inclusion criteria: 1. willing to participate in the clinical study and sign the informed consent; 2. staff members of peking university third hospital, including the hospital headquarters, capital international airport hospital, north hospital, beijing haidian hospital, peking university third hospital yanqing hospital and peking university third hospital chongli hospital; 3. test negative for covid-19 antigen and have no symptoms related to covid-19.

inclusion criteria: 1. willing to participate in the clinical study and sign the informed consent; 2. staff members of peking university third hospital, including the hospital headquarters, capital international airport hospital, north hospital, beijing haidian hospital, peking university third hospital yanqing hospital and peking university third hospital chongli hospital; 3. test negative for covid-19 antigen and have no symptoms related to covid-19.

inclusion criteria: willing to participate in the clinical study and sign the informed consent; staff members of peking university third hospital, including the hospital headquarters, capital international airport hospital, north hospital, beijing haidian hospital, peking university third hospital yanqing hospital and peking university third hospital chongli hospital; test negative for covid-19 antigen and have no symptoms related to covid-19.

inclusion criteria: willing to participate in the clinical study and sign the informed consent; staff members of peking university third hospital, including the hospital headquarters, capital international airport hospital, north hospital, beijing haidian hospital, peking university third hospital yanqing hospital and peking university third hospital chongli hospital; test negative for covid-19 antigen and have no symptoms related to covid-19.