A vaccine administered to adolescent girls and boys has been so successful that herd immunity against a virus that causes cervical cancer is within reach, at least in the United States. However, sustaining community resistance to the human papillomavirus (HPV) will depend on whether the US can maintain high vaccination rates, say experts. HPV is the world's most common sexually transmitted infection  . Vaccines against certain strains of HPV, of which there are hundreds, have existed for almost