Get a Flu Shot and Prevent a CV Event -- It's That Simple

Charles P. Vega, MD


March 24, 2014

In This Article

The Meta-analysis, Deconstructed

Researchers focused on randomized clinical trials of influenza vaccination compared with placebo or standard care. Studies published before August 2013 were eligible for inclusion in the meta-analysis, and all trials included at least 50 patients.

The primary endpoint for the current study was a composite of CV events, including unstable angina, stroke, heart failure, myocardial infarction, urgent coronary revascularization, or cardiovascular death. These events were reported in the original research either as primary outcomes or as part of the assessment of vaccine safety, and all events occurred within 12 months of follow-up.

Of 2189 articles identified in the original search, a total of 12 randomized controlled trials met criteria to be included in the meta-analysis. Furthermore, 6 trials were judged to be of higher quality in adhering to the use of a placebo or usual care control. Research included studies using both the inactivated intramuscular vaccine and live, attenuated nasal vaccine. The 6 higher-quality trials included a total of 6735 participants with a mean age of 67 years. One half (51%) of participants in these trials were women, and 36% had a CVD history.

The other 6 trials compared standard influenza vaccination with experimental forms of vaccination, usually a more potent vaccine. These trials included a total of 16,857 participants with a mean age of 72 years. The mean follow-up duration across all trials was approximately 7 months.

Five trials were judged to be of particularly high quality. When double-blinding (vs single-blinding) was excluded as a quality measure, 4 more studies received the grade of high-quality. Funnel-plot analysis failed to detect publication bias, and there was no significant heterogeneity in comparing results from different studies. The overall number of CV events in the included research was low (246 in total), raising the risk for underpowering to detect the outcomes of the meta-analysis.

Five of the studies that compared the influenza vaccine with placebo or control were published. The rates of CV events in the vaccine and control groups in these studies were 2.9% and 4.7%, respectively (risk ratio [RR], 0.64; 95% confidence interval [CI], 0.48-0.86). The associated number needed to treat with influenza vaccine to prevent 1 CV event was 58. Adding data from the unpublished controlled trial of the influenza vaccine failed to materially alter this outcome.

In subgroup analysis, influenza vaccination was particularly effective in reducing CV risk among patients with a recent history of acute coronary syndrome (ACS) (RR, 0.45; 95% CI, 0.32-0.63; number needed to treat, 8). However, the vaccine was not significantly effective in this outcome among participants with stable coronary artery disease.

The majority of deaths in the included research were due to CVD. However, the influenza vaccine failed to reduce the risk for CV or overall mortality, even among participants with a recent history of ACS.

In the 6 trials comparing more potent influenza vaccines with standard doses, the respective rates of CV events were 0.39% and 0.60% (RR, 0.72; 95% CI, 0.42-1.13). This difference was not significant. In addition, the influenza vaccine appeared to be more effective in preventing CVD in studies in which CVD was measured as a primary outcome vs a safety measure.


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