LONDON — Heart patients who catch the flu might have more to worry about than just a fever or the sniffles: the virus also could spark a heart attack, new research shows.

Amid the global outbreak of swine flu, experts say it's crucial heart patients get vaccinated against both regular flu and swine flu to avoid medical problems. Doctors said swine flu isn't any more dangerous than regular flu, but it's important for heart patients to get vaccinated because more flu viruses will be circulating this year.

British researchers analyzed 39 previous studies of heart patients and found a consistent link between flu and heart attacks. Up to half of all unexpected flu deaths were due to heart disease, the researchers found.

The study was published online today in the British medical journal, The Lancet Infectious Diseases.

"The message here is so strong and so logical that it's hard for us to ignore," said Dr. Ralph Brindis, vice president of the American College of Cardiology. "If we can convince cardiac patients to get a flu vaccine, that could ultimately save lives."

Only about one-third of heart patients in the U.S. regularly get vaccinated.

Doctors have long known flu viruses can worsen existing medical conditions and that heart patients are especially vulnerable during flu pandemics. Flu viruses cause inflammation in the body, usually in the lungs. But they also can cause swelling in the heart itself or in the coronary arteries, which could lead to dangerous clots breaking off and causing a heart attack.

Once heart patients get the flu, they also are more vulnerable to complications like pneumonia and other infections.

"We know influenza vaccine is effective in preventing influenza and therefore in theory, ought to be effective in preventing the complications of influenza," said Andrew Hayward of University College London, one of the study authors. He said two of the studies analyzed showed heart patients who got a flu shot had fewer heart attacks than those who didn't.

Hayward said flu viruses might merely act as triggers for heart attacks in cardiovascular patients.

"Influenza may be bringing forward an event that might have happened anyway," he said, adding there is evidence that when the virus peaks, so too do heart attacks.

Experts are unsure whether the study results apply to otherwise healthy people with no history of heart disease. But they say flu viruses could potentially trigger heart attacks in people with no apparent heart disease, if they have risk factors like high blood pressure or are overweight.