The flu shot may not just protect against influenza. A new study suggests it may also reduce the risk of developing a blood clot by as much as 26 per cent.

Researchers from the University Paris Descartes found that the flu shot was equally effective against two types of blood clots: deep vein thrombosis, which is a blood clot in the leg, and pulmonary embolism, which is a clot in the lung.

A blood clot can be fatal if it breaks loose, travels through the bloodstream and reaches the lungs.

The research, which included more than 1,400 patients, also found that the flu shot:

• Lowered the likelihood of developing a blood clot by 48 per cent among people over age 52.
• Reduced the risk of developing blood clots by 50 per cent among women under 51 years of age and by 59 per cent in women taking oral contraceptives.

"Our study suggests for the first time that vaccination against influenza may reduce the risk of venous thrombotic embolism (VTE)," as blood clots are also known, lead study author Dr. Joseph Emmerich said in a statement.

Emmerich presented the findings at the American Heart Association's Scientific Sessions 2008 in New Orleans, La.

It is still unclear to researchers how exactly the flu vaccine works to lower blood-clot risk, Emmerich said, but his team is planning future studies to try and answer that question.

One theory is that infections such as influenza can trigger inflammation in the body, which in turn may trigger a clot.

In the meantime, the findings suggest that flu vaccination may one day be recommended as a preventative measure, especially in patients who have already had a blood clot.