6 times higher risk of heart attack in the week after flu diagnosis
The risk of having an acute myocardial infarction (AMI) is sixfold higher within the week after laboratory-confirmed influenza infection, reveals a study.
“Our findings are important because an association between influenza and AMI reinforces the importance of vaccination,” said lead author Dr Jeffrey Kwong of the Institute of Clinical Evaluative Science in Toronto, Canada.
The self-controlled case-series study involved 332 patients (median age 77 years) hospitalized for AMI within the 1-year period before or after a positive laboratory test result for influenza. Hospitalization episodes were categorized into those occurring within the “risk interval” if they fell within the first 7 days after respiratory specimen collection, or within the “control interval” if they occurred within the 1 year before or after the risk interval. [N Engl J Med 2018;378:345-353]
A higher rate of AMI hospitalization occurred during the 7 days after laboratory-confirmed influenza infection compared with during the control interval (20 vs 3.3 admissions/week).
This corresponds to a six times greater incidence ratio of AMI hospitalization during the risk interval than the control interval (6.05, 95 percent confidence interval [CI], 3.86–9.50). The risk of AMI was particularly high in the period immediately after influenza diagnosis, with an incidence ratio of 6.30 (95 percent CI, 3.25–12.22) for days 1 through 3 and 5.78 (95 percent CI, 3.17–10.53) for days 4 through 7.
There was no increase in AMI incidence after day 7 of influenza detection (incidence ratios, 0.60 and 0.75 for days 8–14 and days 15–28, respectively).
“The risk of acute MI is greatest in the week after the detection of influenza … After that first week, [the risk] does seem to go back down to baseline,” observed Kwong.
Subgroup analyses showed that AMI incidence increased after influenza infection among adults aged >65 years vs ≤65 years (incidence ratios, 7.31 vs 2.38; p-interaction=0.14) and among those with first hospitalization for AMI vs with prior AMI history (6.93 vs 3.53; p-interaction=0.29), although no significant differences were observed between subgroups.
Also, AMI incidence was higher within 7 days of detection of influenza B compared with influenza A (incidence ratios, 10.11 vs 5.17; p-interaction=0.19), but the difference was not significant. Alternative exposures to respiratory syncytial virus (incidence ratio, 3.51) and other respiratory viruses (incidence ratio, 2.77) were also associated with a higher AMI incidence although the incidence ratios were lower than for influenza infection.
“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 … [such as] persons older than 65 years,” said Kwong. “People at risk of heart disease should take precautions to prevent respiratory infections, and especially influenza, through measures including vaccinations and handwashing.”
Other strategies to minimize the risk of cardiovascular events associated with respiratory infections include promoting infection prevention practices such as respiratory etiquette and social distancing, added Kwong and co-authors.
“In the context of chronic atherosclerotic vascular disease, an infectious illness may cause an acute coronary syndrome through acute inflammation, biomechanical stress, and vasoconstriction,” proposed the researchers on the possible mechanisms underlying the association.
Meanwhile, as the results are based on laboratory-confirmed respiratory infections (indicative of sufficient severity to call for a laboratory testing), the researchers cautioned against generalizing the findings to milder infections which usually do not require testing for respiratory viruses.