The HPV vaccine is a prophylactic vaccine: It is most efficacious when given before exposure to HPV virus, which usually occurs at the onset of sexual activity. Two HPV vaccines currently exist. The quadrivalent vaccine (HPV4) provides protection against HPV types 6, 11, 16, 18 and is approved by the U.S. Food and Drug Administration (FDA) for males (to prevent genital warts) and for females aged 9 through 26 years (to prevent genital warts and genital cancer and precancer). In contrast, the bivalent vaccine (HPV2) provides protection against HPV types 16 and 18, but it is FDA approved only for females through age 26 years (to prevent cervical cancer and precancer). On 17 November 2010, an FDA advisory panel recommended expanding HPV4 indications to include prevention of anal intraepithelial neoplasia in both males and females aged 9 through 26 years.