This protocol provides additional information about the safety and immunogenicity of two RNA-based COVID-19 vaccine candidates. The study is a Phase 1/2, randomized, placebo-controlled, observer-blind, dose-finding, and vaccine candidate-selection trial in healthy adults. The primary objectives are to evaluate the safety, tolerability, immunogenicity, and potential efficacy of up to four different SARS-CoV-2 RNA vaccine candidates against COVID-19. The study will assess these candidates in three age groups: 18 to 55 years, 65 to 85 years, and 18 to 85 years (stratified as ≤55 or >55 years). The study consists of three stages: Stage 1 to identify preferred vaccine candidates, Stage 2 to expand the cohort, and Stage 3 to conduct a final large-scale evaluation. The study will use an internal review committee (IRC) and an external data monitoring committee (DMC) to review data and ensure participant safety. The statistical methods are designed to evaluate safety, immunogenicity, and efficacy, with a focus on local reactions, systemic events, and adverse events. The study will also monitor for potential COVID-19 disease enhancement and pregnancy. Participants will be followed up for up to 26 months, with shorter follow-up periods for those not progressing to the final stage. The protocol includes detailed information on eligibility criteria, enrollment, dosing, and safety assessments.This protocol provides additional information about the safety and immunogenicity of two RNA-based COVID-19 vaccine candidates. The study is a Phase 1/2, randomized, placebo-controlled, observer-blind, dose-finding, and vaccine candidate-selection trial in healthy adults. The primary objectives are to evaluate the safety, tolerability, immunogenicity, and potential efficacy of up to four different SARS-CoV-2 RNA vaccine candidates against COVID-19. The study will assess these candidates in three age groups: 18 to 55 years, 65 to 85 years, and 18 to 85 years (stratified as ≤55 or >55 years). The study consists of three stages: Stage 1 to identify preferred vaccine candidates, Stage 2 to expand the cohort, and Stage 3 to conduct a final large-scale evaluation. The study will use an internal review committee (IRC) and an external data monitoring committee (DMC) to review data and ensure participant safety. The statistical methods are designed to evaluate safety, immunogenicity, and efficacy, with a focus on local reactions, systemic events, and adverse events. The study will also monitor for potential COVID-19 disease enhancement and pregnancy. Participants will be followed up for up to 26 months, with shorter follow-up periods for those not progressing to the final stage. The protocol includes detailed information on eligibility criteria, enrollment, dosing, and safety assessments.