It is generally accepted that persistent infection of the uterine cervix by high-risk types of the sexually acquired human papillomavirus (HPV) is necessary but not sufficient for the development of cervical cancer. Most people at some time in our lives acquire genital infections with HPV. In almost all cases, HPV infection is of no clinical significance, but genital cancers such as cervical and penile cancer can be uncommon consequences of this extremely common sexually-acquired infection. We do not fully understand why so few women develop cervical cancer, or why so few men develop penile cancer, when so many men and women are infected by HPV. Decades may elapse after initial infection with HPV and before the clinical manifestations of cancer. During this lengthy time interval, specific cellular changes occur within the cervix that may be detected by a variety of methods. These specific cellular changes are referred to as “precancerous” cervical lesions.
The development of HPV vaccines that might be useful for the prevention of cervical cancer has been a high-priority global health goal for many years. On 8 June 2006 the U.S. Food and Drug Administration (FDA) approved an HPV vaccine developed by Merck. Merck’s HPV vaccine is effective in preventing precancerous cervical lesions caused by HPV type 16 and HPV type 18 for up to 5 years following vaccination. HPV types 16 and 18 together cause about 70% of all cases of cervical cancer. HPV vaccines provide no protection against the HPV types other than 16 and 18 that, taken together, cause the remaining 30% of cervical cancers. Moreover, HPV vaccines will provide no protection for women who have already been exposed to HPV types 16 and 18 through sexual activity. For women who have already initiated sexual activity (and this group currently constitutes the great majority of the world’s women), Pap screening, rather than HPV vaccination, is the prudent option for effective cervical cancer prevention.
It may be desirable for females who can afford both HPV vaccination earlier in life (before the onset of sexual activity) and Pap screening later in life to access both of these preventive interventions. However, because all females who receive HPV vaccines must continue to be screened, and because it is uncertain whether HPV vaccines will provide any additional measurable protection for females who are also screened, the eventual added benefit of HPV vaccines for cervical cancer prevention is uncertain.
Because HPV vaccines provide no benefit to females who have already been exposed to HPV, HPV vaccines should be administered to females under the age of 13. However, the safety of HPV vaccines administered to females in this age group has not yet been adequately studied. Also, developing countries, including Vietnam, do not currently have the infrastructure to administer vaccines to young adolescents. It will therefore be necessary to choose between investing in the infrastructure to screen adult women, or investing in the infrastructure to vaccinate young adolescents. It is uncertain whether even affluent countries have the resources to invest in both screening and vaccination.
In developing countries such as Vietnam, providing access to HPV vaccination for some females will reduce access to Pap screening for others. From the population perspective of public health, this compromise is not appropriate.