Men who have sex with men (MSM) proved twice as likely as heterosexuals to be infected with multiple HIV-1 variants during a single exposure, according to results of a study by Beatrice Hahn and George Shaw at the University of Alabama and colleagues at other centers.

Hahn and Shaw believe their findings are “consistent with the demonstrably higher epidemiological risk of virus acquisition in MSM and could indicate a greater challenge for HIV-1 vaccines than previously recognized.”

The biology of sexual HIV-1 transmission—vaginal, rectal, penile, or oral—differs between MSM and heterosexuals, and that difference could affect prevention measures. For example, circumcision sharply lowers the risk of HIV acquisition by heterosexual men but does not protect MSM nearly as well, if at all.

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