Influenza-like illness in acute myocardial infarction patients during the winter wave of the influenza A H1N1 pandemic in London: a case-control study

Objective: To investigate recent respiratory and influenza-like illnesses (ILIs) in acute myocardial infarction patients compared with patients hospitalised for acute non-vascular surgical conditions during the second wave of the 2009 influenza A H1N1 pandemic.

Design: Case-control study.

Setting: Coronary care unit, acute cardiology and acute surgical admission wards in a major teaching hospital in London, UK.

Participants: 134 participants (70 cases and 64 controls) aged ≥40 years hospitalised for acute myocardial infarction and acute surgical conditions between 21 September 2009 and 28 February 2010, frequency-matched for gender, 5-year age-band and admission week. PRIMARY EXPOSURE: ILI (defined as feeling feverish with either a cough or sore throat) within the last month. SECONDARY EXPOSURES: Acute respiratory illness within the last month not meeting ILI criteria; nasopharyngeal and throat swab positive for influenza virus.

Results: 29 of 134 (21.6%) participants reported respiratory illness within the last month, of whom 13 (9.7%) had illnesses meeting ILI criteria. The most frequently reported category for timing of respiratory symptom onset was 8-14 days before admission (31% of illnesses). Cases were more likely than controls to report ILI-adjusted OR 3.17 (95% CI 0.61 to 16.47)-as well as other key respiratory symptoms, and were less likely to have received influenza vaccination-adjusted OR 0.46 (95% CI 0.19 to 1.12)-although the differences were not statistically significant. No swabs were positive for influenza virus.

Conclusions: Point estimates suggested that recent ILI was more common in patients hospitalised with acute myocardial infarction than with acute surgical conditions during the second wave of the influenza A H1N1 pandemic, and influenza vaccination was associated with cardioprotection, although the findings were not statistically significant. The study was underpowered, partly because the age groups typically affected by acute myocardial infarction had low rates of infection with the pandemic influenza strain compared with seasonal influenza.