Study of Self-Amplifying Messenger Ribonucleic Acid (samRNA) Vaccines Against COVID-19 in Healthy Adults and People Living With Human Immunodeficiency Virus (HIV)

Inclusion Criteria: * Male or non-pregnant female at least 18 years and no more than 65 years of age at enrollment (Parts A, B, and C only). * No previous SARS-CoV-2 infection or recovered. * HIV-negative status confirmed by laboratory testing. Additional inclusion criteria for PLWH: * Serum positive HIV test or history of HIV infection. * On anti-retroviral therapy for at least 3 months before screening and clinically stable. Additional inclusion criteria for Part D (GRT-R918): * Male or non-pregnant female between 18 and \<60 years of age at enrollment. * Male or non-pregnant female greater than or equal to 60 years of age at enrollment. * Received any authorized SARS-CoV-2 vaccine series at least 2 months prior to study vaccine. Exclusion Criteria: * Current active infection with COVID-19. * Positive for SARS-CoV-2 by nasal swab polymerase chain reaction (PCR) at screening. * Currently receiving treatment or prevention agents with activity against SARS-CoV-2. * Breastfeeding, pregnant, or planning to become pregnant during the course of the study. * Received or plans to receive any non-study provided SARS-CoV-2 vaccine (including boost) during the study period (except for Part D). * Received or plans to receive any live, attenuated vaccine within 28 days before or after study vaccination. * Received or plans to receive any subunit or killed vaccine within 14 days before or after vaccination. * Received or plans to receive immunoglobulins and/or any blood products within the 3 months preceding the planned administration of first study vaccination or at any time during the study. * Currently active viral infection of hepatitis B virus or hepatitis C virus. Additional exclusion criteria for PLWH: * Screening CD4+ T cell count ≤200 cells/mcL. * Viral load ≥10,000 virus particles/mL. * History of opportunistic illness indicative of Stage 3 HIV infection. * Acute febrile illness within 4 weeks before the first vaccination. Additional exclusion criterion for Part D (GRT-R918) Cohorts D3, D4, D7, and D8: \- Received last dose of any authorized SARS-CoV-2 vaccine series within 2 months prior to study vaccine.