Venous Thromboembolism Risk in Patients Vaccinated for Influenza

There is no evidence that inactivated influenza virus is associated with venous thromboembolism (VTE) in adults aged 50 years and older, according to recent research published in Vaccine.

Administration of an influenza vaccine can cause a short-lived increase in the production of proinflammatory cytokines, but there have been few studies analyzing the risk for VTE after vaccination. Therefore, the researchers sought to examine VTE risk (including deep vein thrombosis [DVT] and pulmonary embolism [PE]) after vaccination in adults aged 50 years and older. As a secondary aim, they sought to characterize the risk for DVT and PE separately.

A self-controlled case series was used to examine the risk for VTE in adults aged 50 years and older who had a VTE between 2007 and 2012. They identified patients meeting the criteria through the Vaccine Safety Datalink, a collaborative project funded by the US Centers for Disease Control and Prevention. The researchers found presumptive VTE cases and validated those through diagnostic codes, tests, and prescription medication. They validated potential cases by medical record review. The VTE incidence rate ratio was calculated for confirmed cases1 to 10 days postvaccination relative to all other person-time from September through December.

Findings showed there were 396 confirmed cases of VTE. In the 1 to 10 days after flu vaccine administration, there was no increased risk for VTE (incident rate ratio [IRR] = 0.89; 95% CI, 0.69-1.17) compared with the control period. However, in a post hoc analysis, the researchers found an increased risk for VTE in current smokers (IRR = 2.57; 95% CI, 1.06-6.23).

“These findings are consistent with previous research and support the safety of annual vaccination in this population,” the researchers wrote.

One of the study’s strengths is that the electronic case-finding algorithm aided in efficient detection of incidence VTE cases — nearly all cases that met the electronic criteria were confirmed by review of the medical records.

In terms of limitations, the researchers may have missed some cases of true VTE through the use of the electronic algorithm to identify diagnostic codes, diagnostic tests, and oral anticoagulant prescriptions.

Reference

Vickers E, McClure DL, Naleway AL, et al. Risk of venous thromboembolism following influenza vaccination in adults age 50 years and older in the Vaccine Safety Datalink [published September 6, 2017]. Vaccine. doi:10.1016/j.vaccine.2017.08.086