LONDON (Reuters) - Flu and other serious respiratory infections significantly increase the risk of heart attack or stroke, British researchers said on Thursday.

The risk of suffering from either potentially fatal event doubles in the week following a respiratory infection but recedes thereafter, according to a study published in the European Heart journal.

The finding adds to a growing body of evidence suggesting that people with heart trouble should get a flu shot each year, said epidemiology professor Tom Meade.

Meade was one of the team of researchers from the London School of Hygiene and Tropical Medicine who worked on the study.

The link with respiratory infection -- uncovered through the analysis of a database containing the details of 2 million patients -- might explain why both heart attacks and strokes are more common in the winter.

But the reason for the link remains unclear.

"My own view is that the infection switches on the blood clotting system to an over-active state. It is an inflammatory process," Meade said.

Other studies have shown a link between inflammation and heart attacks.

Meade said the absolute risk of heart attack or stroke for a person with respiratory infection was still low, with less than 2 percent of all heart attacks and strokes resulting from infection.

But the number of heart attacks and deaths that could potentially be prevented by proactive treatment was nonetheless substantial, since both are common conditions.

The danger only appears to be triggered by relatively severe infections, such as influenza, serious bronchitis and pneumonia.

"A head cold or runny nose doesn't seem to be a problem, Meade said.

Vaccination against flu is already recommended for people with heart disease in countries like Britain and the United States, but often people fail to get vaccinated.

"Influenza is a serious infection, particularly in patients with heart disease such as heart failure, and it could even trigger a heart attack," said Mike Knapton of the British Heart Foundation, which funded the study.

"We strongly recommend they (heart patients) take up the offer to give themselves protection against the flu."

(Reporting by Ben Hirschler; Editing by Sarah Marsh)