TORONTO - When it comes to human papillomavirus infection, it appears the adage "it only takes one'' is right on the mark.
A new study of university students shows that nearly one-third of women who reported having ever had only one male sexual partner were infected with an HPV within a year of starting that sexual relationship.
Three years into those partnerships, nearly 50 per cent of the women had been infected at least once, despite the fact they'd still only had a single sexual partner.
"This paper shows that even just with one partner there's a high risk of infection,'' lead author Dr. Rachel Winer said from Seattle, where she teaches at the University of Washington.
"It's unlike other STDs (sexually transmitted diseases) where . . . the virus or bacteria is in core (population) groups. HPV is different in that it's just very common among everyone who's having sex. So even just being exposed to one partner makes you susceptible to infection.''
The study, which was supported by the U.S. National Institutes of Allergy and Infectious Diseases, will be published this week in the Journal of Infectious Diseases.
The findings underscore the wisdom of offering HPV vaccine to girls before they've begun to have sex, said Dr. Monika Naus, director of the immunization program at the British Columbia Centre for Disease Control in Vancouver.
"What this confirms is that you shouldn't wait until you've had one or two partners before you consider HPV vaccine, because there is a risk even with that first partner,'' said Naus, who was not involved in the study.
She noted that data from British Columbia -- drawn from a survey of adolescents and teens -- suggests that by age 12, four per cent of boys and 3.1 per cent of girls have already had sexual intercourse. By age 16, the rate rises to 28.3 per cent of males and 33.4 per cent of females.
The HPV vaccine currently on the Canadian market -- Merck Frosst's Gardasil -- is licensed for use in females aged nine through 26. The National Advisory Committee on Immunization recommends it be given to girls between ages nine through 13, suggesting at this age most Canadian girls aren't yet sexually active.
Four provinces have rolled out publicly funded HPV vaccine programs. Newfoundland and Labrador and Prince Edward Island offer the shots to girls in Grade 6, Nova Scotia to girls in Grade 7 and Ontario to girls in Grade 8.
In the research, Winer and colleagues were looking at an older group of subjects -- women who waited until they were in university to begin having sex.
But a previous study in younger British teenagers found strikingly similar results. The authors of that work followed a group of female teenagers aged 15 to 19 who had just started their first sexual relationship. Among those who had still only had one sexual partner three years later, the HPV infection rate was 46 per cent.
The new research followed 125 women aged 18 to 22 who hadn't had sex yet or had had their first intercourse with a single male partner in the three months prior to the start of the study.
The women were asked to keep Web-based diaries of their sexual activities and to offer their best guess of how many previous sexual partners their boyfriend had had. They also had gynecological examinations every four months.
The researchers stopped collecting data from women in the group if they reported they had started a sexual relationship with a second partner.
The fact that some women were newly infected two and even three years into their first sexual relationship could have been due to a slacking off in condom use, the authors suggested.
Other factors could have been at play as well. The males could have had sex with someone else, the women could have had other partners they didn't report or they could have had sexual contact -- stopping short of intercourse -- that allowed transmission to occur.
The rate of infections among women who guessed their boyfriends had had more than two previous sexual partners was higher than among women who guessed their partners were less experienced.
"That's one of the strong findings that came out of this work: That the more partners the woman's male partner had had, the greater the chance that she got infected with the HPV virus,'' said Ann Burchell, a PhD candidate at McGill University whose own work focuses on sexual transmission of HPV.
Burchell, who wasn't involved in the research, said it also shows that those who suggest women can avoid HPV infection by having only one sexual partner in their lifetime aren't taking an important variable into consideration.
"Telling a women just to have one partner in her whole life and be monogomous is also not necessarily protective,'' she said. So even if a woman waited until marriage and that was the only partner she ever had, she still may get an HPV infection through that partner.''
The reality of human papillomaviruses is that they are out there, Burchell said.
"The analogy that I've heard about HPV is that it's the common cold of the STI (sexually transmitted infections) world. If you leave the house in the winter you're probably going to catch a cold at one point. And HPV is very much like that.''
"If you're sexually active, you're quite likely to get it at at least one point in your life.''
