http://www.cdc.gov/HPV/vaccine.html
Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
HPV vaccines are given as three shots to protect against HPV infection and HPV-related diseases. HPV vaccines offer the greatest health benefits to individuals who receive all three doses before having any type of sexual activity. That’s why HPV vaccination is recommended for preteen girls and boys at age 11 or 12 years.