Flu Shot May Protect Against Vein Problems

Abstract

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 in all patients who have experienced pulmonary embolism or deep vein thrombosis, Joseph Emmerich, MD, PhD, 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. Among the participants, 28.2% of those with VTE and 32.1% of controls had received a flu shot. 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 participants younger than age 52 years—the median age of the study population—the relative risk reduction was even larger, at 48%. The preventive effect was of similar size against pulmonary embolism and deep vein thrombosis, according to Dr. Emmerich, professor of vascular medicine at the René Descartes University, Paris. The protective effect against VTE associated with flu vaccination 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, the relative risk reduction rose to 59%. Dr. Emmerich listed several potential mechanisms for influenza vaccination's apparent protective effect against VTE. One possibility is that by protecting individuals from getting the flu, vaccination prevents them from being immobilized in bed for days, a VTE risk factor. Another possibility is that infection with the influenza virus provokes a systemic inflammatory reaction that could promote thrombosis. Arguing against either of these proposed mechanisms, however, was the fact that VTE events were evenly distributed throughout the year, Dr. Emmerich noted. Dr. Emmerich is planning a definitive multicenter, randomized, prospective study of influenza vaccination for secondary prevention of VTE. His case-control study was funded by the French national medical research foundation and other academic sources. The American Heart Association recommends annual flu shots in patients with cardiovascular disease, as they are at increased risk of death when they do contract influenza. Bruce Jancin is with the Denver bureau of Elsevier Global Medical News.