Flu may increase heart attack risk, study suggests

By Dr Deepu

"Cardiovascular events triggered by influenza are potentially preventable by vaccination," the researchers wrote.

       The study 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 investigators found that 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  above results were based on study involving 148,307 cases of  patients who were tested for influenza. Among all of those tests, 19,729 turned up positive for the flu. And among those cases, there were 332 patients who had at least one heart attack in the year before or after their flu specimen was tested. (The study authors tallied 364 hospitalizations for acute myocardial infarction overall, meaning that some unlucky folks had two or more heart attacks during the two-year observation period.)

Twenty of those heart attacks occurred within one week of a positive flu test. That, of course, was a rate of 20 heart attacks per week.
The other 344 heart attacks happened some other time in the two-year observation period. That worked out to 3.3 heart attacks per week.
That means the risk of a heart attack was six times greater in the first week after flu testing than at other times when the flu was much less likely to be a factor.

The researchers redid their analysis by splitting up that danger week into two parts. They found that heart attack risk was 6.3 times greater during the first three days after a flu test and 5.8 times greater in days four through seven.

About one-quarter of the patients in the study were 65 years old, and the rest were older. When the researchers examined those two groups separately, the link between flu infection and heart attack risk held up only for the older group.

There was no sign of an increased heart attack risk in the rest of the first month after getting a flu test.

The data in the study came from Ontario, Canada, where residents have public health insurance and universal access to medical care. Information on influenza test results came from the Flu and Other Respiratory Viruses Research Cohort, and heart attack hospitalizations were tracked by the Discharge Abstract Database of the Canadian Institute for Health Information.

The researchers, led by Dr. Jeffrey C. Kwong of the University of Toronto, acknowledged that they couldn't do their analysis based on the date when patients were actually infected with the influenza virus, or when they first began having symptoms, because that information was not available. However, in cases where patients get a flu test, they have typically been sick for only one or two days first.

Also, not all flu cases are severe enough to prompt patients to go and get tested. That means the results of this study may not apply to people with milder illnesses, they added.

The researchers did notice that when flu test results came back positive for certain other kinds of respiratory infections instead of for influenza, there was still an increased (though smaller) short-term risk for heart attacks. That suggests that it's not the flu itself that's the problem — it's the biological impact of a respiratory infection.

For instance, an infection can create conditions that make blood clots more likely to form and cause blood vessels to constrict. Infections also cause inflammation and can reduce blood pressure. All of these are risk factors for a heart attack, Kwong and his colleagues wrote.

The study results suggest that people who want to avoid a heart attack should be sure to get a flu shot — and that doctors and public health officials should encourage them to do so.