inclusion criteria: * aged between 18 and 60 years. * able to understand the content of informed consent and willing to sign the informed consent * able and willing to complete all the secluded study process during the whole 6 months study follow-up period. * negative in hiv diagnostic test. * negative in serum antibodies (igg and igm) screening of covid-19. * normal in lung ct images (no imaging features of covid-19 * axillary temperature ≤37.0°c. * the bmi index is 18.5-30.0. * negative in nasopharyngeal swabs / sputum and anal swabs through rt-pcr * laboratory tests such as hematological examination and clinical biochemistry examination are in the normal range or without meaning judged by clinical doctor. * general good health as established by medical history and physical examination.

inclusion criteria: * aged between 18 and 60 years. * able to understand the content of informed consent and willing to sign the informed consent * able and willing to complete all the secluded study process during the whole 6 months study follow-up period. * negative in hiv diagnostic test. * negative in serum antibodies (igg and igm) screening of covid-19. * normal in lung ct images (no imaging features of covid-19 * axillary temperature ≤37.0°c. * the bmi index is 18.5-30.0. * negative in nasopharyngeal swabs / sputum and anal swabs through rt-pcr * laboratory tests such as hematological examination and clinical biochemistry examination are in the normal range or without meaning judged by clinical doctor. * general good health as established by medical history and physical examination.

inclusion criteria: - aged between 18 and 60 years. - able to understand the content of informed consent and willing to sign the informed consent - able and willing to complete all the secluded study process during the whole 6 months study follow-up period. - negative in hiv diagnostic test. - negative in serum antibodies (igg and igm) screening of covid-19. - normal in lung ct images (no imaging features of covid-19 - axillary temperature ≤37.0°c. - the bmi index is 18.5-30.0. - negative in nasopharyngeal swabs / sputum and anal swabs through rt-pcr - laboratory tests such as hematological examination and clinical biochemistry examination are in the normal range or without meaning judged by clinical doctor. - general good health as established by medical history and physical examination.

inclusion criteria: - aged between 18 and 60 years. - able to understand the content of informed consent and willing to sign the informed consent - able and willing to complete all the secluded study process during the whole 6 months study follow-up period. - negative in hiv diagnostic test. - negative in serum antibodies (igg and igm) screening of covid-19. - normal in lung ct images (no imaging features of covid-19 - axillary temperature ≤37.0°c. - the bmi index is 18.5-30.0. - negative in nasopharyngeal swabs / sputum and anal swabs through rt-pcr - laboratory tests such as hematological examination and clinical biochemistry examination are in the normal range or without meaning judged by clinical doctor. - general good health as established by medical history and physical examination.