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Padian revisited

Dec 6, 2001

You provide a wonderful servide and are one of the most knowledgeable people on the subject of HIV transmission that I have read. However your interpetation of the Padian study is in error I believe. You state that Padian found that the risk of transmission from female to male was 1 in a 1000 and that male to female transmission was about 8 times more likely. I think if review the study you will find that the rate of male to female transmission was 1 in a 1000 and that female to male transmission was 8 times more difficult. I agree with you that the study shouldn't be generalized to say these numbers are absolute but that is what this study found. Also you state that most of the people used condoms. The study reported that 23 couples where the woman was HIV positve never used condoms. Out of 82 couples where the women was infected, 2 men became infected in 10 years. One couple had STD's involved and another had a great deal of bleeding. Again you are right in not applying this study to other situations are to draw the conclusion that female to male transmission is so rare. But those are the results of this particular study.

Response from Mr. Kull

Thank you for pointing out my error! I'm always thankful for the intelligent and thoughtful proofreaders out there; it keeps me on my toes! I've gone back and made appropriate corrections.

The abstract of the study actually says that male-to-female transmission is eight times more efficient, and not, as you state, female-to-male is eight times more difficult (it's a subtle difference). Most of the couples did use condoms, condom use increased over time, and at the end of the study 74% of the participants said they used condoms.

I've included the abstract in this response for people to read:

"NS Padian, SC Shiboski, SO Glass and E Vittinghoff Department of Obstetrics, Gynecology and Reproductive Sciences, University of California, San Francisco, USA.

To examine rates of and risk factors for heterosexual transmission of human immunodeficiency virus (HIV), the authors conducted a prospective study of infected individuals and their heterosexual partners who have been recruited since 1985. Participants were recruited from health care providers, research studies, and health departments throughout Northern California, and they were interviewed and examined at various study clinic sites. A total of 82 infected women and their male partners and 360 infected men and their female partners were enrolled. Over 90% of the couples were monogamous for the year prior to entry into the study; < 3% had a current sexually transmitted disease (STD). The median age of participants was 34 years, and the majority were white. Over 3,000 couple-months of data were available for the follow-up study. Overall, 68 (19%) of the 360 female partners of HIV-infected men (95% confidence interval (CI) 15.0-23.3%) and two (2.4%) of the 82 male partners of HIV- infected women (95% CI 0.3-8.5%) were infected. History of sexually transmitted diseases was most strongly associated with transmission. Male-to-female transmission was approximately eight-times more efficient than female-to-male transmission and male-to-female per contact infectivity was estimated to be 0.0009 (95% CI 0.0005-0.001). Over time, the authors observed increased condom use (p < 0.001) and no new infections. Infectivity for HIV through heterosexual transmission is low, and STDs may be the most important cofactor for transmission. Significant behavior change over time in serodiscordant couples was observed."

Let's not get too bogged down with the details of this, or any study. I totally agree that this study found that transmission of HIV from a female to a male during intercourse was highly unlikely, and this is important for our understanding of transmission phenomena.

We have to look at the whole picture and not base our prevention message on one piece of information. Some studies conducted in Africa have shown that the difference between transmission rates to men and women during heterosexual sex are not that significant. In the world outside of the United States, AIDS remains a predominantly heterosexual disease. I'd like to emphasize the implications that African studies on viral load, STDs and circumcision may have in our understanding of HIV transmission (especially female-to-male, which sustains the high rates of heterosexual transmission).

Look at another thing that Nancy Padian herself says about this study: "My fear is that some men will think this is a green light," said Padian. Even if the risk is lower for men, "you don't shoot craps with something that kills you."


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