Two doses of HPV vaccine not 3 may offer similar protection

VANCOUVER, British Columbia, May 8 (UPI) -- Girls who received two doses of human papillomavirus vaccine had immune responses not worse than those who had three doses, Canadian researchers say.

Dr. Simon R. M. Dobson of the University of British Columbia, Vancouver, and colleagues conducted a study to determine whether average antibody levels to HPV-16 and HPV-18 among girls receiving two doses were non-inferior -- not worse than -- to women receiving three doses.

The study authors also looked at antibody levels to HPV-6 and HPV-11, and compared girls given two or three doses. The randomized, phase 3, multicenter study included 830 Canadian females from August 2007 through February 2011. Follow-up blood samples were provided by 675 participants. Girls age 9-13 were randomized 1:1 to receive three doses of quadrivalent HPV vaccine at 0, two, and six months or two doses at 0 and six months. Young women age 16-26 years received three doses at 0, two and six months. Antibody levels were measured at 0, seven, 18, 24, and 36 months.

"Globally, cervical cancer is the second most common cause of cancer morbidity and mortality in women. HPV infection has been identified as a necessary cause for the development of cervical cancer, with HPV genotypes 16 and 18 accounting for approximately 70 percent of cervical cancer cases," the study said. "Global use of HPV vaccines to prevent cervical cancer is impeded by cost. A two-dose schedule for girls might be possible."

"The geometric mean titer ratios for girls at two doses to women at three doses remained non-inferior for all genotypes to 36 months. Antibody responses in girls were non-inferior after two doses vs. three doses for all four vaccine genotypes at month seven, but not for HPV-18 by month 24 or HPV-6 by month 36."