The quadrivalent human papilloma virus vaccine is most effective against cervical dysplasia when given to girls before they become sexually active or develop abnormal cytology, a study found.
A significant percentage of vaccinated women may not be protected against high-grade squamous intraepithelial lesions and lesser dysplasia especially if they were vaccinated after age 18 or had abnormal cytology before vaccination.
Researchers looked at more than 3,500 females 15 years old or older who received the vaccine in Manitoba between September 2006 and April 2010. The group was matched by age to up to 3 nonvaccinated females (n=9,594). The cohort was exmained for atypical squamous cells of undetermined significance, low-grade squamous intraepithelial lesions, and high-grade SILs.
Results appeared in the Journal of Clinical Oncology.
Among girls ages 15 to 17, adjusted efficacy estimates were 35% (95% CI, −19% to 65%) for high-grade squamous intraepithelial lesions, 21% (−10% to 43%) for low-grade ones, and −1% (−44% to 29%) against atypical ones. Estimates among those who had 1 or more Pap smear after enrollment for high-grade lesions (46%; 95% CI, 0% to 71%), low grade lesions (35%; 95% CI, 10% to 54%), and atypical ones of undetemined significance (23%; 95% CI, −8% to 45%.
The vaccine was associated with a 23% (95% CI, −17% to 48%) reduction in high-grade lesions risk among adults with no history of abnormal cytology, but there was no evidence of protection among those with such a history (−8%; 95% CI, −59% to 27%).
Authors wrote, “These findings affirm the importance of vaccinating females at a young age before any significant exposure to HPV occurs and underscores the need for maintaining organized high-quality screening programs that cover all sexually active women, even if they were vaccinated.”