A bad case of the flu can trigger a short-lived, but substantial, spike in some people's, new research suggests. Among 332 heart attack patients, the complication was six times more likely to strike following a bout of the flu, researchers reported.
The findings come in the midst of a particularly brutal flu season.
Across the United States, flu-related hospitalizations are spiking, according to the U.S. Centers for Disease Control and Prevention. The most recent figures put the rate at 31.5 hospitalizations per 100,000 Americans, versus only 13.7 per 100,000 two weeks earlier.
The Canadian researchers said their findings underscore some longstanding advice: Get a yearly, especially if you're at increased risk of a heart attack.
"If you have, you take it as gospel that you should do things like take your cholesterol medication and keep your blood pressure under control," said lead researcher Dr. Jeffrey Kwong.
"You should look at the yearly flu shot that way, too," said Kwong, a scientist at the Institute for Clinical Evaluative Sciences, in Toronto.
Studies have long shown that flu infection is linked to an increased risk of heart attack in people who are vulnerable.
And for years, the American College of Cardiology (ACC) and other groups have recommended that people with heart disease get an annual flu shot.
But the new findings strengthen the theory that a flu infection can actually, according to Dr. Andy Miller, chair-elect of the ACC's Board of Governors. However, the study did not prove that the flu can cause a heart attack.
Still, "there's a growing line of evidence that inflammation, and interventions that reduce inflammation, are important in heart attack risk," said Miller, who was not involved in the study.
He explained how a flu infection could, in theory, cause trouble: If a person already has artery-clogging plaques, a bout of the flu could cause inflammation -- body-wide and within blood vessels -- that then causes a plaque to rupture. When a plaque breaks apart, it can completely block an artery supplying the heart, causing a heart attack.
The new study was able to look at how people's heart attack risk changed in relation to the timing of a flu infection.
Kwong's team started with nearly 20,000 Ontario adults who'd come down with a case of the flu that was confirmed through lab testing. Out of that group, 332 were hospitalized for a heart attack within a year.
On average, the study found, patients were six times more likely to suffer their heart attack in the week after their flu infection was confirmed -- as compared with the year before, or the year after.
The patients also faced a higher risk shortly after coming down with other respiratory infections, the findings showed.
According to Kwong, the patients likely had severe infections -- bad enough to warrant a trip to the doctor and lab testing. So it's not clear, he noted, whether milder cases would carry the same risk.
Nor can the study say what the absolute risk of heart attack would be for any one person who catches the flu, Kwong said.
But, Miller added, out of 20,000 people with serious flu infections, relatively few had a heart attack over the next year.
The flu shot is imperfect: It's no guarantee against infection, and it works better during some flu seasons than others. But, Kwong pointed out, "even some protection is better than no protection."
Still, other measures -- like regular-- are important, too. And everyone can help out by staying home when they are sick and not exposing others to their infection, Kwong advised.
Miller stressed that people at risk of heart attack should focus on all of their risk factors, during flu season and otherwise.
"Address all of the traditional risk factors you have -- obesity, high blood pressure, high cholesterol," he said. "Getting the flu shot is one additional intervention to take."
The findings were published Jan. 25 in the New England Journal of Medicine.