This is a single-arm, open-label, clinical study to evaluate the safety of HER2 tumor vaccine injection alone/in combination with standard of care chemotherapy in patients with HER2/neu overexpressing gastric or gastroesophageal junction (GEJ) adenocarcinoma.

The purpose of this study is to evaluate the safety and tolerability, antitumor activity, The immunoreactivity, pharmacokinetics and of HER2 tumor vaccine.

Inclusion Criteria:

• Male or female aged ≥ 18 years;
• Metastatic gastric or GEJ adenocarcinoma, or locally advanced disease not amenable to surgical resection;
• HER2/neu overexpression (3+ by immunohistochemistry (IHC) or if IHC 2+ confirmed by fluorescent in situ hybridization [FISH];
• ECOG score of 0 ~ 2;
• Adequate bone marrow, hepatic and renal and coagulation function;
• Women of childbearing age who have a negative pregnancy test within 7 days before treatment. Female patients of childbearing age, and male patients with partners of childbearing age must agree to use at least one medically recognized contraceptive method during study treatment and within at least 6 months after the last dose of investigational drug;
• Voluntarily participated in this study, signed the informed；

Exclusion Criteria:

• Continuous systemic treatment with either corticosteroids (>10 mg daily prednisone equivalents) or other immunosuppressive medications within 4 weeks prior to first dose of study treatment. Inhaled or topical steroids and physiological replacement doses of up to 10 mg daily prednisone equivalents are permitted in the absence of active auto-immune disease；
• Subjects who have had major surgery within 4 weeks before HER2 tumor vaccine administration
• Subjects who have received anti-tumor therapy such as chemotherapy, radiotherapy, biological therapy, endocrine therapy, targeted therapy, immunotherapy, etc within 5 elimination half-life prior first dose of HER2 tumor vaccine treatment;
• Subjects with known brain metastasis and/or clinically history tumor brain of metastasis；
• Patients with uncontrollable pleural effusion, abdominal effusion and pericardial effusion;
• Active infection requiring treatment. HIV, HCV, syphilis, CMV, EBV infected patients; Patients with active HBV replication
• Other conditions that the investigator assessed as ineligible for inclusion.