If one person in a heterosexual couple has human papillomavirus (HPV), there's a 20 percent chance his or her partner will pick up the virus within six months, a new study concludes.

The study, the largest-yet analysis of HPV transmission rates, found no difference between male-to-female transmission rates and female-to-male transmission rates.

It also found no link between the number of partners in a person's sexual past and their chances of picking up HPV from a current partner.

"There's been very little work done on how frequently HPV transmits," said study author Ann Burchell of McGill University in Montreal. "Most of the work on HPV has revolved around how common it is within a population." Combining the data on transmission and frequency, she said, can help researchers get a fuller picture of how the virus spreads.

The new study was published Oct. 7 in the Journal of Infectious Diseases.

Catching a virus

HPV infects the genitals of both males and females, and can cause genital warts as well as cervical cancer. It's the most common sexually transmitted virus in the U.S. — around fifty percent of sexually active adults will have HPV at some point in their lives. Most cases only last a year or two, but other cases can linger for longer and lead to cancer.

To study how often HPV spread from an infected person to an uninfected sex partner, Burchell recruited college-age women in relationships. She and her colleagues identified 179 couples in which one person was infected with HPV, but the other wasn't. Four months after the study began, Burchell asked the couples to return to the clinic for follow-up testing and questionnaires.

When the researchers tallied the final numbers of who had been newly infected with HPV, they found that the overall probability of transmission was 20 percent over a six-month period. The couples reported having sex four times a week, on average, and 50 percent said they never used condoms. [Should the HPV Vaccine Be Mandatory? Health Experts Weigh In]

Other smaller studies have suggested that HPV more easily spreads from females to males than from males to females. The new study, however, saw nearly identical rates of transmission.

"Our hypothesis is that female-to-male transmission may occur more often, but results in shorter infections, and by the time we saw these couples again, some of those male infections had cleared," Burchell said.

The incremental nature of follow-up visits is a limitation of all studies that look at the natural course of a disease, said Brenda Hernandez, of the University of Hawaii Cancer Center. Hernandez has led ongoing studies looking at the transmission of HPV and how long infections last.

"Ideally, you'd want to be able to sample individuals every single day," she said.

Researchers had also previously hypothesized that those who've had many sexual partners are more likely to have gained immunity to HPV — so they were thought to be less likely to pick up a new HPV infection from a current partner. When someone is infected with a virus, the body often saves antibodies to fight off the virus in the future.

The new study, however, found no correlation between the number of sex partners and immunity.

Hernandez said HPV doesn't necessarily follow the rules when it comes to antibodies. "We've found that only a little over half of females who have an HPV infection develop antibodies," she said. This lack of antibodies could explain why few people develop natural immunity to HPV.

Vaccination against HPV

In 2006, the first vaccine against HPV was approved for use in females, and in 2009, the approval was extended to males. Understanding the transmission rates of HPV, Burchell said, can help researchers understand how the vaccine should be used to stop the spread of the virus.

The more transmissible a virus is, Burchell explained, the more people in a population that need to be vaccinated to keep the virus from spreading.

"These numbers are really important to understand for vaccine program planning," Burchell said. "The better we can understand how HPV moves around the population, the better we can control it."

Burchell said she also wants to study further the length of infections, how antibodies against HPV affect rates and whether the amount of virus in a person's body affects the likelihood of transmission. Continuing, detailed studies of larger populations are needed to fully understand how HPV spreads, said Hernandez. For example, her team has found that HPV can spread from one location on a person to another location without sexual contact.

"We still don't feel that this research is at the point where it is directly translatable to public policy on how to manage HPV," she said.

Pass it on: There's a 20 percent probability of an HPV-infected person passing the virus to an uninfected partner if they're in a sexual relationship for six months.

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