Circumcision can halve the risk of a man acquiring the HIV infection that leads to Aids, US scientists reported last week. Two major trials, in Kenya and Uganda, have confirmed what doctors and campaigners have thought for several years.

The results have important implications for the fight against Aids in Africa and Asia. Kevin de Cock, head of the World Health Organisation (WHO) HIV/Aids department, said it could reduce the numbers of infected men by "many tens of thousands, many hundreds of thousands and maybe millions over coming years".

The two trials should have gone on into next year but were called to an abrupt halt by the funder, the National Institute of Allergies and Infectious Diseases (NIAID) after an interim review showed a halving of the risk of infection among those circumcised. Now that the point is proven to the satisfaction of scientists, it would be unethical to continue. All the uncircumcised participants will be offered the procedure.

With a vaccine still decades away, the circumcision results are the best news in a long while out of the Aids pandemic. But there are questions still unanswered.

Anthony Fauci, director of NIAID, said the 48% reduction among men in the trail in Rakai, Uganda, and the 53% reduction among those in Kisumu, Kenya, "could be negated by small reductions in condom use or the addition of additional sexual partners". It was vital, he warned, that people understood the need to continue to protect themselves by condom use and safe sex.

There was no sign that the 2,784 men in the Kenyan trial and the 4,996 men taking part in Uganda had become reckless in their sexual practices, said Dr Fauci, "but now the announcement is out, we are cognisant that there could be [an effect]".

There are other serious issues. Circumcision was carried out by skilled medical professionals in the trials and all those involved had aftercare in case of complications. The WHO intends to tell governments to ensure that circumcision is carried out in a similar fashion. But that will necessitate setting up clinics with skilled staff. It is likely that, now the research findings are out, a booming trade in adult male circumcision will develop among those who have no medical qualifications.

There are also cultural obstacles to overcome, because for some groups circumcision is not normal practice. In India Muslims are circumcised while Hindus are not.

Scientists say there are several biological reasons why circumcision may decrease the transmissibility of HIV. The mucosal surface of the foreskin contains large numbers of cells particularly susceptible to being targeted by the virus and the area under the foreskin is moist.