Circumcision, which has helped prevent AIDS among heterosexual men in Africa, doesn't help protect gay men from the virus, according to the largest U.S. study to look at the question.
The research, presented at a conference Tuesday, is expected to influence the government's first guidance on circumcision.
Circumcision "is not considered beneficial" in stopping the spread of HIV through gay sex, said Dr. Peter Kilmarx, of the U.S. Centers for Disease Control and Prevention.
However, the CDC is still considering recommending it for other groups, including baby boys and high-risk heterosexual men.
UNAIDS and other international health organizations promote circumcision, the cutting away of the foreskin, as an important strategy for reducing the spread of the AIDS virus. There hasn't been the same kind of push for circumcision in the United States.
For one thing, nearly 80 percent of American men are already circumcised — a much higher proportion than most other countries. Worldwide, the male circumcision rate is estimated at about 30 percent.
Also, while HIV spreads primarily through heterosexual sex in Africa and some other parts of the world, in the United States it has mainly infected gay men. Only about 4 percent of U.S. men are gay, according to preliminary CDC estimates released at the conference this week. But they account for more than half of the new HIV infections each year.
Previous research has suggested circumcision doesn't make a difference when anal sex is involved. The latest study, by CDC researchers, looked at nearly 4,900 men who had anal sex with an HIV-infected partner and found the infection rate, about 3.5 percent, was approximately the same whether the men were circumcised or not.
Government recommendations on circumcision are still being written and may not be final until next year, following public comment. CDC doctors and many experts believe there is a good argument for recommending that baby boys and heterosexual men at a higher risk for HIV be circumcised.
The definition of "high risk" is still being discussed, said Kilmarx, chief of the epidemiology branch in the CDC's HIV division.
Circumcision is a sensitive issue laden with cultural and religious meaning, particularly when babies are involved, Kilmarx acknowledged.
"It's seen by many as more than just as medical procedure," he said. It's possible the government won't make recommendations but instead will promote an education campaign for parents about the procedure's potential benefits and risks, he added.
The prospect of the government promoting circumcision of infants has already drawn fire from an advocacy group called Intact America. The organization, based in Tarrytown, New York, parked a motorized billboard this week outside the hotel hosting the HIV conference, displaying the message: "Tell the CDC that circumcising babies doesn't prevent HIV."
"It's removing healthy, functioning, sexual and protective tissue from a person who cannot consent. You're mutilating a child," said Georgeanne Chapin, the group's executive director.