The quadrivalent HPV vaccine may not adequately protect a significant number of women from high-grade squamous intraepithelial lesions and lesser dysplasia, according to study results.
The historical cohort study utilized routinely collected health information from several large longitudinal databases in Manitoba, Canada, to evaluate the vaccine effectiveness of the quadrivalent HPV vaccine. Eligible participants included females aged at least 15 years who lived in Manitoba and were registered with the Manitoba Health Population Registry (MHPR) between August 2006 and March 2010.
Researchers identified 3,541 females who had been treated with the quadrivalent HPV vaccine during the enrollment period. Using an MHPR centralized electronic database linking system, the researchers then linked the vaccinated cohort to the MHPR to find 9,594 age-matched females with no record of being vaccinated with the quadrivalent HPV vaccine during the enrollment period.
Vaccinated females were age-matched to up to three non-vaccinated females. Researchers used Cox regression models to calculate HRs for three outcomes: atypical squamous cells of undetermined significance (ASCUS), low-grade squamous intraepithelial lesions (LSILs) and high-grade intraepithelial lesions (HSILs).
Adjusted estimates of vaccine effectiveness among 15- to 17-year-olds were 35% (95% CI, –19 to 65) against HSILs, 21%, (95% CI, –10 to 43) against LSILs and –1% (95% CI, –44 to 29) against ASCUS.
Those who had one or more Pap smears after enrollment had higher vaccine effectiveness estimates: 46% (95% CI, 0-71) for HSILs, 35% (95% CI, 10-54) for LSILs and 23% (95% CI, –8 to 45) for ASCUS.
In women aged at least 18 years with no history of abnormal cytology, the quadrivalent HPV vaccine was found to confer a 23% (95% CI, –17 to 48) decrease in risk for HSILs. However, those with abnormal cytology history did not benefit from this level of protection (–8%; 95% CI, –59 to 27). “For females vaccinated outside of the publicly funded vaccination program, we found that the [quadrivalent] HPV vaccine was moderately effective in preventing HSIL (and to a lesser extent, LSIL and ASCUS) among 15- to 17-year-olds, but far less effective in the older age groups, especially among those with a history of abnormal cytology.”
Disclosure: The researchers report research funding from GlaxoSmithKline, Pfizer and Sanofi-Aventis; consultant roles with GlaxoSmithKline and Merck Frosst Canada; and honoraria and other remuneration from Merck Frosst Canada.