The flu doesn't just make you feel lousy. A study published Wednesday finds it can increase your risk of having a heart attack, too.
"We found that you're six times more likely to have a heart attack during the week after being diagnosed with influenza, compared to the year before or after the infection," says study author Dr. Jeff Kwong, an epidemiologist and family physician with the Institute for Clinical Evaluative Sciences and Public Health Ontario in Canada.
The results appear in this week's New England Journal of Medicine.
Doctors noticed long ago that there was a connection between seasonal flu and cardiovascular deaths, but the association has been hard to nail down. Part of the challenge is that many people with flu symptoms don't get tested for the virus. So Kwong and his colleagues decided to use test results (from flu tests and other viruses, too) and match them with hospital records.
"This is the first time we've had lab-confirmed influenza, so we're certain that these were influenza [viruses] causing the infection," Kwong says.
There's a lot happening in the body during the flu that can help explain the increased risk of a heart attack.
"There's inflammation going on, and your body is under a lot of stress," explains Kwong. Oxygen levels and blood pressure can drop. These changes "can lead to an increased risk of forming blood clots in the vessels that serve your heart."
A young person who is normally healthy is very unlikely to have a heart attack during the flu. "It's all about your baseline risk," Kwong says.
In his study, the people who had a heart attack during the flu were older adults. "Most of them were over 65, and a lot of them had risk factors for heart disease," such as high blood pressure, high cholesterol or diabetes, Kwong says.
The flu vaccine does not always prevent an infection from a flu virus. Some years, the shot offers more protection than others. And only about 30 percent of the study population had been vaccinated against influenza for that season. But, flu trackers say, some protection against the virus is better than none.
"If we can reduce the risk of influenza infection, then we should reduce the risk of heart attacks," Kwong says. So "getting an influenza vaccine is a good idea."
Infectious disease specialists at the Centers for Disease Control and Prevention say it's not too late to get a flu shot this season. There's a lot of flu in the U.S. right now, and the rate of hospitalizations is still increasing. So the storm of infection isn't over yet.
"There could be another wave of flu coming," says Dr. Alicia Fry, of the CDC's Influenza Division. She says the division is seeing outbreaks of Influenza B in some nursing homes. This strain is different from the primary one that has circulated so far this year.
Fry has this advice for people who do get sick with flu-like symptoms: "It's really important to stay home." She says don't go to work — or send your kids to school, if they're sick.
The risk of infecting colleagues or classmates, as the new study shows, can go way beyond making them feel crummy.
MARY LOUISE KELLY, HOST:
In the middle of this bad flu season comes a study offering yet more evidence that it is really important to get a flu shot. Here's one finding. The risk of heart attack spikes in the days following a flu diagnosis. NPR's Allison Aubrey reports.
ALLISON AUBREY, BYLINE: Flu trackers at the Centers for Disease Control and Prevention say they can't say yet whether this will end up as one of the worst flu seasons, but the CDC's Alicia Fry says it is bad, and it's not over yet.
ALICIA FRY: The rate of hospitalizations continues to increase, and we are seeing high rates of deaths and outpatient visits at this point. And we can see that for most of the states, it's at a critical high level.
AUBREY: People who get the flu feel lousy, but now there's new evidence that the risks go beyond feeling sick. Researchers in Canada have documented evidence that in some people, the flu can trigger a heart attack. Here's study author Jeff Kwong of Public Health Ontario.
JEFF KWONG: We found that you are six times more likely to have a heart attack during the week after being diagnosed with influenza compared to the year before or after the infection.
AUBREY: Six times more likely - did that surprise you?
KWONG: Well, yeah, it is quite a - it's a very strong association, and so that was a little bit of surprise.
AUBREY: Kwong says this link has long been suspected. But to confirm it, he and his colleagues decided to match flu test results with hospitalization data, so this way they were certain that it was the influenza virus causing the infection. Now, Kwong says there's a lot happening in the body during the flu that could explain the risk.
KWONG: There's inflammation going on, and your body is under a lot of stress. So that can lower your blood pressure. It can lower the oxygen levels, and it can lead to an increased risk of forming blood clots in your vessels that serve your heart.
AUBREY: Now, a young person who is normally healthy is very unlikely to have a heart attack when they get the flu. In this study, those who ended up suffering a cardiovascular event during the flu were older.
KWONG: Most of them were over 65, and a lot of them also had risk factors for heart disease already.
AUBREY: Such as high blood pressure, high cholesterol or diabetes. Now, the flu vaccine does not always prevent an infection. Some years, it offers more coverage than others. But the CDC's Alicia Fry says it's not too late to get one this year. She says they're still seeing new outbreaks - for instance, cases of influenza B in nursing homes. That's a different strain than the one that's been circulating so far this year.
FRY: There could be another wave of flu coming.
AUBREY: And she has this reminder for those who do end up getting sick.
FRY: It's really important when people start to have symptoms of flu that they stay home.
AUBREY: The risks of infecting a colleague or a classmate can go beyond just making them feel sick. Allison Aubrey, NPR News.
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