Certain types of human papillomavirus (HPV) cause almost all cases of cervical cancer. Vaccines that protect against HPV can substantially reduce the risk of cervical cancer. However, HPV vaccination rates are too low, especially in countries with very high rates of cervical cancer. HPV vaccines are expensive-many countries cannot afford them-more than one dose is needed, and giving multiple doses is difficult. Researchers want to find out if one dose prevents HPV infection. If it does, more people might get the vaccine.