Study of Monovalent and Bivalent Recombinant Protein Vaccines Against COVID-19 in Adults 18 Years of Age and Older

The duration of participation in the initial, double-blind, primary series design of the study will be approximately 365 days post-last injection (ie, approximately 386 days total) for each participant. Based on decisions of the Study OG, Stage 1 and Stage 2 participants will be invited to participate in an unblinded Crossover / Booster study design with duration as follows: * For participants who initially received vaccine: 12 months post-booster (ie, approximately 18 to 24 months) * For participants who initially received placebo: ≥ 4 months post-last dose of the primary series + 12 months post-booster (ie, approximately 28 to 34 months) * For participants who do not consent to continue in the unblinded Crossover / Booster part of the study, all study procedures will be stopped and participants will be discontinued from the study.