Most HPV infections are transmitted through sexual contact. It is estimated that approximately more than 80% of men and women in the U.S. are infected with HPV. most infections occur soon after sexual intercourse. Most people are asymptomatic after infection. It can take 10-20 years from persistent HPV infection to the development of cervical cancer. Therefore, vaccination of young girls before they have sex will provide the best prevention of cervical cancer. There are currently two vaccines approved for the prevention of HPV infection in the United States: Gardasil (recombinant HPV quadrivalent vaccine, types 6, 11, 16, and 18) and Cervarix (recombinant HPV bivalent vaccine, types 16 and 18). Both vaccines are prepared from viral proteins and do not contain viral DNA, so neither will cause HPV infection. Both vaccines can only be used to prevent HPV infection and not to treat pre-existing infections or lesions. Because HPV infection may take more than 10 years to develop into cervical cancer, and precancerous lesions may occur quickly, many clinical trials have set the study endpoint as the incidence of CIN or adenocarcinoma in situ.