mRNA-1273.617.2

This phase 2 clinical trial will evaluate the safety and immunogenicity of additional doses of prototype and variant (alone or in combination) vaccine candidates in previously vaccinated participants with or without prior severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and will evaluate innate, cellular, and humoral immune responses to inform on how to shift the immune response to cover new variants as they emerge. A randomized open-label, non-placebo controlled, multi-site, multi-stage clinical trial in individuals, 18 years of age and older, who are in a stable state of health, has received a complete authorized/approved vaccine series (primary series + booster either with homologous or heterologous vaccine products) >/ = 16 weeks prior to enrollment. Subjects will be stratified by i) age (18-64 years and = 65 years of age) (however arms 16 and 17 or stage 4 will only enroll participants between the ages of 18-49 years) and ii) history of confirmed prior SARS-CoV-2 infection, and randomly assigned to receive one of several variant vaccines. Enrollment will target a goal of approximately 45% of each of the variant vaccine arms to be in older adults (= 65 years of age) for stages 1, 2 and 3 and approximately 20% to have had confirmed COVID-19 for all 4 stages. The primary objective is to evaluate humoral immune responses of candidate SARS-CoV-2 variant vaccines, alone or in combination.