A new study suggests a single HPV vaccination appears just as effective as two doses at preventing the viral infection that causes cervical cancer. HPV, or human papillomavirus, is very common and spread through sex. Most HPV infections clear up on their own. But some linger, causing cancers that appear years later, including cervical cancer in women and rarer cancers in both women and men. On Wednesday, researchers reported a U.S.-funded study in Costa Rica that found a single HPV shot provided similar protection against cervical cancer-causing infection as two doses, for at least five years.