Vaccines against the cancer-causing human papillomavirus appear to be working better than expected in the U.S., given the country's dismal vaccination rates, according to a new study from the U.S. Centers for Disease Control and Prevention.

The HPV infection rate among girls between the ages of 14 and 19 dropped by 56 percent with the arrival of the vaccines Cervarix and Gardasil, a result the CDC calls "encouraging," given that two-thirds of teenage girls skipped all three recommended vaccine doses, according to the study.

A single dose of vaccine cut the risk of HPV infection by 82 percent, the study found.

"This is really exciting news," said ABC News' chief health and medical editor Dr. Richard Besser, noting that it would be years before researchers could measure the impact of the vaccines on cancer rates. "The goal now has to be to increase vaccine coverage during those preteen years."

HPV-related cancers are on the rise as vaccination rates stay low

HPV is the most common sexually transmitted infection, according to the CDC, with more than 40 different strains, some of which have been linked to cancers of the cervix, vagina, vulva, anus and throat in women, as well as penile and anal cancer in men.

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All told, the virus has been implicated in more than 28,000 cancer cases each year, according to the CDC. Here is the CDC's breakdown:

• 12,000 cervical cancers
• 2,100 vulvar cancers
• 500 vaginal cancers
• 600 penile cancers
• 2,800 anal cancers in women
• 1,500 anal cancers in men
• 1,700 oropharyngeal cancers in women
• 6,700 oropharyngeal cancers in men

The agency said that up to 21,000 of those cancer cases could be prevented by the HPV vaccines, which are recommended for both boys and girls 11 and 12 years old, and given in three doses over six months.

"In order for the HPV vaccine to be effective, it has to be given before the virus is acquired," said Besser. "That means giving it before people become sexually active."

The CDC urges boys to get vaccinated against HPV

Despite the CDC recommendations, vaccination rates across the U.S. remain low, reaching just under 50 percent for teenage girls in 2010, according to the study, and that's just for one dose.

"Our low vaccination rates represent 50,000 preventable tragedies," CDC director Tom Frieden said in a statement, adding that Rwanda had a higher HPV vaccination rate than the U.S. "Fifty-thousand girls alive today will develop cervical cancer that would have been prevented if we had reached 80 percent vaccination rates, as Rwanda has. For every year we delay in doing so, another 4,400 women will develop cervical cancer."

While a single dose of the HPV vaccines was found to cut the risk of HPV infection by 82 percent, clinical trials suggest the protection rises to 96 percent with all three doses. But the shots have been slow to catch on in the U.S., with some parents hesitant to take action against a sexually transmitted disease in their preteen kids.

"It's hard for many parents to think about their children one day having sex, but taking this prevention step when kids are young can prevent a cancer when they are adults," said Besser. "That is truly amazing."