inclusion criteria: * healthy adults 18 years of age and above at the time of enrollment; * able to provide consent to participate in and having signed an informed consent form (icf); * able and willing to complete all the scheduled study procedures during the whole study follow-up period; * hiv negative; * no nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, sore throat, etc; * igg ang igm negative for covid-19; * axillary temperature ≤37.0℃; * no contact history of covid-19.

inclusion criteria: * healthy adults 18 years of age and above at the time of enrollment; * able to provide consent to participate in and having signed an informed consent form (icf); * able and willing to complete all the scheduled study procedures during the whole study follow-up period; * hiv negative; * no nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, sore throat, etc; * igg ang igm negative for covid-19; * axillary temperature ≤37.0℃; * no contact history of covid-19.

inclusion criteria: - healthy adults 18 years of age and above at the time of enrollment; - able to provide consent to participate in and having signed an informed consent form (icf); - able and willing to complete all the scheduled study procedures during the whole study follow-up period; - hiv negative; - no nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, sore throat, etc; - igg ang igm negative for covid-19; - axillary temperature ≤37.0℃; - no contact history of covid-19.

inclusion criteria: - healthy adults 18 years of age and above at the time of enrollment; - able to provide consent to participate in and having signed an informed consent form (icf); - able and willing to complete all the scheduled study procedures during the whole study follow-up period; - hiv negative; - no nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, sore throat, etc; - igg ang igm negative for covid-19; - axillary temperature ≤37.0℃; - no contact history of covid-19.