Influenza Vaccination for Secondary Prevention of Cardiovascular Events: A Systematic Review.

BACKGROUND: Influenza is a common respiratory infection that may cause complications, including cardiovascular events. Influenza illness has been shown to double the risk of myocardial infarction, with the highest risk among patients with established cardiovascular disease. Vaccination against influenza has been associated with reductions in myocardial infarction, cerebrovascular disease, and death.

OBJECTIVE: To evaluate the evidence for influenza vaccination as a strategy to reduce cardiovascular events specifically in patients with established cardiovascular disease.

DATA SOURCES AND STUDY SELECTION: MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials were searched with the terms "influenza vaccine" and "cardiovascular disease". Included in this review were randomized controlled trials (RCTs), nonrandomized studies, and meta-analyses that compared influenza vaccination against control in patients with established cardiovascular disease and that reported clinically meaningful cardiovascular outcomes (defined as cardiovascular death, myocardial infarction, and stroke).

DATA EXTRACTION AND SYNTHESIS: The search yielded 10 studies (3 nonrandomized studies, 5 RCTs, and 2 meta-analyses). The nonrandomized studies and the RCTs had inconsistent results with respect to cardiovascular death and adverse cardiovascular events. The 2 meta-analyses, which included the same 4 RCTs involving patients with established cardiovascular disease, showed that the influenza vaccine reduced cardiovascular death by about 50% relative to control. Vaccination also reduced major cardiovascular events by about 43%; the reduction was greater (54%) in the subgroup of patients with recent (≤ 1 year) acute coronary syndrome. However, these data are potentially confounded by small sample sizes, low event rates, and variable outcome reporting. There was also high clinical heterogeneity among the studies, which may not reflect contemporary practice.

CONCLUSIONS: Given the limitations of these data, it is unclear whether the cardiovascular benefit with influenza vaccination in patients with cardiovascular disease is a true effect. Nevertheless, because of the potential benefit and the low risk of adverse events, the annual influenza vaccine should be recommended for all patients with established cardiovascular disease.