CHICAGO -- A study of heterosexual couples in Africa concludes that the chance of catching the AIDS virus from a single sexual encounter with an infected person is one in 588.
This risk is calculated for people who do not use condoms and who have sex regularly with one infected partner.
Experts have long assumed that the risk of a single heterosexual encounter in North America and Europe is about one in 1,000, and the figures that emerge from the new African data are similar.
In this study, researchers followed 174 sexually monogamous couples in Rakai, Uganda, in which one partner had HIV and the other did not. They were given condoms but usually did not use them. Typically the couples had sex nine or 10 times a month, and over time, 38 people became infected.
Earlier data from the same research team showed that the risk of people transmitting HIV is slight if the amount of virus in their bloodstream is low. Those findings have encouraged the belief that the wide use of AIDS drug combinations, which make virus levels fall dramatically, will slow the spread of the disease.
The latest figures were presented by Dr. Ronald H. Gray of Johns Hopkins University at the Eighth Annual Retrovirus Conference in Chicago, which concluded Thursday.
Among the findings:
--Infected teen-agers are three times more likely than people over 40 to spread HIV to others during each sexual encounter. This difference cannot be explained by the fact that young people are more sexually active.
--The risk that an HIV-infected woman will transmit the virus to an uninfected man is one in 454. For an infected man to an uninfected woman, it is one in 769. This difference is not large enough to be statistically meaningful, and many have assumed that HIV spreads more readily from men to women than vice versa.
--The risk of spread depends greatly on how much virus people carry. In those whose level of virus is less than 1,700 copies per milliliter of blood, the risk is one in 10,000. When levels are over 38,500, risk is one in 294.
--The risk of transmission appears to be the same for different subtypes of virus. Some have speculated that AIDS is much more prevalent in Africa because a different variety of the virus dominates there.
--None of the circumcised men in the study caught HIV. Some experts have raised the possibility of promoting circumcision as a way to control the epidemic.
Whether the transmission risk is the same among couples outside Africa is unclear, especially since virus levels may be higher in Africa, where so few infected people get treated. However, Dr. Helene Gayle, AIDS chief at the U.S. Centers for Disease Control and Prevention, said the latest data at least offer a general estimate of this risk.