* history of allergy to any vaccine or its excipients; * presence of severe, uncontrollable or hospitalized diseases; * history of major surgery within 3 months prior to enrollment; * history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers) or covid-19; * congenital or acquired immunodeficiency or autoimmune disease; * any acute diseases or acute attacks of chronic diseases within 7 days prior to enrollment; * receipt of any covid-19 prophylactic medication other than primary series of icv; * long-term receipt (\>14 consecutive days) of glucocorticoids or other immunosuppressive agents within the past 6 months; * receipt of biological agents, immunopotentiators or immunosuppressants within the past 6 months; * receipt of blood or blood-related products within 3 months prior to vaccination; * administration of antipyretics, painkillers or antiallergics within 24 hours prior to vaccination; * participating or planning to participate in other clinical trials during the study period; * pregnant or lactating females, women of childbearing age of pregnancy test positive; * presence of any underlying disease or condition which, in the opinion of the investigator, may place the subject at unacceptable risk, is unable to meet the requirements of the protocol, or interfere with the assessment of vaccine response.

* history of allergy to any vaccine or its excipients; * presence of severe, uncontrollable or hospitalized diseases; * history of major surgery within 3 months prior to enrollment; * history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers) or covid-19; * congenital or acquired immunodeficiency or autoimmune disease; * any acute diseases or acute attacks of chronic diseases within 7 days prior to enrollment; * receipt of any covid-19 prophylactic medication other than primary series of icv; * long-term receipt (\>14 consecutive days) of glucocorticoids or other immunosuppressive agents within the past 6 months; * receipt of biological agents, immunopotentiators or immunosuppressants within the past 6 months; * receipt of blood or blood-related products within 3 months prior to vaccination; * administration of antipyretics, painkillers or antiallergics within 24 hours prior to vaccination; * participating or planning to participate in other clinical trials during the study period; * pregnant or lactating females, women of childbearing age of pregnancy test positive; * presence of any underlying disease or condition which, in the opinion of the investigator, may place the subject at unacceptable risk, is unable to meet the requirements of the protocol, or interfere with the assessment of vaccine response.

history of allergy to any vaccine or its excipients; presence of severe, uncontrollable or hospitalized diseases; history of major surgery within 3 months prior to enrollment; history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers) or covid-19; congenital or acquired immunodeficiency or autoimmune disease; any acute diseases or acute attacks of chronic diseases within 7 days prior to enrollment; receipt of any covid-19 prophylactic medication other than primary series of icv; long-term receipt (>14 consecutive days) of glucocorticoids or other immunosuppressive agents within the past 6 months; receipt of biological agents, immunopotentiators or immunosuppressants within the past 6 months; receipt of blood or blood-related products within 3 months prior to vaccination; administration of antipyretics, painkillers or antiallergics within 24 hours prior to vaccination; participating or planning to participate in other clinical trials during the study period; pregnant or lactating females, women of childbearing age of pregnancy test positive; presence of any underlying disease or condition which, in the opinion of the investigator, may place the subject at unacceptable risk, is unable to meet the requirements of the protocol, or interfere with the assessment of vaccine response.

history of allergy to any vaccine or its excipients; presence of severe, uncontrollable or hospitalized diseases; history of major surgery within 3 months prior to enrollment; history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers) or covid-19; congenital or acquired immunodeficiency or autoimmune disease; any acute diseases or acute attacks of chronic diseases within 7 days prior to enrollment; receipt of any covid-19 prophylactic medication other than primary series of icv; long-term receipt (>14 consecutive days) of glucocorticoids or other immunosuppressive agents within the past 6 months; receipt of biological agents, immunopotentiators or immunosuppressants within the past 6 months; receipt of blood or blood-related products within 3 months prior to vaccination; administration of antipyretics, painkillers or antiallergics within 24 hours prior to vaccination; participating or planning to participate in other clinical trials during the study period; pregnant or lactating females, women of childbearing age of pregnancy test positive; presence of any underlying disease or condition which, in the opinion of the investigator, may place the subject at unacceptable risk, is unable to meet the requirements of the protocol, or interfere with the assessment of vaccine response.