Safety and Immunogenicity of HDT-301 Targeting a SARS-CoV-2 Variant Spike Protein

This is an open-label, dose-ranging, study to evaluate the safety and immunogenicity of the investigational HDT-301 vaccine in healthy (vaccinated or unvaccinated) adult subjects, with a 4:3 recruitment of individual aged 18-49 years and 50-65 years, respectively. The study duration is approximately 13 or 14 months for each subject. Three cohorts of 21 subjects each will be sequentially recruited in the study: 1. Cohort 1 will include individuals with vaccination against COVID-19 who will receive a two-dose schedule of HDT-301 56 days apart. 2. Cohort 2 will include individuals with vaccination against COVID-19 who will receive a one-dose schedule of HDT-301. 3. Cohort 3 will include individuals with no history of vaccination against COVID-19 who will receive a two-dose schedule of HDT-301 56 days apart. Recruiting and screening of participants will be conducted on a continual basis. Subjects confirmed to be Emergency Use Authorization (EUA) SARS-CoV-2 vaccine recipients will first be placed into either Cohort 1 (Study Days 1 and 57; Two-Dose Regimen) or Cohort 2 (Study Day 1; One-Dose Regimen) then subsequently placed into each of the three dose groups. Subjects in the Cohort 3 (naïve, unimmunized) subset will be placed into each of the three dose groups to receive the study vaccine on Study Days 1 and 57 (Two-Dose Regimen). Each cohort will be divided into 3 groups of 7 participants, that will be provided HDT-301 at either a low-, mid- or high-dose. For each of the three-dose groups, a sentinel group will be initially enrolled and followed through Day 8 post-dose one for safety. If no halting rules are met by Day 8 enrollment will proceed to the remaining subjects. Blood samples will be collected during the study for each participant.