Circumcising adult men may cut in half their risk of getting the AIDS virus through heterosexual intercourse, the U.S. government saidWednesday, as it shut downtwo studies in Africa testing the link.

The U.S. 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 studies' uncircumcised men are being offered the chance to undergo the procedure.

The link between male circumcision and HIV prevention was noted as long ago as the late 1980s. The first major clinical trial, of 3,000 men in South Africa, found last year thatcircumcision cut the HIV risk by 60 per cent.

Still, many AIDS specialists had been awaiting the NIH's results asa confirmation.

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

But it's not perfect protection, Fauci stressed. Men who become circumcised must not quit using condoms nor 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 Organization.

There was already enough evidence that circumcision is protective, and programs should be ready to go, said Stephen Lewis, the UN's outgoingspecial envoy for AIDS in Africa.

"The international community, and that includes the UN community, needs to learn to move more quickly, because lives are at stake," Lewis told CBC News.

Male circumcision is common at birth in the United States.

In Canada, infant circumcision rates have fallen sharply from almost 50 per cent in the late 1970s to just over nine per cent in 2005. The main routes for HIV transmission in Canada are also from drug use or anal sex, for which circumcision offers no protection.

In sub-Saharan Africa, home to more than half of the world's almost 40 million HIV-infected people, there are large swaths of populations where male circumcision is rare.

WHO wants safe circumcision

The WHO plans an international meeting early next year to discuss the studies' results and how to translate them into policies that promote safe male circumcision— done by trained health workers with sterile equipment— while teaching men that it won't make them invulnerable.

Why would male circumcision play a role? Cells in theforeskin of the penis are particularly susceptible to the HIV virus, Fauci explained. Also, the foreskin is more fragile than the tougher skin surrounding it, providing a surface that the virus could penetrate more easily.

Researchers enrolled 2,784 HIV-negative men in Kisumu, Kenya, and 4,996 HIV-negative men in Rakai, Uganda, into the studies. Some were circumcised; others were just monitored.

Side-effects rare

University of Manitoba researcher Dr. Stephen Moses was co-principal investigator on the trial in Kenya, which was partially funded by the Canadian Institutes for Health Research.

Over two years, 22 of the circumcised Kenyans became infected with HIV compared with 47 uncircumcised men, a 53 per cent reduction. In Uganda, 22 circumcised men became infected vs. 43 of the uncircumcised, a 48 per cent reduction.

The researchers are offering all of the studies' uncircumcised men the chance to undergo the procedure, and 80 per cent ofthe uncircumcised Ugandans already have agreed, said lead researcher Ronald Gray of Johns Hopkins University.

Side-effects, including some mostly mild infections that were easily treated, were rare. The rate of side-effects was comparable to those seen in circumcised U.S. infants, said Robert Bailey, of the University of Illinois at Chicago, who led the Kenyan trial.