CD4 and COVID-19: Secondary endpoints will include the evaluation of binding antibody (bAB) and neutralizing antibody (nAB) responses at vaccination day (Visits 1) and 30 days after vaccination (Visit 4), the ratio of bAB to nAB titers at Visits 1 and 4, and cellular immune responses (CD3, CD4, CD8, CD45RA, CCR7, IFN-γ, IL-2, IL-4, IL-13, and TNF) at Visits 1 and 4.<h4>Discussion</h4>The study will be critical to determine the safety and immunogenicity of the pDNA vaccine in participants who have previously received a COVID-19 mRNA vaccine.