Flu shots cut the risk of blood clots forming in veins

According to new research by French scientists flu shots may reduce the risk of venous thrombotic embolism.

Venous thrombotic embolism (VTE) occurs when blood clots form in veins and is dangerous because the blood clot can break loose and travel through the circulatory system to the right side of the heart - from there to the lungs (pulmonary embolism), which can be life-threatening or even fatal.

The researchers at the University Paris Descartes say their study suggests for the first time that vaccination against influenza may reduce the risk of VTE.

Dr Joseph Emmerich, lead author of the study and professor of vascular medicine and head of the INSERM Lab 765, which investigates thrombosis, says this protective effect was more pronounced before the age of 52 years.

The researchers conducted a case-control study involving 1,454 patients matched by age and sex with an average age of 52 years, from 11 centers across France who were taking part in the FARIVE study.

The team compared 727 patients with no history of cancer within the last five years, who had initial episodes of VTE, to a matched control group of patients free of venous and arterial thrombotic disease.

Ineligible to participate in the study were patients younger than 18 years old, those who already had VTE, those who had a diagnosis of active cancer or a history of malignancy less than five years previously, or those who had a short life expectancy due to other causes.

Potential control subjects with cancer, liver or kidney failure, or a history of venous and/or arterial thrombotic disease were also ineligible.

The patients were interviewed and a questionnaire covering age, educational level, medication history, personal and familial history of thrombotic disease, and acquired risk factors for VTE, including pregnancy, use of oral contraceptives or estrogen replacement therapy, trauma or surgery less than three months previously, prolonged immobilization, or travel lasting more than five hours, was completed.

Patients were classified as having secondary (provoked) VTE if they had one or more of the above acquired risk factors - all other patients were considered to have had unprovoked episodes of VTE.

Influenza vaccination status during the previous 12 months was also recorded and the outcomes of the cases, documented through half-yearly telephone interviews for five years, found that overall, the risk for developing VTE after having the flu shot was 26% less and the flu shot's protective effect was more pronounced before rather than after age 52 years, with a 48% lower likelihood of VTE in those younger than 52.

For women under 51 years, getting the flu shot reduced the odds of developing VTE by 50% and by 59% for women taking oral contraceptives.

Dr Emmerich says the protective effect of vaccination was similar for different types of VTE but the link between influenza virus infection and the risk of VTE remains unclear.

Dr Emmerich says infections in general increase blood viscosity, and systemic inflammatory reactions to infectious agents can themselves trigger a thrombotic process.

He says however that influenza vaccination might lower the risk of thrombosis in other ways, as suggested by the even distribution of VTE events across the 12 months of the year in both vaccinated and unvaccinated cases in our study.

Dr Emmerich says more research is needed to confirm this relationship between influenza vaccination and VTE, and to explore the underlying mechanisms but there is a possibility that flu vaccination could be recommended after a first VTE event.

The American Heart Association recommends heart patients get annual flu shots to protect against influenza and say patients with cardiovascular disease are more likely to die from influenza than patients with any other chronic condition but such patients should not be given the nasal-spray flu vaccine.

The research was presented at the American Heart Association's Scientific Sessions 2008.