Catching the flu can increase the risk of having a heart attack within the following week six-fold, according to a study involving thousands of flu cases.
The Canadian research team behind the study says its findings provide strong evidence of a link between influenza and heart attacks.
Scientists say the findings, published in the New England Journal of Medicine, reinforce the need for people at risk of heart attacks to have flu jabs.
The Canadian team looked at nearly 20,000 cases of laboratory-confirmed flu infection from 2009 to 20014.
The results showed that for seven days after flu was detected, there was a six-fold increase in the chances of patients suffering heart attacks.
Lead scientist Dr Jeff Kwong, from the Institute for Clinical Evaluative Sciences in Ontario, said: "Our findings, combined with previous evidence that influenza vaccination reduces cardiovascular events and mortality, support international guidelines that advocate for influenza immunisation in those at high risk of a heart attack."
The risk may be higher for older adults, patients with influenza B infections, and those experiencing their first heart attack, said the researchers.
A less extreme raised risk of heart attack was also associated with other respiratory viral infections besides influenza.
Dr Kwong said: "People at risk of heart disease should take precautions to prevent respiratory infections, and especially influenza, through measures including vaccinations and hand washing."
