This Phase 1 clinical trial (CORAL-CEPI) will assess the potential of second-generation Coronavirus Disease 2019 (COVID-19) vaccines. These vaccines use a codon optimized Spike (S) cassette with additional T cell epitopes (TCE) (cassette S-TCE) covering multiple epitopes from non-spike proteins to safely drive strong, broad, and durable B cell and T cell immune responses to SARS-CoV-2. This trial will assess the potential to generate B cell and T cell responses against SARS-CoV-2 in both people living with HIV (PLWH) and HIV-negative participants, in participants who have previously been infected by SARS-CoV-2, and those who are naive to SARS-CoV-2, meaning they have neither been infected with nor vaccinated against SARS-CoV-2. GRT-R912, GRT-R914, and GRT-R918 are vaccines using a samRNA vector based and administered as either a single dose or two dose regimen, providing an option for a potent, single-modality approach.