Researchers took blood samples to measure the number of antibodies made against each viral strain. They continued to take blood samples over time to see how long the antibody response might last.
The younger girls who got two doses of vaccine appeared to make at least as many antibodies against the HPV strains as the teens and young women did on the three-dose regimen. And their protection appeared to last just as long, up to three years after they started their shots, according to the study, published April 30 in the Journal of the American Medical Association.
The researchers cautioned that while the antibody response to two doses of the vaccine looks promising, it doesn't prove that the shorter course actually protected against viral infections or cancers. A much longer study is under way to test that.
But study author Dr. Simon Dobson, a clinical associate professor with the vaccine evaluation center at the University of British Columbia, said the results are encouraging because they suggest that younger girls could get two doses to prime their immune systems against HPV, and then get a later dose to boost that response closer to the time they might become sexually active.
"It raises the possibility that you could give two doses early in preadolescence, and then wait and give the third dose later in adolescence when the girls are going to be closer to the time when they're most likely to need protection against HPV," Dobson said.
Two doses would also save money, a key consideration especially in countries where resources are scarce. According to the CDC, the Gardasil vaccine cost about $130 a dose in 2012.
Until more is known, however, experts say girls should get all three doses.
Kahn said doctors don't know if two doses will be enough to fully protect teenage girls.
"We need data on girls older than 13. Because even though the vaccine is
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