"This is not common," Chapman said. "Very, very rare. Do not get alarmed about the small chance of contracting anal cancer from Gardasil."

The authors, from the University of Pennsylvania, called for a suspension of the trial and retracted the study. The vaccine is still sold in the U.S. as Gardasil.

Chapman's side is leaning heavily on this news, including research showing that only 1 percent of sexually active men will get anal cancer in their lifetime.

"This is not common," Chapman said. "Very, very rare. Do not get alarmed about the small chance of contracting anal cancer from Gardasil."