Even though human papillomavirus vaccination rates remain low in the United States, the shots are dramatically reducing infections with the virus that causes cervical cancer and genital warts, federal data show.

"The impact of the vaccine is larger than we thought we'd see," said Lauri E. Markowitz, an HPV expert with the U.S. Centers for Disease Control and Prevention.

Government vaccine advisers last week began considering whether to recommend two doses of the newest version of the vaccine rather than three, a move that could simplify and boost immunization.

The HPV vaccine, introduced a decade ago, was hailed as a breakthrough in cancer prevention, but doctors and parents of adolescents have been disconcerted by its novelty, complexity, and link to sexual activity.

The vaccine wards off sexually transmitted strains of the human papillomavirus that cause most cervical cancer, as well as some less common cancers of the head, neck, and genitals that occur in both men and women. Merck's Gardasil brand also prevents infection with HPV types that cause genital warts; GlaxoSmithKline's Cervarix does not.

The vaccine is recommended for girls and boys 11 and 12, so they can be protected before they become sexually active. But federal data show that among 13- to 17-year-olds, only 37 percent of girls and 13 percent of boys have gotten all three shots.

However, for two shots, those rates jump to about half of girls and almost a third of boys.

An estimated 14 million Americans become infected with HPV each year. While the vast majority have no symptoms and clear the virus on their own, some develop persistent infections that can gradually turn cells malignant.

Because cancer takes years or decades to arise, the full impact of the vaccine may take a generation to emerge. But benefits are already clear, based on CDC surveys and analyses, including a study published this month in the journal Pediatrics:

Between the prevaccine era and 2012, the prevalence of potentially harmful HPV infections among girls ages 14 to 19 fell from almost 12 percent to about 4 percent - a 64 percent reduction. For the first time, the CDC also found a decline among women in their early 20s, even though they have lower immunization rates. Their infection rate fell more than a third, from 19 percent to 12 percent.

The prevalence of genital warts has fallen among women in their teens and 20s, and among men in their early 20s, based on an analysis of private insurance claims. Although warts are not life-threatening, they can spread and recur, and require removal with laser therapy or freezing.

CDC monitoring programs in California, Connecticut, New York state, and Oregon show fewer women ages 18 to 20 are developing severe precancerous cervical lesions, which can progress to cancer if not treated. Their incidence of lesions fell an average of 87 percent between 2008 and 2012. Smaller declines are starting to show up in women in their 20s and 30s.

Such positive impacts have been measured in Australia, Canada, and parts of Europe where HPV vaccination rates are over 50 percent among women under 20.

The fact that the U.S. is already seeing unexpectedly large impacts may be due to the "herd effect" - once enough people are protected, they protect susceptible members by reducing the spread of the virus.

"There also may be effectiveness for less than three doses," said Markowitz, the CDC epidemiologist who has led HPV impact studies.

A two-dose regimen of the original Gardasil vaccine is already standard in over 80 countries.

CDC's Advisory Committee on Immunization Practices is now considering whether to recommend a two-dose regimen of Merck's new Gardasil, which adds protection against some rare cancer-causing HPV strains. It can prevent 90 percent of cervical cancers, compared with 70 percent with the original product.