inclusion criteria: 1. male or female aged ≥18 years old when signing icf (the specific age range depends on the authorized age range by local regulatory authority); 2. participants who were vaccinated with primary series covid-19 vaccine and/or received booster vaccination, and the last dose of covid-19 vaccine was received more than 3 months; 3. the participant and/or his/her legal guardian and/or his entrusted person can sign written icf, and can fully understand the trial procedure, the risk of participating in the trial, and other interventions that can be selected if they do not participate in the trial; 4. the baseline sars-cov-2 anti-spike igg will be measured at the screening period and an upper igg limit may be used to screen the participants. it is aimed to recruit participants who are susceptible to sars-cov-2 infection. the participant with the igg level higher than the limit may be excluded from the study. the igg limit will be determined based on the average level of igg in a region, and may not be applied if the incidence rate of covid-19 is low; 5. the participant and/or his/her legal guardian and/or his entrusted person have the ability to read, understand, and fill in record cards; 6. healthy participants or participants with pre-existing medical conditions who are in stable condition. the "pre-existing medical conditions" include but not limited to hypertension, diabetes, chronic cholecystitis and cholelithiasis, chronic gastritis that meet the described criteria. a stable medical condition is defined as disease not requiring significant change in therapy or no need for hospitalization as a consequence of worsening disease state for at least 3 months prior to enrollment; 7. fertile men and women of childbearing potential voluntarily agree to take effective contraceptive measures from signing icf to 6 months after the last dose of study vaccination; the pregnancy test results of women of childbearing potential are negative on screening.

inclusion criteria: 1. male or female aged ≥18 years old when signing icf (the specific age range depends on the authorized age range by local regulatory authority); 2. participants who were vaccinated with primary series covid-19 vaccine and/or received booster vaccination, and the last dose of covid-19 vaccine was received more than 3 months; 3. the participant and/or his/her legal guardian and/or his entrusted person can sign written icf, and can fully understand the trial procedure, the risk of participating in the trial, and other interventions that can be selected if they do not participate in the trial; 4. the baseline sars-cov-2 anti-spike igg will be measured at the screening period and an upper igg limit may be used to screen the participants. it is aimed to recruit participants who are susceptible to sars-cov-2 infection. the participant with the igg level higher than the limit may be excluded from the study. the igg limit will be determined based on the average level of igg in a region, and may not be applied if the incidence rate of covid-19 is low; 5. the participant and/or his/her legal guardian and/or his entrusted person have the ability to read, understand, and fill in record cards; 6. healthy participants or participants with pre-existing medical conditions who are in stable condition. the "pre-existing medical conditions" include but not limited to hypertension, diabetes, chronic cholecystitis and cholelithiasis, chronic gastritis that meet the described criteria. a stable medical condition is defined as disease not requiring significant change in therapy or no need for hospitalization as a consequence of worsening disease state for at least 3 months prior to enrollment; 7. fertile men and women of childbearing potential voluntarily agree to take effective contraceptive measures from signing icf to 6 months after the last dose of study vaccination; the pregnancy test results of women of childbearing potential are negative on screening.

inclusion criteria: male or female aged ≥18 years old when signing icf (the specific age range depends on the authorized age range by local regulatory authority); participants who were vaccinated with primary series covid-19 vaccine and/or received booster vaccination, and the last dose of covid-19 vaccine was received more than 3 months; the participant and/or his/her legal guardian and/or his entrusted person can sign written icf, and can fully understand the trial procedure, the risk of participating in the trial, and other interventions that can be selected if they do not participate in the trial; the baseline sars-cov-2 anti-spike igg will be measured at the screening period and an upper igg limit may be used to screen the participants. it is aimed to recruit participants who are susceptible to sars-cov-2 infection. the participant with the igg level higher than the limit may be excluded from the study. the igg limit will be determined based on the average level of igg in a region, and may not be applied if the incidence rate of covid-19 is low; the participant and/or his/her legal guardian and/or his entrusted person have the ability to read, understand, and fill in record cards; healthy participants or participants with pre-existing medical conditions who are in stable condition. the "pre-existing medical conditions" include but not limited to hypertension, diabetes, chronic cholecystitis and cholelithiasis, chronic gastritis that meet the described criteria. a stable medical condition is defined as disease not requiring significant change in therapy or no need for hospitalization as a consequence of worsening disease state for at least 3 months prior to enrollment; fertile men and women of childbearing potential voluntarily agree to take effective contraceptive measures from signing icf to 6 months after the last dose of study vaccination; the pregnancy test results of women of childbearing potential are negative on screening.

inclusion criteria: male or female aged ≥18 years old when signing icf (the specific age range depends on the authorized age range by local regulatory authority); participants who were vaccinated with primary series covid-19 vaccine and/or received booster vaccination, and the last dose of covid-19 vaccine was received more than 3 months; the participant and/or his/her legal guardian and/or his entrusted person can sign written icf, and can fully understand the trial procedure, the risk of participating in the trial, and other interventions that can be selected if they do not participate in the trial; the baseline sars-cov-2 anti-spike igg will be measured at the screening period and an upper igg limit may be used to screen the participants. it is aimed to recruit participants who are susceptible to sars-cov-2 infection. the participant with the igg level higher than the limit may be excluded from the study. the igg limit will be determined based on the average level of igg in a region, and may not be applied if the incidence rate of covid-19 is low; the participant and/or his/her legal guardian and/or his entrusted person have the ability to read, understand, and fill in record cards; healthy participants or participants with pre-existing medical conditions who are in stable condition. the "pre-existing medical conditions" include but not limited to hypertension, diabetes, chronic cholecystitis and cholelithiasis, chronic gastritis that meet the described criteria. a stable medical condition is defined as disease not requiring significant change in therapy or no need for hospitalization as a consequence of worsening disease state for at least 3 months prior to enrollment; fertile men and women of childbearing potential voluntarily agree to take effective contraceptive measures from signing icf to 6 months after the last dose of study vaccination; the pregnancy test results of women of childbearing potential are negative on screening.