The National Institutes of Health closed the studies in Kenya and Uganda early, when safety monitors took a look at initial results this week and spotted the protection. The uncircumcised men who took part in the study are being offered the chance to undergo the procedure.

The link between male circumcision and HIV prevention was noted in the late 1980s. The first major clinical trial, of 3,000 men in South Africa, found last year that circumcision cut the HIV risk by 60 per cent. Still, many Aids specialists had been awaiting the NIH’s results as a final confirmation.

“Male circumcision can lower both an individual’s risk of infection, and hopefully the rate of HIV spread through the community,” said the Aids expert Dr Anthony Fauci, the director of the NIH’s National Institute of Allergy and Infectious Diseases.

But it is not perfect protection, Dr Fauci emphasised. Men who become circumcised must not stop using condoms or take other risks, and circumcision offers no protection from HIV acquired through anal sex or injection drug use, he noted.

“It’s not a magic bullet, but a potentially important intervention,” agreed Dr Kevin De Cock, of the World Health Organisation.

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