Effect of human papillomavirus (HPV) vaccination on clinical indicators of sexual behaviour among adolescent girls: the Ontario Grade 8 HPV Vaccine Cohort Study

Abstract

Background: Suboptimal human papillomavirus (HPV) vaccine coverage in some jurisdictions is partly attributed to fears that vaccination may increase risky sexual behaviour. We assessed the effect of HPV vaccination on clinical indicators of sexual behaviour among adolescent girls in Ontario.

Methods: Using Ontario's administrative health databases, we identified a population-based cohort of girls in grade 8 in the 2 years before (2005/06 and 2006/07) and after (2007/08 and 2008/09) implementation of Ontario's grade 8 HPV vaccination program. For each girl, we then obtained data on vaccine receipt in grades 8 and 9 and data on indicators of sexual behaviour (pregnancy and non-HPV-related sexually transmitted infections) in grades 10-12. Using a quasi-experimental method known as regression discontinuity, we estimated, for each outcome, the risk difference (RD) and relative risk (RR) attributable to vaccination and to program eligibility.

Results: The cohort comprised 260 493 girls, of whom 131 781 were ineligible for the program and 128 712 were eligible. We identified 15 441 (5.9%) cases of pregnancy and sexually transmitted infection and found no evidence that vaccination increased the risk of this composite outcome: RD per 1000 girls -0.61 (95% confidence interval [CI] -10.71 to 9.49) and RR 0.96 (95% CI 0.81 to 1.14). Similarly, we found no discernible effect of program eligibility: RD per 1000 girls -0.25 (95% CI -4.35 to 3.85) and RR 0.99 (95% CI 0.93 to 1.06). The findings were similar when outcomes were assessed separately.

Interpretation: We present strong evidence that HPV vaccination does not have any significant effect on clinical indicators of sexual behaviour among adolescent girls. These results suggest that concerns over increased promiscuity following HPV vaccination are unwarranted and should not deter from vaccinating at a young age.

Figure 1:

Cohort flow diagram. *The total number of exclusions at this stage is less than the sum of exclusions listed because some girls were excluded for more than one reason. †At the time of this study, 2 of Ontario’s 36 Immunization Records Information System (IRIS) databases, representing about 22% of Ontario’s population, had not yet been transferred to the Institute for Clinical Evaluative Sciences and were therefore unavailable for use. IRIS records were also unavailable for girls who emigrated from Ontario before starting kindergarten or immigrated to Ontario after completing high school. ‡A girl’s IRIS record was defined as “up to date” if it had been modified 30 days before cohort entry or later. Otherwise, it was assumed that the girl had moved out of our study area before cohort entry. §Eligible for Ontario’s publicly funded, school-based human papillomavirus vaccination program.

Figure 2:

Probability of exposure to the quadrivalent human papillomavirus vaccine according to birth year quarter (the forcing variable) and program eligibility. See Appendix 3 (available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.140900/-/DC1) for a description of how the forcing variable was operationalized.

Figure 3:

Risk of clinical indicators of sexual behaviour (ascertained for the period between Sept. 1 of grade 10 and Mar. 31 of grade 12), according to birth year quarter (the forcing variable) and eligibility for the human papillomavirus (HPV) vaccination program. (A) Composite outcome of pregnancy and non–HPV-related sexually transmitted infections (STIs). (B) Pregnancy. (C) Non–HPV-related STIs. See Appendix 3 (available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.140900/-/DC1) for a description of how the forcing variable was operationalized.