Inclusion Criteria:  18 ≤age ≤59 years; Vaccinated with two doses of SARS-CoV-2 vaccine (manufactured by Sino Biopharmaceutical Limited or SINOVAC BIOTECH CO., LTD.) (≥6 months from the time of vaccination). Not vaccinated third dose vaccination; Voluntarily participate in the study with signed informed consent form.  Exclusion criteria:  Pregnancy or lactation period; Infected with human immunodeficiency virus (HIV), hepatitis B virus (HBV) or hepatitis C virus (HCV); Plan to vaccinate third dose of SARS-CoV-2 vaccine within 2 months; History of autoimmune diseases (e.g. rheumatoid arthritis, systemic lupus erythematosus, dry syndrome, etc.); Previously infected with SARS-CoV-2 or close contact of SARS-CoV-2 infected individual; Other SARS-CoV-2 vaccination history (not manufactured by Sino Biopharmaceutical Limited or SINOVAC BIOTECH CO., LTD.); End-stage cancer or other terminal diseases with life expectancy <6 month; History of severe cardiovascular and cerebrovascular diseases, such as heart failure, uncontrolled coronary heart disease, cardiomyopathy, uncontrolled arrhythmia, uncontrolled high blood pressure, or history of myocardial or cerebral infarction within past six months; Participating in other clinical trials.