The standard method for assessing the risk of HIV infection through heterosexual intercourse could be badly wrong, according to a study to be presented at the International Aids Conference on Tuesday.
The paper, published online late on Monday by the journal The Lancet Infectious Diseases, takes issue with a yardstick widely used by epidemiologists and says it may have helped fuel misperceptions that HIV is hard to catch for heterosexuals.
This yardstick suggests that transmission of the HIV virus occurs on average once with every 1000 acts of heterosexual intercourse between someone who is infected and another who is uninfected.
But the measurement is based on stable couples where there is a low prevalence of risk factors, according to an overview of the published evidence, led by Kimberly Powers of the University of North Carolina.
In other scenarios, these risk factors can multiply the chances of transmission by a factor of between several times and several hundred times, it suggested.
Powers' team found zero transmission after more than 100 acts of penile-vaginal intercourse among a study group of so-called serodiscordant couples ? couples in which one partner has the human immunodeficiency virus (HIV) while the other does not.
At the other end of the range, infection occurred for every 3.1 acts of heterosexual anal intercourse, the investigators found.
These risks could be amplified by other factors, such as men who were not circumcised ? circumcision has been found to provide some protection against infection ? or if a partner had genital ulcers, or was at the early or late stage of HIV infection, when virus levels are higher.
"The use of a single, 'one-size-fits-all' value for the heterosexual infectivity of HIV-1 obscures important differences associated with transmission cofactors," the study said.
The measurement of one infection per 1000 acts of intercourse "seems to represent a lower bound. As such, this value substantially underestimates the infectivity of HIV-1 in many heterosexual contexts," it said.
The study says there remain many blanks in the knowledge about heterosexual infectivity, such as the area of oral sex.
The 17th International Aids Conference opened on Sunday in the Mexican capital, drawing 22 000 doctors, field workers, scientists and policymakers from around the world. It runs until Friday.