A Study to Evaluate the Immunogenicity and Safety of A Recombinant Protein COVID-19 Vaccine as Booster Vaccines

Inclusion Criteria: * 1\. Aged ≥3 years old when signing ICF; * 2\. Participants who were vaccinated with licenced COVID-19 vaccines with recommended doses and immune schedule, and the interval between the last dose and ICF is ≥6 months. * 3\. The participant and/or his/her legal guardians or entrusted person can sign the 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 participant and/or his/her legal guardians or entrusted person have the ability to read, understand, and fill in record cards; * 5\. Healthy participants or participants with pre-existing medical conditions who are in stable condition. 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; * 6\. Fertile men and women of childbearing potential voluntarily agree to take effective contraceptive measures from signing ICF to 6 months after the study vaccination; the pregnancy test results of women of childbearing potential are negative on screening. Exclusion Criteria: * 1\. Presence of fever within 72 hours before the study vaccination (for participants \>14 years old, axillary temperature ≥37.3℃; for participants ≤14 years old, axillary temperature ≥37.5℃;); * 2\. A positive result of antigen test or nucleic acid …