Grand Rounds

Is Acute Myocardial Infarction Associated with the Flu?

BACKGROUND AND PURPOSE:

Acute myocardial infarction has been associated with respiratory infection especially in older individuals who are at higher CVD risk
Quality studies remain limited
Kwong et al. (NEJM, 2018) evaluated the association between laboratory-confirmed influenza infection and acute myocardial infarction

Self-controlled case-series design
- Index date for defining the exposure (laboratory-confirmed influenza infection) was set as the date the respiratory specimen was obtained
- Observation period was set as the interval from 1 year before to 1 year after the index date
- Included patients who had at least one admission for acute myocardial infarction during this period
Influenza infection was confirmed using high-specificity laboratory results
Hospitalization for acute myocardial infarction was confirmed using administrative data
Participants:
- All Ontario residents registered with publicly funded health insurance
- Those hospitalized for one or more respiratory viruses
  - ≥35 years at testing
- Those hospitalized for an acute myocardial infarction
Risk interval: Defined as the first 7 days after respiratory specimen collection
Control interval: Defined as 1 year before and 1 year after the risk interval
To examine the specificity, analyses were performed looking at viral exposures other than influenza

364 hospitalizations occurred for acute myocardial infarction within 1 year before and 1 year after a positive test result for influenza
Patient demographics
- median age 77 years
- 48% of the patients were female
- 24% had had a previous hospitalization for acute myocardial infarction
- 49% had CVD risk factors including diabetes, dyslipidemia, and hypertension
- 31% had received flu vaccine
- 82% of infections were due to influenza A
Of these, 20 (20.0 admissions per week) occurred during the risk interval and 344 (3.3 admissions per week) occurred during the control interval
The incidence ratio of an admission for acute myocardial infarction during the risk interval as compared with the control interval was 6.05 (95% CI, 86-9.50)
After day 7, no increased incidence was observed
Analysis by virus and type
- 10.11 (95% CI, 4.37 to 23.38) after influenza B detection
- 5.17 (95% CI, 3.02 to 8.84) after influenza A detection
- 3.51 (95% CI, 1.11 to 11.12) after respiratory syncytial virus detection
- 2.77 (95% CI, 1.23 to 6.24) after other viruses were detected

There is a statistically increased incidence of acute myocardial infarction (6x) during the 7 days after laboratory confirmation of influenza infection as during the 1 year time interval before and after a positive flu test.
There appears to be a significant association between respiratory infections, especially influenza, and acute myocardial infarction

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