Ryan,

The Padian Study, which I'm sure you're familar with, concluded that it took on average 1 in 6000 acts of intercourse in order for HIV to be transmitted from a woman to a man. 1 in 6000! That means with every unsafe sexual act of vaginal intercourse with an infected female, one puts oneself at a risk of 0.0001 chance in becoming infected.

If the weatherman said that there was a 0.0001 percent chance of rain, whould you take your umbrella Ryan?

My understanding of Nancy Padian's study is a bit different (Padian N, Shiboski S, Vittinghoff E, Glass S. Heterosexual transmission of HIV: Results from a ten year study. Am J Epidemiol 146:350-357, 1997). In her 10 year study of serodiscordant heterosexual couples (82 infected men and their partners, 360 infected women and their partners), there was approximately a 9 per 10,000 contact risk of HIV transmission to a female (a .00009 risk, which could be rounded to about 1 infection per 1,000 contacts). Male to female transmission was eight times more likely than female to male. Most of the people in the study were white and used condoms with their partners regularly. History of sexually transmitted diseases was strongly associated to transmission.

Nancy Padian herself advises against generalizing the results of this study, which took place in California among mostly white participants, to ALL heterosexual sex in the world: "It's more likely that the epidemiology of a disease would differ in different locations than be the same -- just look at cancer and heart disease." (http://www.thebody.com/schoofs/proof.html).

There is increasing, reliable, scientific evidence that heterosexual men who are uncircumcised have a greater risk of being infected during insertive vaginal intercourse when compared to uncircumcised men. This is clear in many regions in Africa, where the rate of infection is much higher among uncircumcised heterosexual men. The interior of the foreskin is a mucous membrane that contains Langerhans cells, cells that are known to be vehicles for HIV's passage to the immune system. Circumcision is a much more common practice in the United States, which might account for the lower rate of transmission to heterosexual men.

That is not to say that heterosexual men in the United States are at the same risk as men in Africa. U.S. men seem to be at much less risk for infection than heterosexual women and men in other populations. The reasons why that is the case are still unclear. It does not mean that women cannot transmit HIV to their male partners, and that may be a dangerous assumption to make at this point.

So, the weather analogy, while clever, doesn't quite work for me. The abilities of HIV science to predict transmission are still way behind the reliability and accuracy of meteorlogy to predict the weather. And getting wet in the rain is not exactly the same as getting HIV.

RMK