Researchers have also discovered that the vaccine guards against vaginal and vulval cancers. HPV is present in 80 percent of the 6,000 cases of vulval and vaginal cancers diagnosed each year.
It's hard to know how many girls have actually received the vaccine. Merck bases its estimate on anecdotal reports from the field, but the American Cancer Society has no estimates of its own.
"We don't know how many [vaccines] are sitting in a refrigerator somewhere, versus in a girl's arm," Saslow said.
New research is looking at other ways the vaccine could be used to guard against cancer.
For example, Merck is studying whether the vaccine would work in women as old as 45, Dougherty said. The company also is exploring whether the vaccine works against other strains of HPV beyond the four it was originally meant to target.
Research also is under way to see if boys should receive the vaccine, as a way to protect girls from HPV. While males obviously can't get HPV-linked cervical cancer, they can help spread the sexually transmitted virus.
"Because males can transmit HPV to their partner, putting their partner at risk for developing cervical diseases, we have had a male research program going for some time now," Dougherty said.
The vaccine may also prove to be a way to protect men as well. HPV has been shown to be a cause of throat cancer, which affects both sexes.
"We just know the vaccine is safe in males, but we need to prove that it works," Saslow said. Test results for males are still two to three years away, she said.
For older women, the best way to prevent cervical cancer is through the Pap test.
Cervical cancer tends to occur in midlife, with half of women diagnosed between the ages of 35 and 55, according to the American Cancer Society.
Under current cancer society guidelines, all women should begin cervical cancer screening about three years after they begin having vaginal intercourse but no later than age 21. Testing should be done every year with the regular Pap test or every two years using a newer liquid-based Pap test.
Beginning at age 30, most women who have had three normal Pap test results in a row can space out their screenings to every two to three years.
Most women 70 or older who have had three or more normal Pap tests in a row and no abnormal Pap test results in the last 10 years can choose to stop having the test.
More information
To learn more, visit the American Cancer Society.
