The human papillomavirus (HPV) vaccine is proving effective in the United States since it was first introduced in 2006, with a new study saying teen girls infection rates are down by 56 percent.

A new study from the Centers for Disease Control and Prevention (CDC) found that infection rates among teen girls infected with the four HPV strains included in the vaccine (HPV 6, 11, 16, and 18), had dramatically dropped. CDC Director Tom Frieden said the results come as a "wake-up call" and urges more people to get vaccinated against HPV, which is the most common sexually transmitted infection (STI).  

Currently, about one-third of girls age 13-17 are fully vaccinated. That compares with higher vaccination rates in other countries such as Rwanda, where more than 80 percent of teenage girls have been vaccinated. The full series comprises three doses given over six months.

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

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.

HPVs are responsible for nearly all cases of cervical cancer, along with most cases of anal cancer, the National Cancer Institute says. The viruses also cause more than half the cancers in the middle part of the throatand about half of vaginal, vulvar and penile cancers. There are more than 40 HPV types that can infect the genital areas of males and females.  

According to the CDC, there are about 79 million Americans, most in their late teens and early 20s, who are infected with HPV, and every year some 14 million people become newly infected.

 "Our low vaccination rates represent 50,000 preventable tragedies - 50,000 girls alive today will develop cervical cancer over their lifetime that would have been prevented if we reach 80 percent vaccination rates," Frieden said.