Estimating the proportion of HIV transmissions from main sex partners among men who have sex with men in five US cities

Published: January 1, 2009

Background: HIV incidence in the United States among men who have sex with men
(MSM) has been increasing since 2000, and MSM remain the most heavily impacted risk
group in the US HIV epidemic.

Methods: We modeled HIV transmissions, using data from MSM in five US cities from
the National HIV Behavioral Surveillance System, the HIVNET Vaccine Preparedness
Study, and other published data. Annual HIV transmissions were estimated by partner
type (main or casual) and by sex type (receptive anal intercourse, insertive anal
intercourse, or oral sex).

Results: Sixty-eight percent [95% confidence interval (CI) 58–78) of HIV transmissions
were from main sex partners because of a higher number of sex acts with main partners,
more frequent receptive roles in anal sex with main partners, and lower condom use
during anal sex with main partners. By sex type, 69% (95% CI 59–79) of infections were
from receptive anal intercourse, 28% (95% CI 19–38) were from insertive anal
intercourse, and 2% (95% CI 0–5) were from oral sex. The model-based estimated
HIV incidence rate was 2.2% (95% CI 1.7–2.7) per year. Sensitivity analyses demonstrated
estimates of transmission from main sex partners as low as 52% (95% CI 41–62)
and as high as 74% (95% CI 68–80).

Conclusion: According to our model, most HIV transmissions among MSM in five US
cities are from main sex partners. HIV prevention efforts should take into account the
risks of HIV transmissions in male partnerships, and couples-based HIV prevention
interventions for MSM should be given high priority in the US HIV prevention research

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