Cervical Cancer Vaccine Reality Check
period of effectiveness is unknown – long term effects will takes decades to assess
multiple inoculations must be administered before the start of sexual activity
inoculations must be kept current to provide continuing protection
social barriers limit widespread vaccination against sexually transmitted diseases
vaccine is limited to preventing two types of HPV
disease may shift to the HPV types not covered by the vaccine
development and validation cycle for each type is decades – vaccines for other strains are unlikely in the next decade
for some women, the vaccine will likely shift susceptibility to HPV infection to a more dangerous age
vaccine is expensive – $500-$1000 for the initial set of inoculations
Cervical Cancer Vaccine
Recently, Merck has marketed a vaccine against two strains of HPV (human papilloma virus). The news media have reported the product as a “cervical cancer vaccine.” Any vaccine that works against cancer would be a reassuring headline but there are several points which the stories do not cover well. It is essential to note that the vaccine does not prevent cervical cancer in women who have already contracted the virus. Such a vaccine would be a true cancer vaccine. As the U.S. National Institute of Health writes, “Most adults have been infected with HPV at some time in their lives.” Usually persons who contract strains of the virus eventually overcome the virus without any kind of long-term abnormality (see reference). Only persistent infection with particular strains of the virus leads to cervical cancer. The changes in cervical tissue leading to cancer almost always take many years to develop so that cervical cancer occurs most often in women over the age of forty.
There are over 80 types of human papilloma virus (HPV). Approximately 30 types are transmitted sexually and can infect the cervix. At least eighteen of these have been linked to cervical cancer. The vaccine developed by Merck protects against two strains of the virus that some surveys have said account for 65-70% of the cases which lead to cervical cancer in the United States, but the vaccine must be administered before contracting these strains. It is not a therapeutic vaccine but a preventive vaccine. To be effective, a full course of inoculations with the vaccine must be completed before the beginning of sexual activity. Together, this means that if a massive vaccination program was undertaken for preteens, it would mean some reduction in cervical cancer 30 years from now. Barriers to a widespread inoculation program in the United States are not likely to be overcome in the near future. Another factor limiting widespread inoculation is cost. The vaccine price for the initial set of inoculations is $360 in addition to the practitioner charges.
The human papilloma virus is opportunistic, namely, it has taken on many forms adapted to sites on human hosts. Because of this, strains which can lead to cervical cancer, but which in the past have not contributed greatly to the number of cases in the United States, are now becoming more prevalent. One strain has already displaced one of the vaccine strains in some areas of the world. In the journal Sexually Transmitted Diseases (see reference), a study in Kenya found two strains not covered by the vaccine to be dominant in high grade cervical lesions, unlike their role in the United States. The authors conclude there is a need for "further research to tailor vaccines to take into account regional variations." An HPV type (HPV 45) previously infecting only a small number of women in high income countries and not covered by the current vaccine, is now present in 19% of the Finnish invasive cervical cancer cases in fertile-aged women.
Although the Merck vaccine has been shown to be highly effective in the short term, the long term effectiveness of the vaccine is unknown. It is likely that a vaccine providing protection of less than 15 years would generate unexpected outcomes by shifting infection by the virus to women in an older age group, where they would have increased risk of persistence. Likewise, if vaccine boosters shots are neglected, women could contract the HPV strains delayed by the earlier inoculations. The immune system is able to clear HPV infections much more easily at age 20 than after the age of 30.
A clinician’s opinion recently published in the Journal of the American Medical Association (November 2, 2005) states, “In our lifetimes, we will always have some sort of cervical cancer screening in the United States.” In low income countries, screening will be a necessity for a much longer time.