One per cent of individuals receiving post-exposure prophylaxis (PEP) after a possible sexual exposure to HIV went on to become infected with the virus, according to a study conducted in California and presented as a poster to the Eleventh Conference on Retroviruses and Opportunistic Infections in San Francisco.
In two separate studies conducted in 1997 and 2002, investigators from the AIDS Research Institute at the University of California, San Francisco, looked at the efficacy of PEP at preventing infection with HIV following a possible exposure during sex or from injecting drug use. A total of 702 individuals were included in the investigators' analysis. All individuals were tested for HIV after 12 weeks to see if they had been infected with the virus.
As well as seeing how effective PEP was at preventing infection with HIV, the investigators also wished to see how well individuals adhered to therapy and if they engaged in any further HIV risk behaviour after receiving PEP.
Individuals were provided with a PEP regimen consisting of two nucleoside analogues (AZT and 3TC, or d4T and 3TC, or d4T and ddI). When an individual’s partner was known to be HIV-positive with a detectable viral load, a triple regimen including two nucleoside analogues and the protease inhibitor nelfinavir was prescribed.
Of the 702 individuals included in the investigators' analysis, seven (1%) seroconverted. All were gay men, with a median age of 33 years. All the men had requested PEP after having unprotected receptive anal sex with another man (compared to 50% of non-seroconverters, p=0.03), and in four instances their partners were known to be HIV-positive.
All seven seroconverters were provided with their PEP medications within 72 hours of exposure to HIV (median 45.5 hours). Only four of the seven individuals seroconverting were assessed as having excellent adherence to their PEP regimens. Two individuals had poor adherence and one patient’s adherence was assessed as fair.
The investigators also established that all seven individuals had engaged in unprotected anal sex in the six months prior to being provided with PEP. In addition, in the period between being provided with PEP and seroconversion, one individual reported additional high-risk behaviour with an individual known to be HIV-positive and two reported high-risk sex with men of unknown HIV status.
At the time HIV seroconversion was confirmed, six patients had an undetectable HIV viral load and no evidence of resistance to anti-HIV drugs. The other individual had a low viral load (589 copies/mL), and went on to develop the M184V resistance mutation. This patient started PEP within 14 hours of exposure and reported one additional potential HIV exposure within a week of starting PEP.
The investigators conclude that PEP for sexual exposure to HIV is probably not 100% effective. However, studies of the efficacy of PEP for non-occupational exposure to HIV are difficult to design and evaluate, given the high prevalence of HIV risk behaviour both prior to the initiation of PEP and after PEP was prescribed and seroconversion detected.
Further information on this website
Roland ME et al. Seroconversion following non-occupational post-expsure prophylaxis. Eleventh Conference on Retroviruses and Opportunistic Infections, San Francisco, poster 888, 2004.