From July to October 1999, 291 MSM enrolled in the Vax004 trial at 3 Canadian sites (105 in Vancouver, 99 in Montreal and 87 in Toronto). The median age of participants was 37 years, and most had received some college education (). Significant differences were observed across sites with respect to participants' level of education, reasons for joining the study and recent recreational drug use; as well, site-specific differences were observed with respect to recent diagnosis of STDs and percentage of men reporting sexual relations during the 6 months before enrolment with HIV-positive, HIV-negative and status-unknown partners. Among men who had an HIV-positive or status-unknown partner, the percentage engaging in unprotected receptive anal intercourse with each type of partner was not materially different across sites (site-specific data available from the authors upon request).
During 18 months of follow-up, data for 11% of the men were censored because of seroconversion (n = 19) or because of withdrawal from the trial or loss to follow-up (n = 12). The number of men for whom data were censored before their 6-, 12- and 18-month follow-up visits were 6, 19 and 6, respectively. The percentage of men for whom data were censored at each site differed significantly (Montreal 2/99 [2%], Vancouver 12/105 [11%] and Toronto 17/87 [20%], p < 0.001). Compared with the matched controls (who remained HIV-1 seronegative), MSM for whom data were censored were more likely to report recreational use of amphetamines (), as well as unprotected sex with HIV-positive and status-unknown partners during the 6 months before their enrolment and greater numbers of HIV-positive partners ().
At enrolment, 61 (21%) of the 291 men reported having engaged in unprotected receptive anal intercourse during the previous 6 months with partners whose HIV-1 status was positive or unknown. The prevalence of this behaviour at the 6-, 12- and 18-month follow-up visits was 20%, 20% and 18%, respectively, using the most conservative assumption (that men with censored or missing data had engaged in risky behaviour); the matched-pair odds ratios indicated no significant change in participants' behaviour from baseline to any of these time points (). Furthermore, we did not observe any change in this behaviour among subgroups of participants who did and did not report a motivation to enter the trial for protection from HIV-1 infection, a belief that they had received the vaccine, a belief that vaccine efficacy was greater than 50%, or a belief that they had received vaccine and that vaccine efficacy was greater than 50% (data not shown).
The cumulative percentage of men reporting unprotected receptive anal intercourse with an HIV-positive or status-unknown partner at any point during 18 months of follow-up was 27%. The cumulative probability of engaging in this behaviour at any point during 18 months of follow-up was not significantly greater among men who reported being motivated to enrol in the trial for protection from HIV-1 infection, those who believed they had received vaccine, those who believed that vaccine efficacy was greater than 50%, or those who believed that they had received the vaccine and that vaccine efficacy was greater than 50% (). The cumulative percentage of men who reported unprotected receptive anal intercourse with any partner at any point during follow-up was 40%; 13% of men reportedly engaged in this behaviour with HIV-negative partners only, 14% with HIV-positive or status-unknown partners only, and 13% with both HIV-negative and HIV-positive or status-unknown partners. With regard to the latter group, we did not assess the temporal ordering of such encounters.