Inclusion Criteria:  Age 18 to 65 years of age Able to provide informed consent 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 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 Completion of a prior vaccination series with either the BNT162b2, mRNA-1273, or Ad26.CoV.S vaccines at least 6 months prior to study entry  Exclusion Criteria:  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) Persons with a diagnosis of type 2 diabetes mellitus Persons with a diagnosis of chronic kidney disease Persons with a diagnosis of chronic obstructive pulmonary disease (COPD) Persons with a diagnosis of heart conditions to include heart failure, coronary artery disease, prior heart attack, cardiomyopathy Sickle cell disease Current or planned pregnancy during the study Currently breastfeeding Administration of an investigational agent within 90 days of the GLS-5310 booster dose Administration of a vaccine within 2 weeks prior to the GLS-5310 booster dose Administration of immune globulin within 6 months of enrollment Administration of an anti-TNFα inhibitor such as infliximab, adalimumab, etanercept, or anti-CD20 monoclonal antibody rituximab within 6 months from enrollment Current daily treatment of systemic corticosteroids of 20 mg of prednisone or greater; or the equivalent dose of other systemic corticosteroids Treatment within the four weeks prior to enrollment with any drug intended for the prophylaxis or treatment of COVID-19 Any prior treatment with an anti-SARS-CoV-2 monoclonal antibody or immune serum 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) History of malignancy History of transplantation (any organ or bone marrow) Current or planned chemotherapy treatment for hematologic or solid tumor during study period 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 Not willing to allow storage and future use of samples for coronavirus-related research and/or vaccine development Prisoner or subjects who are compulsorily detained for treatment of a psychiatric illness 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 History of chronic rhinosinusitis History of nasal septal defect or deviated nasal septum History of cleft palate History of nasal polyps History of other disorders that, in the opinion of the investigator, may adversely affect administration of intranasal vaccine