More than a quarter of teenage girls and women in the United States may have a common sexually transmitted virus. And most may not even know they have it.
A study published today in the Journal of the American Medical Association, which looked at nearly 2,000 women and girls aged 14 to 59 years, showed 27 percent overall were infected with one or more strains of human papillomavirus (HPV), making it the most common sexually transmitted disease (STD) in the United States.
The research also found that HPV is even more common in younger age groups, with nearly half of all women in their early 20s infected.
According to the study authors, "Our data indicate that the burden of prevalent HPV infection among women was higher than previous estimates."
Recent debate over proposed state programs mandating vaccinations with the HPV vaccine Gardasil for preteen girls has brought the virus into the national spotlight.
Public health experts say the move could protect many of these girls from cervical cancer, which can be caused by certain types of the virus. About 10,000 women in the United States are diagnosed with cervical cancer each year, and 3,700 women die from it.
Approximately 2 percent of women in the study were infected with one of the cancer-causing subtypes. Not all women who are infected with these subtypes will get cervical cancer, but the infection puts them at greater risk.
Many doctors say the findings are not shocking.
"We already knew this infection was common," says Dr. Kevin Ault, associate professor of gynecology and obstetrics at Emory University School of Medicine. "This just adds urgency to getting the vaccine to adolescents and young women."
Dr. Lisa Jones, a gynecologist in New Bedford, Mass., says she hopes the media coverage of HPV and its vaccine will help educate people about how widespread the virus is.
"It is not just 'bad girls' that get HPV," Jones says. "Even women with one partner in their lifetime are also at risk; all it takes is for their partner to have had one other partner."
But even though the findings were little surprise to doctors, they could come as a shock to the public.
"This is a more recent study, and it more accurately reflects what we're seeing in terms of sexual behavior," said Dr. Carol Brown of Memorial Sloan-Kettering Cancer Center in an interview with ABC medical correspondent John McKenzie.
And the figures could have the greatest implications for younger women. Researchers found that among females 14 to 24 years of age, 34 percent were infected with HPV. That suggests 7.5 million teens and young women infected nationwide -- much more than the 4.6 million in previous estimates.
"I think it's really shocking that the virus is so common," Brown told McKenzie.
"It horrifies me. I have a daughter, 18, and a daughter, 22."
Public health experts say information about the prevalence, or infection rate, of HPV is helpful at a time when young women are starting to be vaccinated against the disease.
In an editorial that accompanies the study, Susan Weller and Dr. Lawrence Stanberry of the Sealy Center for Vaccine Development in Galveston, Texas, say the study may provide a helpful baseline to later determine whether or not vaccination programs work.
Such findings may also help determine whether it is cost effective to vaccinate all young women, Weller and Stanberry say.
The vaccine targets four specific subtypes of the virus -- types 6, 11, 16 and 18.
HPV types 16 and 18 cause 70 percent of cervical cancer cases. Types 6 and 11, on the other hand, while much more common are responsible only for genital warts.
The infection rate of the four HPV types contained in the HPV vaccine was 3.4 percent, which suggests that 3.1 million women in the United States may be infected with these strains.
Much of the debate surrounding the HPV vaccine concerns the young age at which females are recommended to receive the shots.
The Centers for Disease Control and Prevention (CDC) currently recommends that all girls ages 11 to 12 be vaccinated. This is because the vaccine works best if the recipient has not yet been exposed to the virus.
According to a CDC spokesperson, the results of the study will not change their vaccination recommendation.
However, some opponents of mandatory vaccination programs for preteen girls argue that vaccinating these girls may encourage them to be sexually active at a younger age.
But previous studies have shown that some girls even younger than 14 are infected with HPV. Dr. Diane Harper, a gynecologist at Dartmouth Medical School, reports, "There is no one age at which all females are not infected with HPV."
And in the current study, at least one of the four HPV types in the vaccine was detected in 6.2 percent of females ages 14 to 19 years.
Harper says vaccination against HPV does not provide 100 percent protection against cervical cancer, and women still need to have regular Pap smears as recommended by their doctor to allow for early detection of changes in the cervix.
"Vaccination is not a substitute or a replacement for Pap testing," she says. "Vaccination without Pap testing will lead to an increase in cervical cancers in the U.S."
Dr. Steven Goldstein, professor of obstetrics and gynecology at the New York University School of Medicine, agrees. "The Pap smear is the greatest triumph epidemiologically in modern medicine, and this whole HPV vaccine threatens to undermine that."