Influenza vaccine shortly after myocardial infarction reduces 1-year mortality (NNT = 50)

Reference

Fröbert O, Götberg M, Erlinge D, et al. Influenza vaccination after myocardial infarction: a randomized, double-blind, placebo-controlled, multicenter trial. Circulation 2021;144(18):1476-1484.

Synopsis

This study identified 2571 adults in 8 countries who were hospitalized with acute myocardial infarction (MI) and had not received the influenza vaccine in the previous 12 months. The patients were randomized to receive the influenza vaccine or placebo within 72 hours of admission or angiography. Participants were only enrolled during the flu season (September through February in the northern hemisphere). The study was terminated prior to achieving the target enrollment of 4372 patients because of the COVID-19 pandemic. The primary outcome of death, MI, or stent thrombosis was based on a telephone interview 12 months after randomization. That is a potential limitation, though the authors performed a masked adjudication of any of these events after reviewing medical records. At baseline, groups were generally similar with a mean age of 60 years, 81% men, and 54% with ST-elevation MI. However, there were more patients with 2-vessel disease in the vaccine group (25.2% vs 21.7%) and correspondingly fewer with 1-vessel disease. The composite of all-cause mortality, stent thrombosis, and recurrent MI was less frequent in the vaccine group (5.0% vs 7.2%; hazard ratio [HR] 0.72; 95% CI 0.52 - 0.99; number needed to treat [NNT] = 45). Most of this difference was due to significantly lower rates of all-cause mortality (2.9% vs 4.9%; NNT = 50) and cardiovascular mortality (2.7% vs 4.5%; NNT = 56). There was no significant difference between the vaccine and placebo groups with regard to MI (2.0% vs 2.4%) or stent thrombosis (0.5% vs 0.2%). There were some interesting subgroup differences, with greater benefit seen for those without a previous MI, for those with non-ST elevation MI, and for nonsmokers. There was a large benefit for the 2017-18 and 2019-20 flu seasons, but not for the 2016-17 and 2018-19 flu seasons. This is probably because the match between the vaccine and the circulating flu strains was better in those two seasons. Harms were limited to mild injection site reactions. Industry support and participation was limited to provision of the influenza vaccine. Adverse events were rare and similar between groups. Finally, approximately 1 in 7 patients in both groups reported seeking a flu shot outside of the trial.