Circumcising men cuts their risk of being infected with HIV in half, and could prevent hundreds of thousands or even millions of new infections, researchers say.
Circumcising men worked so well that the researchers stopped two large clinical trials in Kenya and Uganda to announce the results, although they caution that the procedure does not make men immune to the virus.
Public health leaders hailed the results as pointing to a potentially powerful way to reduce HIV infections in Africa, the continent hardest hit by AIDS.
"It does have the potential to prevent many tens of thousands, many hundreds of thousands and perhaps millions of infections over coming years," says Dr Kevin De Cock, director of the World Health Organization's Department of HIV/AIDS.
The US National Institutes of Health (NIH) study in Kisumu, Kenya, involving 2784 men aged 18 to 24 showed a 53% reduction of HIV infections in circumcised men compared to uncircumcised men.
A parallel study involving 4996 men aged 15 to 49 in Rakai, Uganda, showed circumcised men were 48% less likely than uncircumcised men to become infected.
Researchers previously noticed that in places where circumcision is common, HIV is less common.
Results of the first major study on the issue were reported last year out of South Africa, with researchers seeing a 60% reduction in HIV risk for circumcised men. Researchers view the results of the new trial as strong confirmation.
Dr Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases says the institute ended both trials early and offered circumcision to all men involved in them. The trials began in 2005 and were due to go until mid-2007.
HIV prevention
"These results indicate that adult male circumcision could be an important addition to an HIV prevention strategy for men. Male circumcision can lower both an individual's risk of infection and hopefully the rate of HIV spread through the community," Fauci says.
Experts say the prevalence of male circumcision varies by region in African countries south of the Sahara, with large numbers of men in some areas remaining uncircumcised.
Fauci says while the initial circumcision benefits would be fewer HIV infections in men, the practice could lead to fewer women getting infected in parts of the world like Africa where the virus is passed largely through sex between a man and woman, not homosexual sex.
Another study is under way in Uganda assessing HIV infection risk for women with circumcised partners.
Experts say the reduced HIV risk may be because cells on the inside of the foreskin, the part of the penis cut off in circumcision, are particularly susceptible to HIV infection.
HIV also may survive better in a warm, wet environment like that found beneath a foreskin.
Circumcision just part of the story
Fauci says circumcision is not completely protective "and must be seen as a powerful addition to, not a replacement for, other HIV prevention methods".
He says the benefits could be negated by small decreases in condom use by men or if men add more sexual partners.
"These results only apply to men where the risk of HIV transmission is through the penis. Transmission by injection drug use or receptive anal intercourse will not be affected by adult male circumcision," Fauci adds.
De Cock says public health experts might encounter cultural and social barriers in parts of Africa to male circumcision.
Of the 39.5 million people worldwide infected with the human immunodeficiency virus, 24.7 million are in sub-Saharan Africa. About 25 million people have died from AIDS since it was first identified a quarter century ago.
