inclusion criteria: 1. informed consent: the subject (or the subject's legally acceptable representative, if applicable) must be capable of giving written informed consent and, prior to the commencement of any study-specific procedure, must sign an icf indicating the consent on the subject's voluntary participation in the study and compliance with the requirements and restrictions listed on the icf. 2. bnt162b2 vaccination status: the subject must have received 2 doses of bnt162b2 in hong kong, with the second dose completed at least 180 days prior to the first vaccination. 3. gender and age: male or female, at the age of ≥ 18 and ≤ 75 on the day of signing the icf. 4. body weight and bmi: body weight ≥ 45 kg and bmi ≥ 18.5 kg/m2 and \< 25 kg/m2 at screening and baseline. 5. medical conditions or diagnoses: existence of all of the following medical conditions or diagnoses: 1. generally in good health with no clinically significant abnormality, as determined by medical history, physical examination, 12-lead ecg and clinical laboratory tests at screening and baseline; 2. normal vital signs at screening and baseline, as defined by: * body (tympanic) temperature ≤ 37.5 oc; * resting pulse rate ≥ 50 and ≤ 100 bpm; and * dbp ≥ 50 and ≤ 90 mmhg and sbp ≥ 90 and ≤ 140 mmhg. 6. contraception: willingness and agreement to undertake measures to avoid pregnancy of the subject or the subject's sexual partner(s) as detailed below: 1. a female subject who is a woman of childbearing potential (wocbp) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from at least 30 days prior to the first vaccination until 60 days after the second vaccination; 2. a male subject (i) who is sexually active with a wocbp (except who is permanently sterile by bilateral orchiectomy or vasectomy) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from the first vaccination until 60 days after the second vaccination; and (ii) must be willing and agree to refrain from sperm donation during the aforesaid period. 7. breastfeeding: a female subject must be willing and agree to avoid engagement in breastfeeding at any time from the first vaccination until 60 days after the second vaccination. 8. blood donation: willingness and agreement to avoid blood donation from screening to the end of the period of participation in this study.

inclusion criteria: 1. informed consent: the subject (or the subject's legally acceptable representative, if applicable) must be capable of giving written informed consent and, prior to the commencement of any study-specific procedure, must sign an icf indicating the consent on the subject's voluntary participation in the study and compliance with the requirements and restrictions listed on the icf. 2. bnt162b2 vaccination status: the subject must have received 2 doses of bnt162b2 in hong kong, with the second dose completed at least 180 days prior to the first vaccination. 3. gender and age: male or female, at the age of ≥ 18 and ≤ 75 on the day of signing the icf. 4. body weight and bmi: body weight ≥ 45 kg and bmi ≥ 18.5 kg/m2 and \< 25 kg/m2 at screening and baseline. 5. medical conditions or diagnoses: existence of all of the following medical conditions or diagnoses: 1. generally in good health with no clinically significant abnormality, as determined by medical history, physical examination, 12-lead ecg and clinical laboratory tests at screening and baseline; 2. normal vital signs at screening and baseline, as defined by: * body (tympanic) temperature ≤ 37.5 oc; * resting pulse rate ≥ 50 and ≤ 100 bpm; and * dbp ≥ 50 and ≤ 90 mmhg and sbp ≥ 90 and ≤ 140 mmhg. 6. contraception: willingness and agreement to undertake measures to avoid pregnancy of the subject or the subject's sexual partner(s) as detailed below: 1. a female subject who is a woman of childbearing potential (wocbp) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from at least 30 days prior to the first vaccination until 60 days after the second vaccination; 2. a male subject (i) who is sexually active with a wocbp (except who is permanently sterile by bilateral orchiectomy or vasectomy) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from the first vaccination until 60 days after the second vaccination; and (ii) must be willing and agree to refrain from sperm donation during the aforesaid period. 7. breastfeeding: a female subject must be willing and agree to avoid engagement in breastfeeding at any time from the first vaccination until 60 days after the second vaccination. 8. blood donation: willingness and agreement to avoid blood donation from screening to the end of the period of participation in this study.

inclusion criteria: informed consent: the subject (or the subject's legally acceptable representative, if applicable) must be capable of giving written informed consent and, prior to the commencement of any study-specific procedure, must sign an icf indicating the consent on the subject's voluntary participation in the study and compliance with the requirements and restrictions listed on the icf. bnt162b2 vaccination status: the subject must have received 2 doses of bnt162b2 in hong kong, with the second dose completed at least 180 days prior to the first vaccination. gender and age: male or female, at the age of ≥ 18 and ≤ 75 on the day of signing the icf. body weight and bmi: body weight ≥ 45 kg and bmi ≥ 18.5 kg/m2 and < 25 kg/m2 at screening and baseline. medical conditions or diagnoses: existence of all of the following medical conditions or diagnoses: generally in good health with no clinically significant abnormality, as determined by medical history, physical examination, 12-lead ecg and clinical laboratory tests at screening and baseline; normal vital signs at screening and baseline, as defined by: body (tympanic) temperature ≤ 37.5 oc; resting pulse rate ≥ 50 and ≤ 100 bpm; and dbp ≥ 50 and ≤ 90 mmhg and sbp ≥ 90 and ≤ 140 mmhg. contraception: willingness and agreement to undertake measures to avoid pregnancy of the subject or the subject's sexual partner(s) as detailed below: a female subject who is a woman of childbearing potential (wocbp) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from at least 30 days prior to the first vaccination until 60 days after the second vaccination; a male subject (i) who is sexually active with a wocbp (except who is permanently sterile by bilateral orchiectomy or vasectomy) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from the first vaccination until 60 days after the second vaccination; and (ii) must be willing and agree to refrain from sperm donation during the aforesaid period. breastfeeding: a female subject must be willing and agree to avoid engagement in breastfeeding at any time from the first vaccination until 60 days after the second vaccination. blood donation: willingness and agreement to avoid blood donation from screening to the end of the period of participation in this study.

inclusion criteria: informed consent: the subject (or the subject's legally acceptable representative, if applicable) must be capable of giving written informed consent and, prior to the commencement of any study-specific procedure, must sign an icf indicating the consent on the subject's voluntary participation in the study and compliance with the requirements and restrictions listed on the icf. bnt162b2 vaccination status: the subject must have received 2 doses of bnt162b2 in hong kong, with the second dose completed at least 180 days prior to the first vaccination. gender and age: male or female, at the age of ≥ 18 and ≤ 75 on the day of signing the icf. body weight and bmi: body weight ≥ 45 kg and bmi ≥ 18.5 kg/m2 and < 25 kg/m2 at screening and baseline. medical conditions or diagnoses: existence of all of the following medical conditions or diagnoses: generally in good health with no clinically significant abnormality, as determined by medical history, physical examination, 12-lead ecg and clinical laboratory tests at screening and baseline; normal vital signs at screening and baseline, as defined by: body (tympanic) temperature ≤ 37.5 oc; resting pulse rate ≥ 50 and ≤ 100 bpm; and dbp ≥ 50 and ≤ 90 mmhg and sbp ≥ 90 and ≤ 140 mmhg. contraception: willingness and agreement to undertake measures to avoid pregnancy of the subject or the subject's sexual partner(s) as detailed below: a female subject who is a woman of childbearing potential (wocbp) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from at least 30 days prior to the first vaccination until 60 days after the second vaccination; a male subject (i) who is sexually active with a wocbp (except who is permanently sterile by bilateral orchiectomy or vasectomy) must be willing and agree to remain abstinent or practise at least one effective contraceptive method from the first vaccination until 60 days after the second vaccination; and (ii) must be willing and agree to refrain from sperm donation during the aforesaid period. breastfeeding: a female subject must be willing and agree to avoid engagement in breastfeeding at any time from the first vaccination until 60 days after the second vaccination. blood donation: willingness and agreement to avoid blood donation from screening to the end of the period of participation in this study.