A Cochrane review included 8 studies with a total of 12 029 subjects. Four trials (n = 10 347) focused on prevention of influenza in the general or elderly population and reported cardiovascular outcomes among their safety analyses; four trials (n = 1 682) focused on prevention of cardiovascular events in patients with established coronary heart disease. Follow-up continued between 42 days and one year. Cardiovascular mortality was reported by four secondary prevention trials and was significantly reduced by influenza vaccination overall (RR 0.45, 95% CI 0.26 to 0.76), and by three trials reporting cardiovascular mortality as part of their safety analyses when the numbers of events were too small to permit conclusions. In studies of patients with coronary heart disease, composite outcomes of cardiovascular events tended to be decreased with influenza vaccination compared with placebo.
In a pooled analysis, cardiovascular death occurred in 11 of the 476 vaccinated participants and in 28 of the 483 placebo treated controls (RR 0.39, 95% CI 0.20 to 0.77). However, there was significant heterogeneity observed between the studies. Acute myocardial infarction occurred 16 times in the vaccinated group and 19 times in the placebo group (RR 0.85, 95% CI 0.44 to 1.62).Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment) and inconsistency.