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Abstract
Cardiac complications of influenza infection, such as myocarditis, are well recognised,
but the role of influenza as a trigger of acute myocardial infarction is less clear.
We did a systematic review of the evidence that influenza (including influenza-like
illness and acute respiratory infection) triggers acute myocardial infarction or cardiovascular
death. We examined the effectiveness of influenza vaccines at protecting against cardiac
events and did a meta-analysis of data from randomised controlled trials. 42 publications
describing 39 studies were identified. Many observational studies in different settings
with a range of methods reported consistent associations between influenza and acute
myocardial infarction. There was weaker evidence of an association with cardiovascular
death. Two small randomised trials assessed the protection provided by influenza vaccine
against cardiac events in people with existing cardiovascular disease. Whereas one
trial found that influenza vaccination gave significant protection against cardiovascular
death, the other trial was inconclusive. A pooled estimate from a random-effects model
suggests a protective, though non-significant, effect (relative risk 0.51, 95% CI
0.15-1.76). We believe influenza vaccination should be encouraged wherever indicated,
especially in people with existing cardiovascular disease, among whom there is often
suboptimum vaccine uptake. Further evidence is needed on the effectiveness of influenza
vaccines to reduce the risk of cardiac events in people without established vascular
disease.