inclusion criteria: subject who met all of the following criteria may be enrolled. 1. age more than or equal 18 years on screening. 2. judged by the investigator to be healthy based on medical history, physical examination and vital signs performed at screening. 3. able to provide informed consent. 4. able and willing to comply with all study procedures over follow up period of approximately 12 months. 5. received 2 or more doses of inactivated covid-19 vaccine as primary series with their last dose at least 3 months prior to enrollment. 6. axillary body temperature ≤37.5℃. 7. sars-cov-2 test was negative for nasopharyngeal swabs by rt pcr. 8. non-pregnant and not lactating women. 9. for female subjects of childbearing potential: must agree to avoid pregnancy from study day 0 to study day 60 during the study. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period. acceptable methods of avoiding pregnancy include a sterile sexual partner, sexual abstinence (not engaging in sexual intercourse), hormonal contraceptives (oral, injection, transdermal patch, or implant), vaginal ring, intrauterine device (iud), or the combination of a condom or diaphragm with spermicide. 10. male subjects who are sexually active with a woman of childbearing potential and have not had vasectomy must agree to practice a highly effective form of contraception with their female partners of childbearing potential during the trial, starting after screening until 60 days after receiving the last vaccination. 11. men must be willing to refrain from sperm donation, starting after screening until 60 days after receiving the last vaccination.

inclusion criteria: subject who met all of the following criteria may be enrolled. 1. age more than or equal 18 years on screening. 2. judged by the investigator to be healthy based on medical history, physical examination and vital signs performed at screening. 3. able to provide informed consent. 4. able and willing to comply with all study procedures over follow up period of approximately 12 months. 5. received 2 or more doses of inactivated covid-19 vaccine as primary series with their last dose at least 3 months prior to enrollment. 6. axillary body temperature ≤37.5℃. 7. sars-cov-2 test was negative for nasopharyngeal swabs by rt pcr. 8. non-pregnant and not lactating women. 9. for female subjects of childbearing potential: must agree to avoid pregnancy from study day 0 to study day 60 during the study. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period. acceptable methods of avoiding pregnancy include a sterile sexual partner, sexual abstinence (not engaging in sexual intercourse), hormonal contraceptives (oral, injection, transdermal patch, or implant), vaginal ring, intrauterine device (iud), or the combination of a condom or diaphragm with spermicide. 10. male subjects who are sexually active with a woman of childbearing potential and have not had vasectomy must agree to practice a highly effective form of contraception with their female partners of childbearing potential during the trial, starting after screening until 60 days after receiving the last vaccination. 11. men must be willing to refrain from sperm donation, starting after screening until 60 days after receiving the last vaccination.

inclusion criteria: subject who met all of the following criteria may be enrolled. age more than or equal 18 years on screening. judged by the investigator to be healthy based on medical history, physical examination and vital signs performed at screening. able to provide informed consent. able and willing to comply with all study procedures over follow up period of approximately 12 months. received 2 or more doses of inactivated covid-19 vaccine as primary series with their last dose at least 3 months prior to enrollment. axillary body temperature ≤37.5℃. sars-cov-2 test was negative for nasopharyngeal swabs by rt pcr. non-pregnant and not lactating women. for female subjects of childbearing potential: must agree to avoid pregnancy from study day 0 to study day 60 during the study. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period. acceptable methods of avoiding pregnancy include a sterile sexual partner, sexual abstinence (not engaging in sexual intercourse), hormonal contraceptives (oral, injection, transdermal patch, or implant), vaginal ring, intrauterine device (iud), or the combination of a condom or diaphragm with spermicide. male subjects who are sexually active with a woman of childbearing potential and have not had vasectomy must agree to practice a highly effective form of contraception with their female partners of childbearing potential during the trial, starting after screening until 60 days after receiving the last vaccination. men must be willing to refrain from sperm donation, starting after screening until 60 days after receiving the last vaccination.

inclusion criteria: subject who met all of the following criteria may be enrolled. age more than or equal 18 years on screening. judged by the investigator to be healthy based on medical history, physical examination and vital signs performed at screening. able to provide informed consent. able and willing to comply with all study procedures over follow up period of approximately 12 months. received 2 or more doses of inactivated covid-19 vaccine as primary series with their last dose at least 3 months prior to enrollment. axillary body temperature ≤37.5℃. sars-cov-2 test was negative for nasopharyngeal swabs by rt pcr. non-pregnant and not lactating women. for female subjects of childbearing potential: must agree to avoid pregnancy from study day 0 to study day 60 during the study. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period. acceptable methods of avoiding pregnancy include a sterile sexual partner, sexual abstinence (not engaging in sexual intercourse), hormonal contraceptives (oral, injection, transdermal patch, or implant), vaginal ring, intrauterine device (iud), or the combination of a condom or diaphragm with spermicide. male subjects who are sexually active with a woman of childbearing potential and have not had vasectomy must agree to practice a highly effective form of contraception with their female partners of childbearing potential during the trial, starting after screening until 60 days after receiving the last vaccination. men must be willing to refrain from sperm donation, starting after screening until 60 days after receiving the last vaccination.