Risk of venous thromboembolism following influenza vaccination in adults aged 50years and older in the Vaccine Safety Datalink.

BACKGROUND: Influenza-like illness and inflammation are known risk factors for venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE). However, few studies have characterized the risk of VTE following influenza vaccination. We examined VTE risk after vaccination in adults 50years old and older within the Vaccine Safety Datalink (VSD).

METHODS: We used the self-controlled case series method to determine the risk of VTE among age-eligible adults who received influenza vaccine (with or without pandemic H1N1) and experienced a VTE during the months of September through December in 2007 through 2012. Presumptive VTE cases were identified among VSD participants using diagnostic codes, diagnostic tests, and oral anticoagulant prescription. Potential cases were validated by medical record review. The VTE incidence rate ratio was calculated among confirmed cases for the risk window 1 to 10days after vaccination relative to all other person-time from September through December.

RESULTS: Of the 1,488 presumptive cases identified, 508 were reviewed, of which 492 (97%) were confirmed cases of VTE. The analysis included 396 incident, confirmed cases. Overall, there was no increased risk of VTE in the 1 to 10days after influenza vaccination (IRR=0.89, 95% CI 0.69-1.17) compared to the control period. Results were similar when all person-time was censored before vaccination. A post hoc analysis showed an increased risk among current tobacco smokers (IRR=2.57, 95% CI 1.06-6.23). No clustering of VTE was observed in the 1-42days after vaccination.

DISCUSSION: Overall, there was no evidence that inactivated influenza vaccine was associated with VTE in adults ≥50years old. An increased risk was found among current smokers in a post hoc analysis. These findings are consistent with previous research and support the safety of annual vaccination in this population.