Evaluation of the risk of venous thromboembolism after quadrivalent human papillomavirus vaccination among US females.

After the Food and Drug Administration (FDA) licensed quadrivalent human papillomavirus vaccine (HPV4) in 2006, reports suggesting a possible association with venous thromboembolism (VTE) emerged from the Vaccine Adverse Event Reporting System and the Vaccine Safety Datalink. Our objective was to determine whether HPV4 increased VTE risk. The subjects were 9-26-year-old female members of five data partners in the FDA's Mini-Sentinel pilot project receiving HPV4 during 2006-2013. The outcome was radiologically confirmed first-ever VTE among potential cases identified by diagnosis codes in administrative data during Days 1-77 after HPV4 vaccination. With a self-controlled risk interval design, we compared counts of first-ever VTE in risk intervals (Days 1-28 and Days 1-7 post-vaccination) and control intervals (Days 36-56 for Dose 1 and Days 36-63 for Doses 2 and 3). Combined hormonal contraceptive use was treated as a potential confounder. The main analyses were: (1) unadjusted for time-varying VTE risk from contraceptive use, (2) unadjusted but restricted to cases without such time-varying risk, and (3) adjusted by incorporating the modeled risk of VTE by week of contraceptive use in the analysis. Of 279 potential VTE cases identified following 1,423,399 HPV4 doses administered, 225 had obtainable charts, and 53 were confirmed first-ever VTE. All 30 with onsets in risk or control intervals had known risk factors for VTE. VTE risk was not elevated in the first 7 or 28 days following any dose of HPV in any analysis (e.g. relative risk estimate (95% CI) from both unrestricted analyses, for all-doses, 28-day risk interval: 0.7 (0.3-1.4)). Temporal scan statistics found no clustering of VTE onsets after any dose. Thus, we found no evidence of an increased risk of VTE associated with HPV4 among 9-26-year-old females. A particular strength of this evaluation was its control for both time-invariant and contraceptive-related time-varying potential confounding.