changes in risky behaviour with steady partners thus have a greater impact on HIV incidence than the equivalent changes among casual partners.” They add that the potential for HAART to reduce the infectiousness of HIV could be cancelled out by increases in the amount of risky sex men have with steady partners, noting “the model also shows that increases in risky behaviour may counterbalance the positive effects of HAART, although such increases could be outweighed by increased HIV testing and HAART administration.”
Further information on this website
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Xirirdou M et al. The contribution of steady and casual partnerships to the incidence if HIV infection among homosexual men in Amsterdam. AIDS 17: 1029 – 1038, 2003.