The number of US girls with the sexually transmitted disease HPV has dropped by about half even though relatively few youths are getting the vaccine, research showed on Wednesday.

Human papillomavirus is the most common sexually transmitted infection in the United States and has been linked to cervical cancer in women as well as in oral, head and neck cancers, particularly among men, health officials said.

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Since a vaccine against HPV was introduced in 2006, 56 percent fewer girls age 14-19 have become infected, said the research announced by the US Centers for Disease Control and published in The Journal of Infectious Diseases.

CDC Director Tom Frieden described the findings as a “wake-up call” that the vaccine works and should be more widely used. Currently, about one-third of girls age 13-17 are fully vaccinated.

“It is possible to protect a generation from cancer and we have got to do it,” Frieden told reporters.

The study used nationwide survey data to compare HPV rates before the HPV vaccine was widely available (2003-2006) to afterward (2007-2010).

“The decline in vaccine type prevalence is higher than expected,” said lead author Lauri Markowitz.

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Possible reasons for the decline include “herd immunity, high effectiveness with less than a complete three-dose series and/or changes in sexual behavior we could not measure,” she said.

US health officials urge routine vaccination for both boys and girls age 11-12, before sexual activity begins. A series of three shots is recommended over six months.

The CDC says about 79 million Americans, most in their late teens and early 20s, are infected with HPV, and every year some 14 million people become newly infected.

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Often the virus clears the body on its own but some strains, if left unchecked, can lead to cancer.

HPV is believed to cause 19,000 cancers per year among women in the United States, with cervical cancer being the most common.

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About 8,000 cancers caused by HPV occur in men each year in the United States, mainly throat cancers.