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.

Inclusion Criteria:

1. ACS patient aged from 18 to 80 years.
2. Volunteer for the study and written informed consent.

Exclusion Criteria:

1. Participate in any drug clinical trials within 3 months.
2. Patients with life-threatening complications, or the researchers determined that the survival time of patients with no more than 1 years.
3. Serious neurological disease (Alzheimer's disease, Parkinson syndrome, progressive lower limbs or deaf patients).
4. Previous history of cancer or tumor, or pathological examination confirmed precancerous lesions (such as breast ductal carcinoma in situ, or atypical hyperplasia of the cervix).
5. Patients refused to comply with the requirements of this study to complete the research work.
6. Patients Received influenza vaccination within 1 year.
7. Patients with contraindications for influenza vaccination.