--Dr. Laura Koutsky, epidemiologist at the University of Washington
I get asked this question a lot by women who are already sexually active including some who have had abnormal Pap smears as a result of infections by the human papillomavirus (HPV). HPV infections cause virtually all cervical cancer, and bad actor HPV types 16 and 18 are responsible for 70% of these malignancies. The Gardasil vaccine (and the not yet approved Cervarix vaccine) is highly effective at inducing immunity against these carcinogenic viruses; in fact, this vaccine is the first one to specifically designed to prevent cancer caused by a virus.
Dr. Koutsky and company (an enormous panel of clinical investigators) published the results of their FUTURE II trial, aka Females United to Unilaterally Reduce Endo/Ectocervical Disease, in a May, 2007 edition of the NEJM(1). While the vaccine prevented 98% of cervical lesions--precancerous and malignant--in subjects who tested negative for exposure to HPV types 16 and 18 at the time of entry into the study, it was only 44% protective in women previously infected with these cancer-causing viruses.
The ideal population, therefore, that will benefit from this vaccine is those girls/women not yet exposed to the virus. The CDC's Advisory Committee on Immunization Practices has recommended without reservation that girls 11 and 12 years of age receive this shot.
(1)The FUTURE II Study Group. Quadrivalent Vaccine against Human Papillomavirus to Prevent High-Grade Cervical Lesions. NEJM,Volume 356:1915-1927, May 10 2007.