The study is published in the Feb. 24 issue of The Lancet.
Researchers conducted a clinical trial enrolling 2,784 HIV negative, uncircumcised men between 18 and 24 years old in Kisumu, Kenya, where an estimated 26 percent of uncircumcised men are HIV infected by age 25. The majority of the men in the study were Luo, an ethnic group that does not traditionally practice circumcision.
Half the men were randomly assigned to circumcision and half the men remained uncircumcised for two years.
Participants received free HIV testing and counseling, medical care, tests and treatment for sexually transmitted infections, condoms and behavioral risk counseling during periodic assessments throughout the study.
The clinical trial found that 47 of the 1,391 uncircumcised men contracted HIV, compared to 22 of the 1,393 circumcised men.
"Our study shows that circumcised men had 53 percent fewer HIV infections than uncircumcised men," said Bailey. "We now have very concrete evidence that a relatively simple surgical procedure can have a very large impact on HIV."
The study also measured adverse events related to surgery. According to Bailey, in 1.7 percent of the surgeries there were minor complications -- usually bleeding or a mild infection. There were no severe adverse events.
Bailey cautions that circumcised men may feel they are protected from becoming HIV infected and may be more likely to engage in risky behavior.
"Circumcision is by no means a natural condom," said Bailey. "We do know that some circumcised men become infected with HIV. But we did find that the circumcised men in our study did not increase their risk behaviors after circumcision. In fact, all men in the trial increased their condom use and reduced their number of sexual partners."
The authors conclude that circumcision will be most effective if it is integrated with other prevention and reproductive health services.
"We have wonderful new drugs to at least maintain people for many years with the virus, but meanwhile more people are getting infected at a rate of approximately 5,000 persons per day," said Bailey. "We cannot treat our way out of this epidemic. Prevention of new infections is crucial."
Simulation models estimate that millions of new HIV infections, tens of thousands of deaths, and several million dollars could be saved if male circumcision became routine in sub-Saharan Africa.
"This is really the first good news we've had in quite a long time. If we can reduce the risk of infection by such a substantial amount then we can save a lot of lives."
In December the National Institutes of Health halted Bailey's trial of male circumcision after the study's independent data safety monitoring board reviewed the preliminary results and recommended that all men enrolled in the study who remain uncircumcised be offered circumcision due to the clearly protective effect.