The figures represent the government's first substantial study of vaccination rates for the Gardasil vaccine - Merck & Co.'s heavily advertised, three-shot series that targets the sexually transmitted human papillomavirus, HPV.

The vaccine, which is made at Merck's West Point plant in Montgomery County, protects against strains of the virus that cause about 70 percent of cervical cancers and 90 percent of genital warts, the company says.

Health officials recommend that girls get the shots when they are 11 or 12, if possible, before they become sexually active. Also, at 11, children are generally due for another round of vaccinations.

The survey only covered girls 13 to 17.

Vaccine proponents had been hoping for much higher vaccination rates, saying the shots could dramatically reduce the nearly 4,000 cervical-cancer deaths that occur each year in the United States.

But many families are cautious about the safety of new vaccines, said Patti Gravitt, a Johns Hopkins University epidemiologist.

The vaccine is expensive, retailing for about $375, although many health insurers now cover it. Girls who get the vaccine should still receive pap smears, because the shots are only 70 percent effective against cancer, experts say.

And there are questions about whether it confers lifetime immunity or requires a booster shot.

"Some parents may be adopting the attitude with their daughters that, 'Well, you're still young. I can wait a couple more years before you're sexually active,' " said Gravitt, who was not involved in the research.

"My personal opinion is that this seems quite reasonable after the first year," Gravitt said, of the 25 percent vaccination rate.

Merck officials said they were pleased with the rate.

The Centers for Disease Control and Prevention based the study on household telephone surveys done in late 2007. The survey results cover the time frame from when the vaccine came on the market in mid-2006 to when the survey questions were answered.

The results are based on nearly 3,000 girls, 13 to 17, for whom the researchers could verify vaccination information through medical records.

Of the girls in the survey, 25 percent had received at least one Gardasil shot. That's about 2.5 million of the 10 million girls in that age group.