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By Brenda Goodman
HealthDay ReporterTUESDAY, April 30 (HealthDay News) -- Preteen girls may get the same immune response against human papillomavirus (HPV) with two doses of vaccine as young women get with the full three-shot series, a new study suggests.
HPV is a sexually transmitted infection that causes cervical cancer, the second biggest cancer killer in women around the world.
The HPV vaccine, approved by the U.S. Food and Drug Administration in 2006, is given in a three-shot series. The first and second doses are given one to two months apart, followed by a third dose six months later.
Current government guidelines advise parents to vaccinate boys and girls against HPV starting at age 11. That's partly to ensure they're protected against HPV before their first sexual encounter, but it's also to take advantage of biology.
"The immune response wanes with age. So the younger you are, the better immune response you have," said Dr. Jessica Kahn, a pediatrician at Cincinnati Children's Hospital Medical Center in Ohio. Kahn wrote an editorial on the study, but she was not involved in the research.
Although HPV vaccines are considered highly effective, fewer than one-third of U.S. teens receive all three doses, according to the U.S. Centers for Disease Control and Prevention's National Immunization Survey of Teens.
Doctors have wondered if a different dosing schedule might save money and time while still providing the protection of the three-dose regimen.
To find out, researchers in Canada, where vaccines are delivered through school-based programs, randomly assigned 520 girls aged 9 to 13 to receive either two or three doses of the Gardasil vaccine, which protects against four HPV strains. The girls who got two doses got their shots six months apart. The girls who got three doses got the vaccines on the regular schedule.
The younger girls were compared to 310 young women aged 16 to 26 who got three doses of the vaccine on the regular schedule.
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.
