"Sharing towels and razors and certain types of sex may transmit HPV, but the virus doesn't hang around and cause an infection," Widdice said. "So the person can be exposed to the virus but doesn't get infected. When the couple is tested, they don't share the same types."

It's also possible that the immune systems of men and women respond differently to the virus, she said, with men perhaps being able to quickly eliminate it from their bodies.

Dr. Anna Giuliano, who studies HPV, said that while the new study is useful, researchers still need to figure out exactly how and when HPV is transmitted between couples.

"How many sex acts does it take to get a transmission event to occur? And over what period of time within a relationship: two weeks, one month? What is the probability that an infection will be transmitted from one partner to the next? That's what we really need," said Giuliano, chair of the Department of Cancer Epidemiology and Genetics at the H. Lee Moffitt Cancer Center in Tampa, Fla.

That kind of information will shed light on things like the cost effectiveness of vaccinating both men and women against HPV, she said.

SOURCE Journal of Adolescent Health