A significant and stable number of MSM (approximately 39% in 2007 and 2009) were classified as engaging in serosorting with casual partners in Switzerland. Estimates concerning the number of persons involved in various levels of risk were provided.
Risk reduction practices are often analysed in publications as several overlapping questions regarding respondents’ sexual behaviour: the question of UAI with steady and/or casual partners, paired with the presumed or proven serostatus of the respondent and the supposed serostatus of the respondent’s partners. Serosorting assumes that the protagonists have disclosed their respective HIV statuses beforehand with the explicit aim of avoiding HIV infection. However, this intention concept is often not made explicit in the serosorting definition or is entirely missing from questionnaires. The Swiss GaySurvey focused on risk reduction practices with steady and casual partners in the last two survey waves , and asked specifically whether the respondent acted with the purpose of preventing HIV transmission. The word “serosorting” itself was deliberately not used in the questionnaire in order not to influence the respondents.
Multivariate analysis tends to indicate that serosorting may be practiced as a structured, planned strategy, when we consider factors negatively and positively associated with serosorting. MSM who reported themselves as HIV-positive and that they have had UAI with partners of different or unknown HIV status are indeed less likely to engage in serosorting. Regularly visiting sex-on-premises venues is also negatively associated with serosorting. This negative association might be explained by the difficulty of disclosing one’s HIV status in places (e.g., backrooms, darkrooms, or saunas) where verbal interactions are not encouraged .
Serosorting was positively associated with belonging to a gay organisation, possibly owing to existing debates on risk reduction within these organisations in Switzerland, and more informed choices resulting from these discussions. Similarly, frequent use of the internet to select partners is associated with serosorting. Partner selection through the internet may seem an appropriate method a priori because it can be easier to declare one’s HIV status anonymously, rather than face-to-face, or it may simply be faster to find a partner of same HIV status. However, these conclusions contrast with findings from Berry et al., who showed that internet usage was significantly associated with an increased likelihood of UAI with potentially discordant partners among HIV-negative MSM .
As expected, our data revealed a positive association between serosorting and reporting an STI in the last 12 months . Serosorting was much more practiced by HIV-negative men tested in the past 12 months than by HIV-positive MSM (45.3% vs. 17.8%). This result was expected because of the wording of the original question. However, 21.1% of serosorters declared themselves to be HIV-negative without having been tested during the past 12 months (268 < N < 980), and 15.8% reported not knowing their HIV status (192 < N < 767) and may be considered at risk of being infected with HIV or of infecting other people with HIV. This finding is disturbing, particularly regarding the high mean number of sexual partners reported.
The particular risk profile of these serosorters who have UAI with casual partners (multiple partners, UAI with partners of different or unknown status, STI history, and partially inadequate testing frequency) requires preventive interventions tailored to HIV status.
Our study focused on MSM who have casual male partners, and does not go into detail about any relationship with a steady partner among these men. Moreover, a certain proportion of MSM serosorters also have sex with women. We can make the assumption that the practice of serosorting carries a risk for both sexes, as well as for both homosexual and heterosexual couples. This component should also be taken into account within prevention programmes.
Our results confirm those of several authors, notably Heymer et al., who concluded that serosorting has a real potential to increase risk and should not be promoted as a public-health strategy. HIV testing alone is not a panacea, and frequent testing for HIV and other STIs, behavioural interventions, and emphasis on primary infections should be jointly promoted.
Our study has limitations: Gaysurvey data are not representative of the entire MSM population. The broad dissemination of our questionnaire likely attenuated selection bias. However, this method may overestimate levels of risky behavior, given that several of the sites or newspapers used for recruitment are also used to contact partners. We also do not know how often serosorting occurs, or the absolute number of partners with whom serosorters engaged in serosorting. MSM who responded to Gaysurvey may be numerous to be concerned by serosorting; the intensity of risk remains unknown.
We did not exclude from the 2009 dataset those who reported having participated in 2007. The proportion of serosorters in 2009 who had reported having participated in the 2007 survey was not significantly different from the non-serosorters in this situation (respectively 23.2% and 25%). Furthermore, most of the variables associated with serosorting were variables measuring behaviours reported over a period of 12 months (last 12 months).
The quality of the extrapolation of the numbers of serosorters to the MSM population in Switzerland is dependent upon the quality of the GaySurvey samples used in the computations, which remains unknown. However, we also relied on data from a health survey in the general population, with a restricted and an enlarged definition of MSM, to compute these extrapolated estimates. That survey is a random probability survey which does not suffer from the same weaknesses as GaySurvey and is uncorrelated to it.