Over 50% of HIV transmissions in US gay men are from main sexual partners, investigators report in a study published in the online edition of AIDS. The researchers also found that 46% of infections were from partners who were believed to be HIV-negative and that approximately a quarter of transmissions were the result of insertive anal sex.

Gay men remain the group most affected by HIV in the US. There is some evidence that the number of new HIV infections in this group has increased since 2000. This has prompted calls for a renewed focus on the continuing HIV epidemic amongst gay men and the development of new prevention interventions.

Research from the Netherlands suggests that most HIV infections from young men are contracted from main sexual partners. To gain a better understanding of the dynamics of HIV transmission amongst gay men in the US, investigators designed a model using information obtained from the National HIV Behavioral Surveillance System (NHBS), involving gay men in five large US cities between 2003 and 2005, and the HIVNET Vaccine Preparedness Study (VPS), which recruited gay men in six US cities between 1995 and 1997.

Data from the NHBS study were used to estimate the number of gay men with main and casual partners in a year, the number of casual partners, the HIV status of partners, and sexual behaviour with partners. Information from the VPS was used to estimate the number of sexual episodes per year for men with main and casual partners.

A total of 3652 men from the NHBS study were included in the investigators’ analysis. These men reported 2395 main partners and 2850 casual partners. Based upon information from the VPS, the investigators estimated that the combined total of sex acts with main partners was 195,000 per year and 175,000 per year for casual partners.

The majority of HIV transmissions (68%) were estimated to be from main partners, with 32% thought to be from casual partners. This trend was observed in all age groups. The investigators also estimated that receptive anal intercourse was the main (69%) mode of HIV transmission. However, they attributed 28% of infections to insertive anal sex and 3% to oral sex.

Of the HIV transmissions from main partners, 46% were from individuals believed to be HIV-negative, with 38% from partners of unknown status, and 16% from partners who were known to be HIV-positive. The bulk (63%) of HIV transmissions from casual partners were from partners of unknown status. The investigators also found that more infections occurred from casual partners thought to be HIV-negative (24%) than HIV-positive (16%).

Annual HIV incidence was calculated at 2.2%.

The investigators believe that there are three reasons why there are more HIV transmissions from main rather than casual partners:
• Men had more sex with their main partner than casual partners.
• Men were more likely to have receptive anal sex with their main partner than a casual partner (40% of men reported that the last sex they had with a main partner involved receptive anal sex compared to 26% for a casual partner).
• Men were more likely to report unprotected receptive anal sex with their main partner than a casual partner.

The finding that 28% of HIV transmissions appear to be the result of insertive anal sex is stressed by the investigators. They comment “prevention counsellors should continue to emphasise that the risk of insertive anal intercourse, although lower than the risk of receptive anal intercourse, is still an important concern for men who have sex with men”.

They also emphasise that “among main partners, transmissions mostly arise from partners believed to be HIV-negative”. Furthermore, they note that individuals of unknown status accounted for most transmissions from casual partners. The group least likely to be responsible for HIV transmission to main or casual partners were men with diagnosed HIV infection.

Although over 50% of new HIV infections in the US are amongst gay men, the investigators note that only 22% of HIV prevention interventions designated as 'best-evidence' by the US Centers for Disease Control and Prevention have been tested with gay men.

“New HIV prevention interventions for US men who have sex with men [MSM] are needed and should be responsive to current behavioral risk patterns and transmission characteristics of the US MSM epidemic”, conclude the investigators. “Our data suggest that the development and testing of HIV prevention interventions for MSM couples, a historically understudied area of HIV prevention, may be important as we approach the fourth decade of the US HIV epidemic.”

Reference
Sullivan PS et al. Estimating the proportion of HIV transmissions from main sex partners among men who have sex with men in five US cities. AIDS Vol. 23 (online edition), 2009.

This article was first published by NAM/Aidsmap.com in May 2009 and is republished with permission.