Serosorting is, however, considerably less effective in reducing the chances of acquiring HIV than four other strategies: 100% condom use, monogamy, only having insertive sex, or ‘seropositioning’ (only taking the bottom role with partners known not to have HIV and being top with partners of positive or unknown status). Interestingly, 100% condom use was the least effective of these other four strategies.
‘Seroadaptive’ behaviours include any method of attempting to reduce the risk of HIV acquisition or transmission by altering one’s sexual behaviour according to the HIV status of partners. The term ‘serosorting’ has been used in various different ways. Most commonly, it means restricting unprotected anal sex to partners known to have the same HIV status as yourself. When unprotected sex between HIV-negative men is confined to a primary relationship, with condoms used in all other encounters, this has been called ‘negotiated safety’.
While some studies have found serosorting in HIV-negative men to be effective, others have not. Attempted serosorting by HIV-negative people has an inherent drawback that serosorting by HIV-positive people lacks: people can only be certain of their status up to the first time they risk exposure to HIV after their last negative HIV test. Research indicates that a large minority of people in high-risk communities who assume they are HIV negative in fact have HIV, and that a large proportion of men who ‘know’ their partner’s HIV status have, in fact, tried to guess it.
Nonetheless, though serosorting is fallible, a recent meta-analysis of studies presented at CROI found that serosorting halved the likelihood of acquiring HIV compared to having no strategy at all.
The study pooled behavioural data and HIV incidence rates from four different studies in gay men:
- The HIVNET 001 Vaccine Preparedness Study (VPS), an observational study that took place in eight cities in the US between 1995 and 1997.
- VAX 004, the first phase III efficacy trial of a candidate HIV vaccine, which took place at 61 sites in the US, Canada and the Netherlands between 1998 and 2001.
- The EXPLORE study, a randomised controlled trial of a behavioural HIV-prevention intervention that took place in six US cities between 1999 and 2003.
- The STEP study, a phase III trial of another candidate vaccine, which took place in North and South America and Australia between 2004 and 2007.
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