Sexual risk behaviour of Canadian participants in the first efficacy trial of a preventive HIV-1 vaccine

Background

Phase I and phase II HIV-1 vaccine trials have revealed increases in risky sexual activity among study subjects during the trials, perhaps because the subjects believe that the vaccine being tested is efficacious; subjects may thus suffer harm from their participation. We evaluated the sexual behaviour of Canadian men who have sex with men (MSM) who participated in the phase III Vax004 trial of an HIV-1 vaccine.

Methods

Using self-reports of sexual behaviours during the 6 months before trial entry as a baseline, we determined changes in reported sexual behaviour after 6, 12 and 18 months of participation in the trial.

Results

Of 291 HIV-seronegative MSM enrolled from July to October 1999, 260 (89%) completed 18 months of follow-up, 19 (7%) experienced seroconversion, and 12 (4%) did not complete follow-up. Unprotected receptive anal intercourse during the previous 6 months with partners whose HIV-1 serostatus was positive or unknown was reported by 21% of men at enrolment and by 27% at any point during 18 months of follow-up. No increase in this behaviour from baseline was reported by participants, including among men who were motivated to enrol because of expected protection from HIV-1 infection, men who believed they had received the vaccine, men who believed that the vaccine had greater than 50% efficacy, or men who believed that they had received the vaccine and that vaccine efficacy was greater than 50%.

Interpretation

MSM can be successfully enrolled in HIV-1 vaccine efficacy trials without evident increases in those sexual behaviours most associated with HIV-1 risk.Development of preventive HIV-1 vaccines requires clinical trials that effectively recruit, enrol and retain high-risk subjects, including men who have sex with men (MSM). Since candidate vaccines may prove to have little or no efficacy, these trials must also strive to minimize harms associated with participation. A major concern has been that trial participants might believe vaccination affords some protection and therefore increase their sexual risk-taking.^1,2 This concern derives in part from increases in unprotected anal intercourse observed during phase I and phase II vaccine trials. For example, self-reports of unprotected insertive anal intercourse during the previous 6 months increased among 44 gay men enrolled in San Francisco trials, from 9% at enrolment to 20% at the 12-month assessment; however, the HIV status of sexual partners was not assessed.¹ The world''s first phase III trial to evaluate a candidate preventive HIV-1 vaccine was recently completed in North America and Europe.² A consortium sponsored by the Canadian Network for Vaccines and Immunotherapeutics of Cancer and Chronic Viral Diseases (CANVAC) was formed to assess participation, retention and change in sexual risk behaviour at trial sites in this country. We report here the Canadian experience in this trial through 18 months of follow-up and assess trends in high-risk sexual behaviour reported by participants.