Circumcision reduces the risk of HIV infection by half, according to a new study conducted among nearly 8,000 adult males in Kenya and Uganda, researchers reported Wednesday.
Circumcision proved so effective that the study was halted a year early and the procedure was offered to all study participants.
Previous research has suggested that circumcision is beneficial, but the new trial is "definitive," according to Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, which cosponsored the study with the Canadian Institutes of Health Research.
"It's not a magic bullet," said Dr. Kevin De Cock, director of the World Health Organization's department of HIV/AIDS, but it has the potential to prevent "many hundreds of thousands and perhaps millions of infections over coming years."
The findings, which apply only to heterosexual transmission of HIV from women to men, will have less impact in the United States, where nearly 80% of males are circumcised and homosexual contact between men still plays a major role in transmission of the virus.
But they could have a major effect in the rest of the world, where heterosexual contact is the major form of transmission.
Nonetheless, experts cautioned that circumcision does not eliminate risk. The procedure "has to be integrated with all the other things that we do to prevent new HIV infections," said epidemiologist Robert Bailey of the University of Illinois at Chicago, who led the Kenya study.
Circumcision rates vary widely in Africa. The average is just over 60% for the entire continent, but less than 20% in South Africa, where the AIDS epidemic is most severe.
Early small trials of circumcision had shown a beneficial effect from the procedure, in which the foreskin is removed from the penis, but experts feared that the benefits might be related to cultural biases rather than the procedure itself.
Even a 2005 study of more than 3,000 men in South Africa, which showed that circumcision reduced HIV transmission by 60%, failed to move funding agencies, which feared that the findings could not be reproduced at other sites.
The new results appear to have tipped the balance. UNAIDS and the WHO said Tuesday that they would rapidly convene a panel of experts to determine how to implement the findings.
The Ugandan trial in Rakai enrolled 4,996 men ages 15 to 49, half of whom underwent circumcision. All the men in both studies also received counseling on HIV risk reduction and were advised to wear condoms.
