Scientists studied insurance records of 271,976 girls and women in the United States who received an initial vaccination from 2006 to 2009. Ideally, the three shots should be given to 11- and 12-year-old girls within a six-month window. Catch-up shots are advised up to age 26.
The rate at which the young women completed the series within a year dropped to less than 22 percent in 2009 from more than 50 percent in 2006.
Getting one shot is not enough, according to the senior author, Dr. Abbey B. Berenson, a professor at the University of Texas Medical Branch. “All the data is based on three injections,” she said. “Getting one shot does not protect, based on the data we have now.”
ETA: Wonder why? Cost? Reactions? (I've heard it is a painful shot), Inconvenience?