Do parents have a moral duty to vaccinate their children to improve "herd immunity"?

My doctor is currently making the case for my son (age 12) getting the Gardasil/HPV vaccination, arguing that even though HPV won't really harm him, he could become a carrier and spread HPV to women he has sex with at some time in the future, and thereby harm them. I don't think he has a duty to become one of the "immunized herd" (referring to the idea of "herd immunity" regarding vaccines) and therefore I am not inclined to have him vaccinated against HPV. Should he choose to do so at a later time, he is free to make that decision. Does my son -- or do I as a parent -- have an obligation to vaccinate purely to promote "herd immunity"? If not in this case, where there is a clear issue of undergoing the vaccination primarily for the sake of risk to others, then what about in other cases of vaccines? Does a person have an obligation to society in general to become part of the immunized herd, even if taking a vaccination is probably at low risk to that person's health?