Evaluation of COVID-19 Immune Barrier and Reinfection Risk

Inclusion Criteria: 1. No age limit, no gender limit; 2. Medical staff, administrative and logistics staff who work in the Third Affiliated Hospital of Sun Yat-Sen University or other participants who can cooperate with the follow-up for 48 weeks; 3. The participants need to be sure whether they have ever been infected with COVID-19, and need to be clearly remember the time when they first infected with COVID-19. Exclusion Criteria: 1. Have the following serious respiratory diseases: such as asthma, bronchiectasis, chronic obstructive pulmonary disease, pulmonary interstitial disease, tuberculosis and other respiratory diseases that may interfere with symptom observation; 2. Those with other serious diseases or disease history that affect immune function, including but not limited to uncontrolled and unresectable malignant tumors,hematological diseases, cachexia, active bleeding, severe malnutrition, mental diseases, autoimmune diseases, HIV, etc. 3. Those who have long-term assignment plans and cannot return to the hospital regularly for follow-up.