inclusion criteria: - aged 18 to 55 years old in mid group; aged 6-17 years old in min group; aged 56 years and above in old group; aged in ebo group is not limited. - 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. - axillary temperature ≤37.0°c. - general good health as established by medical history and physical examination.

inclusion criteria: - aged 18 to 55 years old in mid group; aged 6-17 years old in min group; aged 56 years and above in old group; aged in ebo group is not limited. - 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. - axillary temperature ≤37.0°c. - general good health as established by medical history and physical examination.