NEW ORLEANS — Influenza vaccination reduced the incidence of venous thromboembolism by 26% in a case-control study.
This novel finding requires confirmation. Should that happen, it would become appropriate to recommend a change in clinical practice incorporating routine administration of influenza vaccine by injection as a means of preventing repeat venous thromboembolism (VTE) episodes, Dr. Joseph Emmerich said at the annual scientific sessions of the American Heart Association. He presented an 11-center retrospective case-control study involving 727 French adults with a first objectively documented episode of VTE and no history of cancer within the prior 5 years, and an equal number of VTE-free controls.
Participants were asked if they had received a flu shot—the nasal spray flu vaccine isn't available in France—within the past 12 months, and 28.2% of those with VTE and 32.1% of controls said they had. After adjustment in a multivariate regression analysis for potential confounders including age, sex, body mass index, use of oral contraceptives, education level, and varicose veins, influenza vaccination was associated with a highly significant 26% relative risk reduction in VTE. In those aged younger than 52 years, the median age of the participants, relative risk reduction was 48%.
The preventive effect was of similar size against pulmonary embolism and deep vein thrombosis, said Dr. Emmerich, professor of vascular medicine at the René Descartes University, Paris.
The protective effect was particularly robust in women. In women younger than 51 years, the unadjusted risk of VTE associated with flu vaccination was reduced by 50%. After adjustment for oral contraceptive use and other potential confounders, it was 59%.
Dr. Emmerich said one possible mechanism behind the vaccine's apparent protective effect is that if individuals don't get the flu they are not immobilized in bed for days, a VTE risk factor. Another is that infection with the influenza virus provokes a systemic inflammatory reaction that could promote thrombosis. But arguing against these mechanisms was the fact that VTE events were evenly distributed throughout the year, with no reduction during flu season in vaccinated individuals.
In an interview, he said he believed the stronger protective effect in those younger than 52 was a result of the French policy of mandatory flu vaccination of people aged 60 and older.
Dr. Emmerich is planning a definitive multicenter, randomized, prospective study of flu vaccination for secondary prevention of VTE. His case-control study was funded by the French national medical research foundation and other academic sources.
The protective effect of the flu shot was particularly robust in women. ©Pix by Marti/