exclusion criteria of arm a: history of covid-19 of less than 6 months prior to enrollment received partial or complete course of any type of covid-19 vaccine. history of close contact with a person infected or suspected of covid-19 not less than 14 days prior to study enrollment. positive test for sars-cov-2 infection, including but not limited to rt-pcr /rapid antigen test at screening, before the first dose (day 0) and before the second dose (day 7). for more exclusion criteria, please contact the investigator or sponsor. exclusion criteria of arm b: history of close contact with a person infected or suspected of covid-19 not less than 14 days prior to study enrollment. positive test for sars-cov-2 infection, including but not limited to rt-pcr/rat at screening, before the first dose (day 0) . have received three and more doses of covid-19 vaccine. pregnant or breastfeeding or intending to become pregnant or father children within the projected duration of the trial. for more exclusion criteria, please contact the investigator or sponsor.

exclusion criteria of arm a: history of covid-19 of less than 6 months prior to enrollment received partial or complete course of any type of covid-19 vaccine. history of close contact with a person infected or suspected of covid-19 not less than 14 days prior to study enrollment. positive test for sars-cov-2 infection, including but not limited to rt-pcr /rapid antigen test at screening, before the first dose (day 0) and before the second dose (day 7). for more exclusion criteria, please contact the investigator or sponsor. exclusion criteria of arm b: history of close contact with a person infected or suspected of covid-19 not less than 14 days prior to study enrollment. positive test for sars-cov-2 infection, including but not limited to rt-pcr/rat at screening, before the first dose (day 0) . have received three and more doses of covid-19 vaccine. pregnant or breastfeeding or intending to become pregnant or father children within the projected duration of the trial. for more exclusion criteria, please contact the investigator or sponsor.