Condom use – not serosorting – is the most likely reason why HIV incidence is stable among lower-risk men who have sex with men (MSM) in Amsterdam but rising among higher risk men.
Serosorting - where MSM discuss their HIV status and have sex with men of the same status – is thought to be the reason why HIV incidence in cities such as San Francisco has stopped rising despite increases in risky behaviour and sexually transmitted infections (STIs).
But the behaviour relies on a high level of HIV testing among the MSM population, for example in cities like San Francisco and Sydney where testing rates are over 90%.
Serosorting as a risk reduction behaviour has also been studied in London where testing rates are around 75% and now Dutch researchers have studied the behaviour in Amsterdam where HIV testing rates are even lower at 70%.
Contrary to the stable HIV incidence seen in some US cities, the Amsterdam STI outpatient clinic has seen a continued rise in HIV among MSMs above the age of 35. But HIV incidence in the general MSM Amsterdam population has remained roughly stable since 1991.
The researchers compared behaviour and HIV infection rates in two groups of MSM – 281 men classed as “lower risk” and 232 men classed as “high risk”. Behaviour was assessed by surveying the two groups of men during 2004 and 2006.
The men classed as lower risk were from the Amsterdam Cohort Study – an ongoing study of mainly gay men – which is estimated to have had a relatively stable HIV incidence at 1.24 per 100 person years from 1999 to 2005.
Men classed as higher risk were recruited from the Amsterdam STI Outpatient Clinic population, in which HIV incidence is estimated to have risen in the same period to 3.75 per 100 person years. In the high risk group 46% were HIV-positive compared to 7% in the lower risk group.
In both groups, men who discussed their HIV status with their partners were more likely to have unprotected anal sex (p < 0.001).
Fifty per cent of the lower risk and 72% of the high risk MSM who were HIV-negative reported having unprotected anal sex with partners of the same HIV status (concordant partners). HIV-positive men in both groups were more likely to have unprotected anal sex with concordant partners – 72% concordant and 22% discordant in the lower risk group (p = 0.07) and 82% concordant and 36% discordant in the high risk group (p < 0.001).
These findings suggest there is some degree of serosorting practised in both groups.
But men in the high-risk group were still more likely to have unprotected anal sex with partners who were either traceable but of a different HIV status or non-traceable anonymous partners of unknown HIV status.
The researchers say this suggests the stability in HIV incidence seen in the general population - compared to those attending the STI clinic - is due to condom use rather than differences in serosorting behaviour.
They conclude that – despite a lower rate of HIV testing – serosorting is practised in Amsterdam in both high and low risk groups but that the former are still more likely to have unsafe sex with HIV discordant or anonymous partners.
Van de Bij AK et al. Condom use rather than serosorting explains differences in HIV incidence among men who have sex with men. J Acquir Immune Defic Syndr 45: 574-580, 2007.