SUNDAY, Nov. 9 (HealthDay News) -- People who get their annual flu shot may reap an extra benefit: a reduction in their risk of developing a blood clot.

The benefit appeared stronger in those under the age of 52, according to research that was presented Sunday at the American Heart Association's annual scientific sessions, in New Orleans.

The findings, the first to demonstrate such an effect, may help explain why the flu shot lessens the risk of cardiovascular events in people with coronary artery disease, but the real current value of the data may lie in it convincing more people to get their annual shot.

"This kind of data is super helpful to me with patients in the clinic, particularly if they've had a blood clot," said Dr. Ann Bolger, the William Watt Kerr professor of clinical medicine at the University of California, San Francisco, said during a news conference on Sunday. "It's another nail to hammer on."

Venous thromboembolism (VTE) refers to the development of a blood clot in a vein. Such a clot can be life-threatening if breaks off and travels to the lung (pulmonary embolism).

Experts had previously known that the flu vaccine can reduce the risk of cardiovascular events in people with coronary artery disease, but it wasn't clear why.

"We had interesting epidemiological data before that flu increased cardiovascular deaths, but we didn't know where from," Bolger said. "This interesting observation implies that if you get the flu shot and avoid infection, you're less likely to get clots in the veins and arteries."

The authors compared 727 patients with one documented episode of VTE (deep vein thrombosis or pulmonary embolism) and no history of cancer within the past five years (cancer is a risk factor for clots) with an equal number of controls with no history of clots.

Participants filled out questionnaires regarding past medical history, especially risk factors for VTE, and whether or not they had had a flu shot in the past year. They were followed for five years.

In all, 28 percent of people who had had a clot and 32 percent of controls had been vaccinated.

Individuals who had had a flu shot were 26 percent less likely to develop a blood clot.

People younger than 52 were 48 percent less likely to form a blood clot. In women under the age of 51, the risk reduction was 50 percent, and in women under 51 taking birth control pills or estrogen replacement therapy, the risk reduction was 59 percent.

Researchers can only speculate as to why the flu shot might have this protective effect.

"There is an indication that influenza can induce systemic inflammation by increasing interleukin 6," said study author Dr. Joseph Emmerich, a professor of vascular medicine at the University Paris Descartes and head of INSERM Lab 765, which investigates thrombosis. "If it was only due to the prevention of influenza in vaccinated people, we would have a much more important decrease in the incidence [of clots] during the winter season compared to other parts of the year."

In fact, there was no such seasonal swing.

The heart association recommends that individuals with heart disease get an annual flu shot. Patients with cardiovascular disease should not get the nasal spray vaccine.

More information

See the American Heart Association guidelines on flu shots.