The study, which followed infection rates in more than 3,000 heterosexual men over nearly two years, found that circumcision reduced a man's risk of acquiring HIV by 60%. Scientists had suspected circumcision might offer some protection against the virus after noticing differences in HIV infection between groups where circumcision was a cultural right of passage and others where few were circumcised. Until now, no large-scale studies had been carried out to investigate the effect.  Adrian Puren at the National Institute for Communicable Diseases in Johannesburg and a team of researchers in Paris recruited 3,274 uncircumcised volunteers from South Africa aged 18 to 24, who were considering circumcision. Half underwent the operation. The researchers then monitored both groups for HIV infection over the next 21 months. So marked was the difference in infection between the groups that the study was halted on ethical grounds. Of those who had been circumcised, 20 tested positive for HIV while 49 of the uncircumcised group had contracted the virus.  Writing in the journal Public Library of Science Medicine today, the authors say that circumcision appears to reduce the risk of acquiring HIV by 61%, "equivalent to what a vaccine of high efficacy would have achieved".

Other scientists sought to clarify that while circumcision appeared to offer a protective effect over the duration of the study, a high-quality vaccine would be more effective in the longer term. Peter Cleaton-Jones, the chairman of the human research ethics committee at the University of Witwatersrand in Johannesburg, said: "Circumcision is not going to prevent HIV infection in the long run. If circumcised men think they're protected against HIV, they're fooling themselves. If they don't practise safe sex, they'll still be at risk, it's just a lower risk." Why circumcision should offer some protection is not well understood, but researchers know that the part of the foreskin that is removed in the operation is rich in Langerhans cells that the virus strongly attaches to. "HIV has to gain access to the body and to do that it binds to particular cell types," Dr Puren said. "By removing the skin that contains those cells, you remove the tissue the virus would normally bind to." Similar trials are ongoing in Kenya and Uganda and are expected to end within the next year.

The World Health Organisation said: "If male circumcision is confirmed to be an effective intervention to reduce the risk of acquiring HIV, this will not mean that men will be prevented from becoming infected with HIV during sexual intercourse through circumcision alone. Nor does male circumcision provide protection for sexual partners against HIV infection."  Will Nutland of the Terrence Higgins Trust said circumcision would not have much effect in the UK, where most new infections were in homosexual men who had receptive anal sex.

British scientists yesterday announced a separate trial to test a microbicide developed to stem the spread of HIV. Thousands of women in Uganda and South Africa are being enrolled to test Pro 2000, a gel applied to the vagina, that has been shown to block the entry of the virus. The study will be coordinated by the MRC Clinical Trials Unit and Imperial College London.