Experts say the prevalence of male circumcision varies by region in African countries south of the Sahara, with large numbers of men in some areas remaining uncircumcised.

Fauci said while the initial circumcision benefits would be fewer HIV infections in men, the practice could lead to fewer women getting infected in parts of the world like Africa where the virus is passed largely through sex between a man and woman, not homosexual sex.

Another study is underway in Uganda assessing HIV infection risk for women with circumcised partners. Experts say the reduced HIV risk may be because cells on the inside of the foreskin, the part of the penis cut off in circumcision, are particularly susceptible to HIV infection.

HIV also may survive better in a warm, wet environment like that found beneath a foreskin. Fauci said circumcision is not completely protective "and must be seen as a powerful addition to, not a replacement for, other HIV prevention methods."

Fauci said the benefits could be negated by small decreases in condom use by men or if men add more sexual partners. "These results only apply to men where the risk of HIV transmission is through the penis.

Transmission by injection drug use or receptive anal intercourse will not be affected by adult male circumcision," Fauci added. De Cock said public health experts might encounter cultural and social barriers in parts of Africa to male circumcision.

Of the 39.5 million people worldwide infected with the human immunodeficiency virus, 24.7 million are in sub-Saharan Africa.