Influenza Vaccination After Acute Coronary Syndrome

The goal of this clinical trial is to learn if influenza vaccination can prevent adverse cardiac events in Chinese acute coronary syndrome patients. The main questions it aims to answer are: * Whether influenza vaccination can decrease events of cardiovascular death, MI, or stroke? * Whether influenza vaccination can decrease events of all cause death, unplanned revascularization, unplanned hospitalization for heart failure or for arrhythmia, stent thrombosis? If there is a comparison group: Researchers will compare influenza vaccination and placebo to see if adverse cardiac events decrease. Participants will receive an influenza vaccination or placebo after enrollment and phone calls for follow-up at 1 month, 3 months, 6 months and 1 year after discharge.