Researchers say the findings raise the possibility that vaccination against the virus, known as the human papillomavirus (HPV), could help curb the world's HIV pandemic.

The investigators found that among 2,168 Kenyan men between the ages of 18 and 24, half tested positive for HPV at the start of the study. Over the next 3.5 years, nearly 6 percent of those men became infected with HIV, versus just under 4 percent of those who had tested HPV-negative at the outset.

When the researchers controlled for a number of HIV risk factors, men with HPV were still 80 percent more likely than their HPV-negative counterparts to become infected with HIV, suggesting the genital wart virus itself may boost a person's susceptibility to HIV.

The findings are published in the Journal of Infectious Diseases.

There are more than 100 strains of HPV, some of which cause genital and anal warts. In most people, the immune system clears the infection fairly rapidly. However, persistent infection with certain HPV strains can eventually lead to cancer in some cases.

Persistent HPV infection is the primary cause of cervical cancer, and it can also lead to cancers of anus and penis.

The current findings come from a larger clinical trial that, along with two other trials in Africa in 2005 and 2006, found that circumcision lowered men's risk of acquiring HIV through heterosexual sex. Subsequent studies of those men have shown that circumcision may also lower the odds of HPV infection.

In this latest study, HPV infection itself was linked to a higher risk of acquiring HIV even when the researchers factored in circumcision, as well as the men's reported sexual history and whether they had the genital herpes virus -- which has already been linked to an increased risk of HIV infection.

All of this suggests that HPV vaccination, along with circumcision, could help stem the HIV pandemic, according to lead researcher Dr. Jennifer S. Smith, of the University of North Carolina at Chapel Hill.

Finding a vaccine to prevent HIV is the greatest hope for curbing the world's AIDS pandemic, but so far there is no such vaccine, Smith said in a written statement from the university. However, there is a vaccine to prevent specific types of HPV infection, and vaccinating young men before they become sexually active could potentially help prevent the spread of HIV.

There are two vaccines that can prevent infection with certain cancer-related strains of HPV: Gardasil (from Merck) and Ceravix (from GlaxoSmithKline). In the U.S., the vaccines are approved for girls and young women as young as 9, and up to the age of 26. Last year, regulators approved Gardasil for boys and men in the same age group.

The current findings, according to Smith's team, warrant clinic trials to test whether HPV vaccination can lower the risk of HIV infection.

If vaccination does prove effective, cost could stand as a major obstacle to bringing it to developing nations where HIV transmission rates are high. The required three doses of the HPV vaccines cost roughly $400 in the U.S.

It's not clear why HPV infection might increase the odds of HIV infection, but it is biologically plausible, according to Smith's team. Skin lesions caused by HPV, for example, might act as portals of HIV entry, the researchers note. In addition, HPV may induce the production of certain inflammatory proteins in the genital area, which may in turn boost susceptibility to HIV infection.

The study was funded by the U.S. National Institutes of Health and the Canadian Institutes of Health Research.

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