Study bolsters link between flu and heart attack

Evidence of an association between flu and heart attacks has been building over the past several years, but a new study based on lab-confirmed flu and hospitalized people found a significant link not only for flu, but for other respiratory infections as well.

The risk of heart attack increases sixfold during the first 7 days after a lab-confirmed flu diagnosis in adults 35 and older, researchers from Canada reported yesterday in the New England Journal of Medicine. They said stronger evidence that flu can trigger cardiac events may drive more efforts to elevate vaccine coverage in people who are at risk for heart attacks.

Highest risk: week after flu diagnosis

Using a database of respiratory virus testing from 19 labs in Ontario, the research team identified 20,000 adults infected with flu in Ontario from 2009 to 2014. From a separate database of hospital admissions, they found 332 Ontario residents age 35 and older who were hospitalized for acute myocardial infarction 1 year before or after lab-confirmed flu.

Then they compared the incidence of heart attack hospitalization during the risk interval of 1 to 7 days after flu was diagnosed with two control intervals—the 52 weeks before hospitalization and the 51 weeks after.

Of 364 heart attack hospitalizations, 344 occurred during the control interval, for a rate of 3.3 admissions per week. However, 20 occurred during the flu risk interval, for an admission rate of 20 per week. However, the team didn't see a similar increase for the two intervals that followed, days 8 to 14 and days 15 to 28 after flu diagnosis.

The risk may be higher for older adults, those sick with influenza B, and people experiencing their first heart attack, the group found. Also, they determined that the incidence of heart attack was elevated after flu, regardless of flu vaccination status or history of admission before the study period.

The researchers also found a smaller but significantly increased incidence of heart attack admission after positive tests for respiratory syncytial virus (RSV) and other viruses. Compared with the 6.05 incidence ratio for flu, the incidences were 3.51 for RSV, 2.77 for other respiratory viruses, and 3.30 for unidentified respiratory viruses.

They concluded that there is a significant association between respiratory infections, particularly flu, and heart attacks.

Many possible triggers

Several physiologic factors involved in infection might explain why flu could be a heart attack trigger when patients have atherosclerosis, including inflammation, biomechanical stress, or vasoconstriction, the authors wrote. Also, infections lead to platelet activation and endothelial dysfunction that can support thrombogenic conditions.

They also noted that infections can boost metabolic demand, induce hypoxemia or hypotension, or produce other vascular system stress that creates conditions ripe for thrombus occlusion that would provoke a heart attack.

Findings strengthen vaccine advice

Jeff Kwong, MD, MSc, lead author of the study and a researcher at the Institute for Clinical Evaluative Sciences and Public Health Ontario (PHO), said in a PHO press release that the findings support international guidelines that push for flu vaccination in those at risk for heart attack. "Our findings are important because an association between influenza and acute myocardial infarction reinforces the importance of vaccination."

Aside from vaccination and taking other precautions to avoid flu, researchers said patients shouldn't delay seeking medical care for cardiac symptoms, especially within the first week of an acute respiratory infection.

See also:

Jan 24 N Engl J Med abstract

Jan 24 PHO press release

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