"I was a little bit surprised by the strength of the association.
It's not every day you see a six-fold increase in the risk during
the first seven days of lab-confirmed influenza," chief author Dr.
Jeffrey Kwong told Reuters Health in a telephone interview. "We were
also surprised the risk dropped off to nothing by day 8 and beyond."
He and his Canadian team also found that other respiratory diseases
can also increase the chance of a heart attack, but not as nearly as
dramatically.
The group, reporting in The New England Journal of Medicine, did not
examine whether flu-associated heart attacks are deadlier.
The new study reinforces the importance of the flu vaccine and
protective measures such as regular hand washing to guard against
influenza and other infections, said Dr. Kwong, a scientist at the
Institute for Clinical Evaluative Sciences in Toronto.

Doctors have suspected a link between flu and heart attacks since
the 1930s, but in that era it was hard to know if the influenza
virus or a flu-like illness had made a patient sick prior to the
heart attack.
Kwong and his colleagues used confirmed cases of flu, analyzing 364
heart attacks from mid-2008 through mid-2015 among Ontario residents
age 35 or older who were registered with the province's publicly
funded health insurance program.
The heart attack rate was 20.0 admissions per week during the seven
days after diagnosis of the flu, versus 3.3 per week during the 52
weeks before and 51 weeks after that seven-day window.
The risk dropped off dramatically by the eighth day after diagnosis.
Dr. Erica Jones, director of the HeartHealth Program at Weill
Cornell Medicine and New York-Presbyterian Hospital, who was not
connected with the study, said the results aren't surprising based
on her experience with hospitalized heart attack patients.
"This time of year we frequently had people on the floor after the
flu," she told Reuters Health by phone. "It was often associated
with pneumonia."
The flu "is a stressor to the system. It can increase inflammation.
When you get an infection your heart is beating faster. It can
activate platelets, increasing the chance that blood clots will form
in the arteries that serve the heart. All of these can increase the
chance of having a heart attack," Dr. Kwong said.
Among the 332 people in the study who developed at least one heart
attack while recovering from the flu, 69 percent had not received a
flu shot. For 76 percent, it was their first heart attack,
technically known as an acute myocardial infarction.
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The heart attack risk increased slightly for adults over 65 and for
people infected with influenza type B. But those increases were not
statistically significant. The risk was 10 times higher with
influenza B, five times higher with influenza A (the most common
type during that period), 3.5 times greater for respiratory
syncytial virus and nearly three times higher for other viruses.
The study "should not be interpreted as evidence of a lack of
vaccine effectiveness, because this study was not designed to
evaluate the effectiveness of influenza vaccines," the researchers
said. "Rather, since vaccination of adults is only approximately 40%
to 60% effective in preventing laboratory-confirmed influenza
infection, this study shows that if vaccinated patients have
influenza of sufficient severity to warrant testing, their risk of
acute myocardial infarction is increased to a level that is similar
to that among unvaccinated patients."
"We can't say it enough - get a flu shot," said Dr. Jones. "Even if
the flu shot isn't perfect, it may protect at least somewhat and the
flu could be less severe, although this study didn't address that.
Also, wash your hands all the time and stay away from people who you
know are sick."
And if you get the flu, "don't ignore symptoms" that might suggest a
heart attack, she said. "Chest pains, shortness of breath might be
more than you think."
The time it takes for the flu to produce symptoms is about 1.4 days
after infection with influenza A and 0.6 days with influenza B. Once
symptoms begin, it only takes a day or two for them to peak.

The researchers cautioned that the people in their study were not
suffering from mild flu symptoms.
"These are people who are sick enough to see a doctor and the doctor
was worried enough to actually swab the patient" to test for the
virus, said Dr. Kwong. "We don't know if these results apply to
people who have milder infections."
SOURCE: http://bit.ly/2FU4ylA The New England Journal of Medicine,
online January 24, 2018.
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