Could something as seemingly innocuous as the seasonal flu increase the possibility of a heart attack for people at risk of heart disease? New research suggests that may indeed be the case.

Researchers from the Institute for Clinical Evaluative Sciences (ICES) and from Public Health Ontario (PHO)—both in Ontario, Canada—have made a surprising finding, looking at the health risks associated to an influenza diagnosis.

Influenza, usually referred to simply as "the flu," is a contagious disease caused by influenza viruses picked up from the atmosphere, or through close contact with infected individuals. Flu viruses usually infect the nose and throat, causing sneezing, coughing, a sore throat, and sometimes fever.

Lead author of the study, Dr. Jeff Kwong, and a team of researchers from ICES and PHO noted that groups at risk of heart disease seem to have increased chances of experiencing a heart attack in the first week after becoming infected with the flu.

According to the CDC, approximately 735,000 people in the United States experience a heart attack each year.

"Our findings are important because an association between influenza and acute myocardial infarction reinforces the importance of vaccination," Dr. Kwong notes.

The researchers recently published a paper detailing their study's results in the New England Journal of Medicine.

Respiratory infections as a risk factor

In their study, Dr. Kwong and team analyzed the cases of almost 20,000 adults based in Ontario between 2009 and 2014. These were all cases of influenza, as confirmed by laboratory tests.

Out of this total, the researchers singled out 332 individuals who had been admitted to the hospital to be treated for a heart attack within only a year from getting infected with influenza.

The analysis conducted by Dr. Kwong and colleagues revealed a significant link between a diagnosis of acute respiratory infection—and influenza in particular—and a heightened risk for acute myocardial infarction, or heart attack.

The possibility of experiencing a heart attack is increased six-fold in the first week from the detection of infection with a flu virus and, the researchers note, certain groups are more exposed to this risk than others.

Most vulnerable seem to be seniors (adults aged 65 or older), individuals infected with B strains of the influenza virus, and those who have not previously experienced a heart attack.

An increased risk for heart attack was also noted in the case of individuals affected by other types of respiratory viruses, though the exposure was somewhat less significant.

At-risk groups should not postpone flu shot

These results confirm the outcomes of previous studies that outlined correlations between getting a flu vaccine and a lower risk of adverse cardiovascular events.

"Our findings, combined with previous evidence that influenza vaccination reduces cardiovascular events and mortality, support international guidelines that advocate for influenza immunization in those at high risk of a heart attack."

Dr. Jeff Kwong

The researchers strongly recommend that we do everything we can in order to prevent infection, by paying particular attention to hygiene and making sure we get the seasonal flu shot.

"People at risk of heart disease should take precautions to prevent respiratory infections, and especially influenza, through measures including vaccinations and handwashing," stresses Dr. Kwong.

Data from the CDC reveal that only 67.2% of all US adults aged 65 and over have received a flu shot during the past year.

Dr. Kwong also urges individuals who may be at risk of heart disease to play it safe and get evaluated for any telling symptoms if they receive a flu diagnosis.