The finding adds to a growing body of evidence suggesting that people with heart trouble should get a flu shot each year, said professor Tom Meade, one of the 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 two million patients - might explain why both heart attacks and strokes are more common in the winter. But the reason for the link remains unclear.

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 - such as vaccination against flu - 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.