Sex with older partners is associated with a significantly increased risk of HIV infection for younger gay men, and other men who have sex with men (MSM), investigators from North Carolina report in the online edition of the Journal of Acquired Immune Deficiency Syndromes.
After taking into account other risk factors for infection with HIV, the investigators found that a man aged under 23 who reported a sex partner five years older than himself, more than doubled his risk of HIV.
“This study demonstrates that selection of an older sex partner is significantly associated with primary HIV infection among a sample of young men who have sex with men in North Carolina”, comment the investigators.
Gay and other men who have sex with men remain one of the groups most affected by HIV. Prevalence increases in this population according to age. Research conducted in the US in the mid 1990s found that men who reported older male sex partners were significantly more likely to be HIV-positive.
Almost two-thirds of new HIV diagnoses among gay and other men who have sex with men in North Carolina are amongst the under 25s.
Therefore, investigators from the Social Networks and Partnership Study (SNAP) undertook a study to see if the age of sex partners was a risk factor for recent infection with HIV.
A total of 74 men were recruited to the study, twenty of whom had recently been infected with HIV. The 54 men were HIV-negative.
The men provided socio-demographic information, details of their sexual and drug use behaviours, and the age of their last three sex partners.
Three-quarters of the men were under 25 years of age. The median age of those with recent HIV infection was 25 years, whereas the HIV-negative men had a slightly lower median age of 22.5 years.
There were significant racial differences between the two study groups. Men with HIV were significantly more likely to be non-white than the HIV-negative men (60 vs 33%, p = 0.03).
Men who reported having sex after using drugs or alcohol were more likely to have been recently infected with HIV (p < 0.001). Unsurprisingly, unprotected anal sex with an HIV-positive partner, or partner of unknown status, was a significant risk factor for recent infection with HIV (p < 0.05). The investigators also found that the men with primary infection were more likely to report sex with a partner who was known to have HIV (p = 0.01).
Age also emerged as a significant risk factor.
The partners of men in recently infected with HIV were on average six years older than the partners of the HIV-negative men (30 years vs 24 years, p < 0.01).
HIV-negative men reported sex partners whose mean age was within four months of their own. However, the mean age of the partners of the HIV-positive men was almost four years older.
After adjusting for race, sex under the influence of drugs or alcohol, and serodiscordant intercourse, the investigators found that the risk of infection with HIV was doubled if a man reported a sex partner five years his senior (95% CI: 1.2 to 3.3).
The men were then divided into two groups: those aged under and those over 23.
A man aged 23 or younger had a 2.5-fold increase in the risk of HIV if he had a sexual partner aged 28 or above (95% CI, 1.2 to 5.4). For a man aged 24 years and above, the risk was increased twofold (95% CI, 0.06 to 6.1), but this was not significant.
“In summary, young men who have sex with men in North Carolina who select older sex partners have a significantly greater odds of acquiring HIV, even after controlling for specific high-risk behaviors”, conclude the investigators.
In an accompanying Invited Response article the epidemiological modellers, Brian Coburn and Sally Blower, note that simply based on the age distribution of men who have sex with men, young men are more likely to choose an older partner than one of their own age.
Modelling based on the San Francisco population by Sally Blower published in 1997 found that young MSM were four times more likely to choose an older partner than one in their own age range, and the authors' own analysis of the risk of HIV acquisition in the San Francisco Men's Health Study found that, for a man aged below 30, the risk of HIV acquisition doubled if any of his partners was aged over 30, and increased fivefold if all his partners were aged over 30.
They comment: "Public health officials need to make MSM aware that age mixing is a risk factor for HIV infection. Furthermore, young MSM need to be told the magnitude of the risk of HIV infection due to age mixing in comparison with the magnitude of risk due to other traditional risk factors... Hopefully the study will cause this unacknowledged risk factor to receive the attention it deserves."