Observational evidence is mounting that influenza and acute respiratory infections can precipitate major adverse cardiovascular events such as acute myocardial infarction (AMI) in vulnerable groups. Influenza viruses cause seasonal epidemics. Around 50% of infections are asymptomatic1; clinically apparent influenza infection is usually mild and self-limiting, with severe systemic illness affecting a small proportion of people. Deaths typically occur in people at the extremes of age or in those with underlying medical conditions.2 Cardiac pathology in acute influenza infection is thought to result either from direct effects of the virus on the myocardium, for example, influenza-associated myocarditis, or through exacerbation of underlying cardiovascular disease.3 Acute infections such as influenza have transient vasoconstrictive and systemic prothrombotic effects, which may contribute to coronary artery plaque disruption and thrombosis.4

Globally, ischaemic heart disease—predominantly due to AMI—is the leading cause of death and is projected to remain so in 2030.5 AMI remains a key challenge in the clinical management of ischaemic heart disease, especially as it can occur in individuals not previously identified to be at high cardiovascular risk. Influenza is associated with all-cause mortality, with the majority of deaths due to respiratory and, to a lesser extent, cardiovascular causes.6 The impact of influenza on AMI burden remains poorly understood. Key questions for patients, clinicians and public health policymakers include what is the evidence that AMI risk is associated with acute respiratory infections such as influenza? Who is most at risk? Which interventions are most effective and cost-effective? This important area is the focus of a case–control study by MacIntyre et al.7

A major difficulty for studies investigating associations between influenza and acute cardiovascular events is how to define recent influenza. While laboratory testing is usually …