Women who received the HPV vaccination as part of a school-based program had fewer high-grade cervical anomalies and higher rates of cervical screening, according to the results of a case–control study conducted in Canada.
The risk reduction occurred exclusively in women who received the recommended number of vaccine doses, results showed.
“In 2008, Alberta implemented a school-based HPV vaccination program for female students in grade 5 (age 10-11 years) and, in 2009, added a 3-year catch-up program for female students in grade 9 (age 14-15 years),” Huiming Yang, MD, medical officer of health and medical director of screening programs at Alberta Health Services, and colleagues wrote. “The HPV vaccination program expanded to include male students in 2014. The current program provides three doses (in months 0, 2 and 6) of quadrivalent vaccine that includes protection against two oncogenic HPV types (HPV-16 and HPV-18), which together account for 70% of cases of cervical cancer.”
Yang and colleagues sought to determine the effect of the vaccination protocol on cervical cancer screening cytology results. They established a cohort of 10,204 women born between 1994 and 1997, who resided in Alberta and underwent at least one Pap test between 2012 and 2015.
Fifty-six percent (n = 5,712) of the study cohort were unvaccinated; the remaining 44% (n = 4,492) received at least one dose of the HPV vaccination. Vaccinated women had a higher rate of screening (13% vs. 11.4%; P < .001), with a median time from last vaccination to Pap test of 1,374 days (interquartile range, 1,119-1,603).
A total of 8,723 women showed no cervical abnormalities and were deemed controls. The remaining 1,481 patients had cervical abnormalities during screening.
Women with cervical abnormalities tended to be older (P = .046) and were more likely to have had their tests processed at laboratories located in Calgary (P < .001).
The majority of patients with abnormalities (93.5%; n = 1,384) had low-grade anomalies, such as atypical squamous cells of undetermined significance or low-grade squamous intraepithelial lesions; the other 6.5% (n = 97) had high-grade anomalies — including atypical squamous cells in which high-grade lesions could not be ruled out — or high-grade squamous intraepithelial lesions.
The rate of complete vaccination with three or more doses was 83.3% (n = 3,675). Younger women appeared more likely to have received at least one dose (18 years vs. 21 years, 76.6% vs. 20.4%; P < .001).
A greater percentage of unvaccinated women had abnormal cytology results than women who received the full vaccination dose (16.1% vs. 11.8%). Overall, women who received the full vaccination dose had a lower likelihood of having abnormal cytology results (OR = 0.72; 95% CI, 0.63-0.82).
Women who received a partial vaccination with two doses had an adjusted OR of 1.08 (95% CI, 0.84-1.38) for abnormal cytology results, which suggested a lack of efficacy for this regimen. The OR for high-grade abnormalities among partially vaccinated women was 0.16 (95% CI, 0.02-1.17).
A sensitivity analysis that separated high- and low-grade cytology cases showed an OR of 0.5 (95% CI, 0.3-0.85) for high-grade cases among vaccinated women, which correlated with a 50% (95% CI, 15-70) effectiveness rate.
The researchers cited their lack of pathology outcomes data as a study limitation.
“Quadrivalent HPV vaccination significantly reduced high-grade cervical abnormalities but required three doses,” Yang and colleagues wrote. “Evidence from this study and future studies can be used to improve integration of HPV vaccination and screening programs for preventing cervical cancer.” – by Cameron Kelsall
The researchers report no relevant financial disclosures.