Circumcising men cuts their risk of being infected with the Aids virus in half, and could prevent hundreds of thousands or even millions of new infections, researchers said yesterday.

Circumcising men worked so well that the researchers stopped two large clinical trials in Kenya and Uganda to announce the results, although they cautioned that the procedure does not make men immune to the virus.

Experts say the reduced risk may be because cells on the inside of the foreskin are susceptible to HIV infection.

A US National Institutes of Health study in Kisumu, Kenya, involving 2,784 men aged 18 to 24 showed a 53 per cent reduction of HIV infections in circumcised men compared to uncircumcised men. A parallel study involving 4,996 men aged 15 to 49 in Rakai, Uganda, showed circumcised men were 48 per cent less likely than uncircumcised men to become infected.

Researchers previously had noticed that in places where circumcision is common, HIV was less common.

Results of the first major study on the issue were reported last year in South Africa, with researchers seeing a 60 per cent reduction in HIV risk for circumcised men. Researchers viewed the new trials as strong confirmation.

Public health leaders said the results indicated a potential way to reduce HIV in Africa. "It does have the potential to prevent many tens of thousands, many hundreds of thousands and perhaps millions of infections over coming years," Dr Kevin De Cock, the director of the World Health Organisation's Department of HIV/Aids, told reporters.