Partial HPV Vaccine Series & Girls' Genital Warts
But Swedish study didn't address more critical issue of cervical cancer prevention
"This study does not provide any information on how effective two versus three doses of vaccine will be in preventing the more serious disease of cervical or other cancer," he said.
For the study Arnheim-Dahlstrom and colleagues collected data from national Swedish health registries on more than 1 million girls and women aged 10 to 24 who were followed between 2006 and 2010.
Of more than 20,000 cases of genital warts, only 322 happened after a female had received at least one dose of the vaccine, the researchers found.
Although the maximum reduction in risk was found after three doses, two doses also were protective, but to a lesser degree.
According to the researchers, the difference between three and two doses was that with two doses there would be 59 more cases of genital warts per 100,000 women who were observed for one year.
Since the vaccine is most effective before exposure to HPV, the U.S. Centers for Disease Control and Prevention recommends that boys and girls get the full three doses of the vaccine starting at age 11 and 12.
According to the CDC, in 2012, about 54 percent of U.S. girls have had at least one dose of the HPV vaccine, but only 33 percent have received all three recommended doses.
Among boys, about 20 percent had received one dose of the vaccine, but slightly less than 7 percent had received all three doses, the agency noted.
Commenting on the study, Dr. Mitchell Maiman, chairman of obstetrics and gynecology at Staten Island University Hospital in New York City, said, "This is very preliminary. The only protocol that's approved right now is three shots, and we are not doing that great with three shots," he said.
Maiman noted that the three shots cost about $400, and fewer shots could be less expensive, but whether or not fewer doses would be protective against cancer isn't known.