They said their findings, which supported previous indications of a link, reinforced the importance of flu vaccination among those who might have a higher cardiovascular risk.
They noted that previous studies had suggested an association between the two, but the earlier research had used non-specific measures of infection or study designs that were susceptible to bias. For their study, they looked at nearly 20,000 adult cases of laboratory-confirmed influenza infection during 2009-14 and identified 332 patients admitted for a heart attack within a year of flu diagnosis. They found that the chances of a heart attack were increased six-fold during the first seven days after detection of laboratory-confirmed influenza infection.
As well as influenza, the researchers also found elevated risk – albeit not as high – with infection from other respiratory viruses. However, no increased incidence of myocardial infarction was observed after day seven of flu infection, noted the study authors. “We found a significant association between respiratory infections, especially influenza, and acute myocardial infarction,” they stated. Lead study author Dr Jeff Kwong, from the Institute for Clinical Evaluative Sciences and Public Health Ontario, said the new findings were “important” because finding an association between influenza and acute myocardial infarction reinforced the “importance of vaccination”. “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,” he said. He urged patients at risk of heart disease to take precautions to prevent respiratory infections, especially flu, through both vaccination and handwashing. The researchers added that patients should not delay seeking clinical assessment for heart symptoms, particularly within the first week of an acute respiratory infection. Philippa Hobson, senior cardiac nurse at the British Heart Foundation, said: “There is evidence that heart attacks happen more often during or immediately after an acute inflammatory illness, such as flu. “Heart patients are particularly vulnerable to the flu, in part because the virus can cause inflammation, increasing the risk of a heart attack,” she added.