What's the probability of HIV transmission?

An HIV-positive German pop star stands accused of having unprotected sex without informing her partners about her status. She is charged with grievous bodily harm and attempted bodily harm.

The case against Nadja Benaissa, 28, former singer in the girl band No Angels, has generated much ethical and legal controversy. There are also some scientific questions.

What’s the probability of a female with HIV transmitting the virus to her male partners through unprotected sex? Is there a difference between female-to-male versus male-to-female transmission?

Girl band singer accused of infecting partner with HIV

“As with many things in clinical science, there is no black and white,” said Dr. Charles Hicks, professor of medicine at the Duke University Medical Center.

The scientific literature provides conflicting findings. Some studies report that there are big differences in virus transmission rates between females-to-males versus males-to-females. Others show that they are about equal.

Despite different findings, Hicks said, “My opinion is that it’s not enormously different—the idea that men very rarely get infected is a dangerous myth.”

“It used to be thought that it was unidirectional with very little female-to-male transmission,” said Dr. John Bartlett, professor of medicine in the division of infectious diseases at the Johns Hopkins University School of Medicine. “There is a large Uganda study that showed it was much more equal. The number that is quoted is 2-to-1; in other words, male to female is twice as efficient or more likely to transmit HIV, as female to male.”

It also depends on viral load (the amount of HIV measured in his blood), which can be suppressed by taking medication and condom use.

It’s a topic that doctors aren’t eagerly discussing, but the rate of transmitting HIV is low during heterosexual intercourse- estimates are from about 1 in 1,000 to as low as 1 in 10,000. Anal intercourse is believed to be more efficient in transmission.

Determining such probability is an inexact science. Safe sex is heavily stressed, because, Bartlett said, not doing so is “Russian roulette."