When do physicians know enough about the beneficial effectsof a new medical intervention to start recommending or usingit? When is the available information about harmful adverseeffects sufficient to conclude that the risks outweigh the potentialbenefits? If in doubt, should physicians err on the side ofcaution or on the side of hope? These questions are at the coreof all medical decision making. It is a complicated processbecause medical knowledge is typically incomplete and ambiguous.It is especially complex to make decisions about whether touse drugs that may prevent disease in the future, particularlywhen these drugs are given to otherwise healthy individuals.Vaccines are examples of such drugs, and the human papillomavirus(HPV) vaccine is a case in point.
zur Hausen, winner of the Nobel Prize in Physiology or Medicine in 2008, discovered that oncogenic HPV causes cervical cancer.1,2,3,4 His discovery led to characterization of the natural history of HPV infection, an understanding of mechanisms of HPV-induced carcinogenesis, and eventually to the development of prophylactic vaccines against HPV infection.
The theory behind the vaccine is sound: If HPV infection can be prevented, cancer will not occur. But in practice the issue is more complex. First, there are more than 100 different types of HPV and at least 15 of them are oncogenic. The current vaccines target only 2 oncogenic strains: HPV-16 and HPV-18. Second, the relationship between infection at a young age and development of cancer 20 to 40 years later is not known. HPV is the most prevalent sexually transmitted infection, with an estimated 79% infection rate over a lifetime5,6 The virus does not appear to be very harmful because almost all HPV infections are cleared by the immune system.7,8 In a few women, infection persists and some women may develop precancerous cervical lesions and eventually cervical cancer. It is currently impossible to predict in which women this will occur and why. Likewise, it is impossible to predict exactly what effect vaccination of young girls and women will have on the incidence of cervical cancer 20 to 40 years from now. The true effect of the vaccine can be determined only through clinical trials and long-term follow-up.