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A study by Ontario researchers showing influenza can trigger a heart attack, especially in older adults, reinforces the importance of getting the flu shot.
The paper, published in the New England Journal of Medicine, on Jan. 25 found the risk of heart attack increased sixfold in the seven days after detection of lab-confirmed influenza.
“By showing that there is a link between influenza and heart attacks, some heart attacks could be preventable by people getting their flu shots,” says lead author Dr. Jeff Kwong, a scientist at Public Health Ontario and the Institute for Clinical Evaluative Sciences.
“If you really don’t want to have a heart attack this is one other thing you can do,” says Kwong.
The study, done by researchers from the two organizations, comes during what appears to be the peak of this winter’s flu season. And with a couple of months left, it’s still worth getting vaccinated, Kwong says.
Other studies have made links between respiratory infection and heart attack, but this one focuses specifically on the flu and relied on lab-confirmed tests. Heart disease is a leading cause of death worldwide.
Kwong and the team of researchers looked at nearly 150,000 cases of people in Ontario aged 35 and older, who had been tested for the flu — Influenza A and Influenza B — or other respiratory viruses between May 2009 and May 2014.
They found 19,729 tests were positive for the flu. Among the people who tested positive, 332 of them were hospitalized for a heart attack within one year of testing positive, accounting for 364 hospital admissions.
Researchers zeroed in on the date that lab tests confirmed the flu. Then they looked at the year before and the year after that testing date.
They found there were 20 hospital admissions for heart attack in the week after flu testing. In 30 per cent of those cases the individual died within one month of admission for heart attack.
By comparison, there were 344 hospitalizations during the rest of the time, which accounts for 3.3 admissions per week.
Put simply, there was a noticeable spike in the number people being hospitalized for a heart attack in the days immediately following a flu diagnosis.
“In that one week right after testing, there were six times the number of heart attacks that you’d expect compared with the rest of the time period,” said Kwong.
They also discovered an increased risk for heart attack, albeit not as high, from other respiratory viruses, such as respiratory syncytial virus (RSV) and coronavirus.
Dr. Jacob Udell, a cardiologist with Peter Munk Cardiac Centre and Women’s College Hospital who was not involved in the research, called the study “a game changer,” noting the benefits of the flu shot extend well beyond protection from influenza.
In recent days, Udell has seen five confirmed cases of influenza causing a heart attack or heart failure, and another five patients whom he suspects also have the flu.
“I’ve seen 10 people and I’m only halfway through my week,” he said on Wednesday, calling this flu season a “pretty nasty one.”
He says the flu vaccine should be considered heart protection therapy, calling it a tool that’s “right at the top” of his tool box when it comes to helping patients. It’s safe, you take it just one time and it has minimal side-effects.
So why does the flu trigger a heart attack? Kwong explains that infections cause inflammation, which can lead to damage of the inner lining of the blood vessels that serve the heart and can increase the chances of blood clots forming there.
Also, there are increased metabolic demands on the heart, meaning it has to work harder to compensate for things such as lower blood pressure and lower oxygen levels in the blood caused by the flu. That extra effort by the heart can cause a heart attack.
The study was based on people with severe symptoms sick enough to see a doctor and get tested and did not look at mild flu symptoms.
Of the 332 patients that researchers focused on, about three quarters of them were older than 65, with a median age of 77. Nearly half were female and almost a quarter had been previously hospitalized for heart attack. Many of them had risk factors for heart attack, such as diabetes, high blood pressure and high cholesterol.
And, almost one third had been vaccinated against influenza. But Kwong notes that because the vaccine isn’t 100 per cent effective, some folks will get the flu even though they’ve had the shot.
The Ministry of Health and Long-Term Care recommends that anyone older than six months get the publicly funded flu vaccine. Children under age 5 and adults older than 65, nursing home residents and those with underlying medical conditions are at high risk of developing flu-related complications.
Besides the flu shot, Kwong says regular handwashing is a good way to cut your chances of getting a respiratory virus infection.
“And,” he says, “If you are sick, stay home so you don’t get other people sick.”