A significant percentage of women vaccinated with Gardasil® (quadrivalent human papillomavirus recombinant vaccine) may not be protected against high-grade squamous intraepithelial lesions (HSIL) and lesser dysplasia—especially if they were vaccinated after age 18 or had abnormal cytology before vaccination, according to the results of a study published in the Journal of Clinical Oncology.

HPV is the most common sexually transmitted infection in the United States. There are more than 100 different strains of HPV. Some types of HPV cause warts on the hands or feet; others cause genital warts; and some have been linked with cancer, most notably cervical cancer. In fact, HPV is believed to be the leading cause of cervical cancer.

Gardasil® (quadrivalent human papillomavirus [types 6, 11, 16, 18] recombinant vaccine) is one of two FDA-approved vaccines that protect against the types of HPV associated with cervical cancer. The vaccine is approved for females age 9 to 26. The Centers for Disease Control (CDC) recommends that girls get the shots around age 11 or 12, prior to the beginning of sexual activity.

Researchers from the University of Manitoba evaluated the effectiveness of Gardasil in a study that included over 13,000 women over the age of 15. The study included 3,541 females who were vaccinated with Gardasil and 9,594 age-matched, non-vaccinated females. The researchers estimated the hazard ratios for three outcomes: atypical squamous cells of undetermined significance (ASCUS), low-grade squamous intraepithelial lesions (LSILs), and high-grade SILs (HSILs).

They estimated that among the 15 to 17-year-olds in the study, the vaccine reduced the risk of HSIL by 35 percent, LSIL by 21 percent, and ASCUS by -1 percent. The corresponding estimates were higher among those who had at least one Pap smear after enrollment: 46 percent for HSIL, 35 percent for LSIL, and 23 percent for ASCUS.

In contrast, the vaccines effectiveness was lower among subjects 18 and older. In these women, the vaccine was associated with a 23 percent reduction in HSIL if they had no history of abnormal cytology; however, the vaccine appeared to offer no protection in those with a history of abnormal cytology.

The researchers concluded that “a significant percentage of vaccinated women may not be protected against HSIL and lesser dysplasia, especially if they were vaccinated at older age or had abnormal cytology before vaccination.” They suggest that their findings indicate the importance of early vaccination—before exposure to HPV.


Mahmud SM, Kliewer EV, Lambert P, et al: Effectiveness of the Quadrivalent Human Papillomavirus Vaccine Against Cervical Dysplasia in Manitoba, Canada. Journal of Clinical Oncology. Published early online January 6, 2014. doi: 10.1200/JCO.2013.52.4645

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