Editor,—To reduce the risk of HIV transmission, some homosexual men have adopted a strategy whereby they only have unprotected anal intercourse (UAI) with a person of the same HIV status (known as “concordant UAI”).¹ In London, homosexual men in a relationship are more likely to know the HIV status of their UAI partner than men not in a relationship² and so establish concordance. However, this was not examined for HIV positive and negative men separately. A survey conducted in January-February 2000 among homosexual/bisexual men attending one of six gyms in central London, as part of an ongoing behavioural surveillance programme,^{2, 3} has allowed risk reduction strategies to be considered by HIV status. A total of 792 homosexual men (median age 35 years) completed a confidential questionnaire (estimated response rate 50–60%^{2, 3}); 126 (16.0%) were HIV positive, 477 (60.2%) HIV negative, while 169 (21.3%) had never had an HIV test (data missing for 20 men). Just over half the men (55.2%) said they were currently in a relationship with another man; this did not differ significantly by HIV status (p=0.1).

Our analysis focused on how sexual risk behaviour varied both by HIV as well as by relationship status. For HIV negative and positive men, UAI was classified as either concordant (UAI with a partner of the same HIV status) or non-concordant (UAI with a partner of unknown or discordant HIV status). Men reporting more than one UAI partner were classified as concordant only if all UAI partners were of the same HIV status as themselves. Men also indicated whether they had had UAI with a main partner only, casual partner(s), or both. One third of all men (32.9%, 259) reported UAI in the previous 3 months; HIV positive men 42.1% (53/126), HIV negative 34.7% (165/475, data missing for two men) (p=0.1). Overall, concordant UAI was reported by 18.7% (89) of HIV negative and 21.4% (27) of HIV positive men (p=0.4). For HIV negative men, concordant UAI was predominantly reported by those in a relationship and rarely by men who were not (28.6% v 5.0%, p<0.001) (table 1). Concordant UAI was usually with a main partner alone. By way of comparison, HIV positive men were just as likely to report concordant UAI whether they were in a relationship or not (22.2% v 20.6%, p=0.9), often with a casual rather than main partner. The observation that HIV negative men were more likely to report concordant UAI in the context of a relationship while HIV positive men were just as likely to report concordant UAI whether they were in a relationship or not was confirmed in a multivariate model. With HIV status and relationship as independent variables and concordant UAI as the dependent variable, the interaction between HIV status and relationship was highly significant (p=0.001).

Seroconcordance among negative men can only be established with confidence if both men test for HIV together. For this reason it is difficult for HIV negative men to establish concordance with a casual partner. On the other hand, HIV positive men can establish concordance, be it with a casual or regular partner, simply by mutual disclosure. This requires no confirmatory test. Although seroconcordant UAI among positive men carries no risk of HIV transmission to an uninfected person, it raises the possibility of reinfection and drug resistance for the men themselves.⁴

These data provide further evidence that HIV positive and negative homosexual men have both adopted HIV risk reduction strategies.^{1, 2} None the less, high risk sexual behaviour (that is, non-concordant UAI) was reported. Overall, non-concordant UAI was reported by 15.8% (75) of HIV negative and 20.7% (26) of HIV positive men (p=0.2). No significant differences were seen when stratified by either relationship or HIV status (table 1). In the multivariate model there was no significant association between non-concordant UAI and either HIV status (p=0.4) or being in a relationship (p=0.7).

Non-concordant UAI was usually reported with a casual partner with one notable exception. HIV negative men in a relationship were equally likely to report non-concordant UAI with a main partner alone (8.0%) as with a casual partner (6.5%) highlighting the continuing risk for HIV transmission between regular partners.^{2, 5} However, for most men the risk of HIV transmission occurred in the context of a casual sexual encounter. Surveys conducted in the gyms in 1998 and 1999 revealed similar patterns of sexual risk behaviour (data available from authors).

In conclusion, HIV negative and positive homosexual men have adopted different strategies for reducing the risk of HIV transmission with their sexual partners. HIV negative men predominantly reported concordant UAI with a main partner in the context of a relationship while HIV positive men were more likely to report concordant UAI with a casual partner. HIV prevention programmes need to reinforce risk reduction strategies, tailored to a person's HIV status, while simultaneously addressing high risk sexual behaviour.⁶