One in four U.S. women ages 14 to 59 is infected with the sexually transmitted virus that in some forms can cause cervical cancer, according to the first broad national estimate.
The figure is mostly in line with previous assessments. The highest prevalence -- nearly 45 percent -- was found in young women within the age range recommended for a new virus-fighting vaccine, according to a report from the federal Centers for Disease Control and Prevention.
Researchers have estimated that 20 million Americans have some form of human papillomavirus or HPV. The study concluded that 26.8 percent of U.S. women are infected, a figure that is comparable to earlier estimates using smaller groups.
"We expected the prevalence of any HPV infection would be high and that's what we found," said CDC researcher Dr. Eileen Dunne, the study's lead author.
Just 3.4 percent of the women studied had infections with one of the four HPV strains that the new vaccine protects against. But that does not mean the vaccine should be written off, said Dr. Yvonne Collins, an assistant professor of gynecologic cancer at the University of Illinois at Chicago.
For one thing, Collins said, that relatively small percentage corresponds with a lot of women -- about 3 million, according to the report. And it does not include those with past infections that have cleared up.
The number of women with HPV strains targeted by the vaccine was lower than in some previous, less comprehensive estimates. And the overall HPV prevalence among the youngest women studied, 14- to-24-year-olds, was substantially higher than in previous estimates, 7.5 million versus 4.6 million.
Dunne attributed those variations to different study populations and different HPV detection methods. She said the results should not be interpreted to mean infection prevalence has changed in recent years.
The new nationally representative report is based on vaginal swab specimens from 1,921 women tested in 2003-04.
The report appears in Wednesday's Journal of the American Medical Association.
There are dozens of strains of HPV. Low-risk forms can cause genital warts and non-cancerous changes in cells in the cervix, and often clear without treatment. Several high-risk forms have been linked with cervical cancer.
Dunne said HPV prevalence is thought to be high in men as well, but none were studied.
Cervical cancer will be diagnosed in an estimated 11,150 U.S. women this year, and about 3,670 will die from it. Numbers are much higher worldwide, especially in developing countries where Pap tests to detect cervical cancer are not routine.
The new vaccine, Merck's Gardasil, was approved last June for girls and women aged 9 to 26. It protects against two HPV strains believed responsible for about 70 percent of cervical cancer cases, and two other strains that cause 90 percent of genital wart cases.
Other vaccines are in the works to protect against other HPV strains, Collins said.
Women aged 20 to 24 had the highest overall HPV prevalence in the study, 44.8 percent. Prevalence increased each year from ages 14 to 24, then dropped off gradually, confirming that young, sexually active women face the greatest risk of infection.
The study underscores the need for young women to get vaccinated, and to get routine Pap tests, said Dr. Howard Jones, a gynecologic cancer specialist at Vanderbilt University.