To Evaluate Safety & Immunogenicity of DelNS1-2019-nCoV-RBD-OPT1 for COVID-19 in Healthy Adults Received 2 Doses of BNT162b2

Inclusion Criteria:

1. Informed Consent: The subject (or the subject's legally acceptable representative, ifapplicable) must be capable of giving written informed consent and, prior to thecommencement of any study-specific procedure, must sign an ICF indicating the consenton the subject's voluntary participation in the study and compliance with therequirements and restrictions listed on the ICF.
2. BNT162b2 Vaccination Status: The subject must have received 2 doses of BNT162b2 inHong Kong, with the second dose completed at least 180 days prior to the firstvaccination.
3. Gender and Age: Male or female, at the age of ≥ 18 and ≤ 75 on the day of signing theICF.
4. Body Weight and BMI: Body weight ≥ 45 kg and BMI ≥ 18.5 kg/m2 and < 25 kg/m2 atscreening and baseline.
5. Medical Conditions or Diagnoses: Existence of all of the following medical conditionsor diagnoses:
6. Generally in good health with no clinically significant abnormality, asdetermined by medical history, physical examination, 12-lead ECG and clinicallaboratory tests at screening and baseline;
7. 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 ofthe subject or the subject's sexual partner(s) as detailed below:
7. A female subject who is a woman of childbearing potential (WOCBP) must be willingand agree to remain abstinent or practise at least one effective contraceptivemethod from at least 30 days prior to the first vaccination until 60 days afterthe second vaccination;
8. A male subject (i) who is sexually active with a WOCBP (except who is permanentlysterile by bilateral orchiectomy or vasectomy) must be willing and agree toremain abstinent or practise at least one effective contraceptive method from thefirst vaccination until 60 days after the second vaccination; and (ii) must bewilling and agree to refrain from sperm donation during the aforesaid period.
9. Breastfeeding: A female subject must be willing and agree to avoid engagement inbreastfeeding at any time from the first vaccination until 60 days after the secondvaccination.
10. Blood Donation: Willingness and agreement to avoid blood donation from screening tothe end of the period of participation in this study.

Exclusion Criteria:

1. Medical History: History of any of the following diseases or conditions:
2. COVID-19;
3. SARS;
4. Any significant respiratory diseases (e.g. COPD, asthma);
5. Any significant cardiovascular disease (e.g. angina, cardiac arrhythmias);
6. Blood dyscrasias or any significant disorder of coagulation;
7. Any chronic liver disease (e.g. autoimmune hepatitis and cirrhosis);
8. Any chronic infection (e.g. hepatitis B, hepatitis C and HIV);
9. Any malignant neoplastic disease;
10. Encephalopathy, neuropathy or unstable central nervous system (CNS) pathology;
11. Any psychiatric disorder, psychotic disorder, major affective disorder orsuicidal ideation;
12. Any immunodeficiency or autoimmune disease;
13. Any severe allergic reaction (e.g. anaphylaxis) to any vaccine or substance,which requires hospitalization or emergency medical care;
14. Any nasal septal defect, cleft palate, nasal polyps or other nasal abnormalitythat might affect vaccine administration;
15. History of alcohol or illicit drug abuse, or used any illicit drug within 6months prior to screening.
16. Medical Conditions or Diagnoses: Existence of any of the following medical conditionsor diagnoses:
17. Positive serum pregnancy test at screening or positive urine pregnancy test atbaseline (for WOCBP);
18. IgE level > 1,000 IU/ml at screening;
19. Positive SARS-CoV-2 test result in deep throat saliva (DTS) within 4 days priorto the first vaccination;
20. Positive HIV test result at screening;
21. Positive HBsAg test result at screening;
22. Positive HCV antibody test result at screening;
23. Positive urine drug screen test result or positive blood alcohol test result atscreening or baseline;
24. Clinically significant abnormality of T3, T4 or TSH at screening;
25. Clinically significant abnormality of PT (INR) or aPTT at screening.
26. Prior/Concomitant Interventions: Use of or undergoing any of the following prior orconcomitant medications, therapies or interventions:
27. Any COVID-19 or coronavirus vaccine at any time prior to the first vaccination,except BNT162b2 or planned use of any such vaccine throughout the study;
28. Any vaccine other than COVID-19 or coronavirus vaccines within 28 days prior tothe first vaccination, or planned use of any such vaccine up to 28 days after thesecond vaccination;
29. Any immune-modifying medication/therapy (e.g. immunomodulator andimmunosuppressant) within 6 months prior to the first vaccination, or planned useof any such medication/therapy throughout the study;
30. Any blood product (including blood transfusion) or immunoglobulin within 3 monthsprior to the first vaccination, or planned use of any such therapy throughout thestudy;
31. Any anticoagulation medication within 28 days prior to the first vaccination, orplanned use of any such medication up to 28 days after the second vaccination;
32. Any psychotropic medication within 28 days prior to the first vaccination, orplanned use of any such medication up to 28 days after the second vaccination;
33. Regular use of any inhaled/nebulized corticosteroid;
34. Any intranasal preparation within 48 hours prior to the first vaccination, orplanned use of any such preparation up to 48 hours after the second vaccination;
35. Any influenza antiviral medication within 48 hours prior to the firstvaccination, or planned use of any such medication up to 14 days after the secondvaccination;
36. Any prescription or over-the-counter medication within 7 days prior to the firstvaccination, unless with the investigator's approval;
37. Donated ≥ 450 ml of blood within 28 days prior to the first vaccination;
38. Prior nasal surgery or nasal cauterization.
39. Prior/Concurrent Clinical Study: Prior or concurrent participation in any otherclinical study, including:
40. Prior or current participation in another COVID-19 vaccine study;
41. Prior participation in any interventional clinical study and use of anyinvestigational intervention within 90 days prior to the first vaccination;
42. Concurrent participation or plan for participation in another interventionalclinical study during participation in this study.
43. Other Significant Medical Conditions: Any clinically significant concomitant diseaseor condition that, in the reasonable opinion of the investigator, may interfere withthe subject's participation in this study or pose an unacceptable safety risk for thesubject's participation in this study.
44. Special Conditions: Existence of any of the following special conditions:
45. Close contact with anyone known to have COVID-19 within 30 days prior to thefirst vaccination;
46. Travelled outside Hong Kong within 14 days prior to the first vaccination;
47. Planned to travel outside Hong Kong at any time during the period from screeningto Day 50(±3) visit.