Circumcision may somehow protect men from sexual transmission of the AIDS virus, researchers said on Sunday, but they admitted they do not have a clue why.
A study in Uganda aimed at examining how couples infect one another found two things seemed to protect people - being older and being circumcised.
"Acquisition of HIV did not occur in any of the circumcised men," Dr. Thomas Quinn of Johns Hopkins University in Baltimore, who led the study, told the 7th Conference on Retroviruses and Opportunistic Infections, a meeting of AIDS researchers.
"Age, independent of viral load, appeared to have a protective effect," Quinn added. The highest transmission rate was in people 15 to 29 years old.
Quinn's team, working with 15,000 people in the Rakai district of Uganda, also found that people did not pass on the virus to their partners if they had a naturally low level of HIV in the blood - in this case, 1,500 copies according to standard measures.
He found that the more virus people had in their blood, the more likely they were to pass it on. There were no differences in women infecting men or men infecting women.
Quinn said his team was one of the first to actually go out and test the idea where HIV is raging the worst. More than 23 million people in sub-Saharan Africa are infected with HIV.
The findings might suggest ways of stemming the epidemic.
Telling people to abstain from sex or use condoms has not worked, and the drugs that keep the virus at bay in some patients in rich countries are not available in the poor countries hardest hit by the epidemic.
But the study suggests that using drugs to keep the virus at lower levels, or a vaccine that might do the same without quite curing a patient, might help.
Quinn said he was at a loss to explain why circumcision might affect a man's risk of being infected by a woman.