Inclusion Criteria:

1. Age 18 to 65 years of age

2. Able to provide informed consent

3. Able and willing to comply with study procedures and agree to refrain from obtaining
a booster vaccination with a non-study vaccine through to the 1 month post-boost
vaccination visit

4. For women of childbearing potential, able and willing to use an approved form of
pregnancy prevention for at least 4 weeks from study drug administration

5. Completion of a prior vaccination series with either the BNT162b2, mRNA-1273, or
Ad26.CoV.S vaccines, with or without subsequent booster vaccination, with most
recent vaccination at least 6 months prior to study entry

Exclusion Criteria:

1. Persons with symptoms in the past 2 weeks consistent with possible acute SARS-CoV-2
infection (https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/symptoms.html)

2. Persons with a diagnosis of type 2 diabetes mellitus

3. Persons with a diagnosis of chronic kidney disease

4. Persons with a diagnosis of chronic obstructive pulmonary disease (COPD)

5. Persons with a diagnosis of heart conditions to include heart failure, coronary
artery disease, prior heart attack, cardiomyopathy

6. Sickle cell disease

7. Current or planned pregnancy during the study

8. Currently breastfeeding

9. Administration of an investigational agent within 90 days of the GLS-5310 booster
dose

10. Administration of a vaccine within 2 weeks prior to the GLS-5310 booster dose

11. Administration of immune globulin within 6 months of enrollment

12. Administration of an anti-TNFα inhibitor such as infliximab, adalimumab, etanercept,
or anti-CD20 monoclonal antibody rituximab within 6 months from enrollment

13. Current daily treatment of systemic corticosteroids of 20 mg of prednisone or
greater; or the equivalent dose of other systemic corticosteroids

14. Treatment within the four weeks prior to enrollment with any drug intended for the
prophylaxis or treatment of COVID-19

15. Any prior treatment with an anti-SARS-CoV-2 monoclonal antibody or immune serum

16. Prior treatment with an anti-IL-6 inhibitor, anti-IL-1 inhibitor, anti-TNF
monoclonal antibody, or anti-JAK inhibitor (see Appendix B exclusionary period for
specific drugs)

17. History of malignancy

18. History of transplantation (any organ or bone marrow)

19. Current or planned chemotherapy treatment for hematologic or solid tumor during
study period

20. History of other congenital or acquired immunodeficiency, excluding those with HIV
infection who are taking highly active antiretroviral therapy and who have
documentation of undetectable serum viral load and who have a CD4 count > 200
cells/μL on two measures at least 3 months apart

21. Not willing to allow storage and future use of samples for coronavirus-related
research and/or vaccine development

22. Prisoner or subjects who are compulsorily detained for treatment of a psychiatric
illness

23. Any illness or condition that, in the opinion of the investigator, may affect the
safety of the subject or the evaluation of a study endpoint

24. History of chronic rhinosinusitis

25. History of nasal septal defect or deviated nasal septum

26. History of cleft palate

27. History of nasal polyps

28. History of other disorders that, in the opinion of the investigator, may adversely
affect administration of intranasal vaccine