Inclusion Criteria:  For all participants: Ability and willingness of participant (or legally authorized representative) to provide informed consent prior to initiation of any study procedures. For all participants: Individuals ≥18 years of age For participants who are in, or who have completed, the ACTIV-2/A5401 trial: Receipt of all selected investigational therapy or active comparator/placebo for that therapy at selected sites. For participants who are in, or who have completed, the ACTIV-2/A5401 trial and will be receiving study-provided Moderna mRNA-1273 COVID-19 vaccine: Receipt of the last dose of investigational therapy or active comparator/placebo for that therapy ≥60 days and ≤240 days prior to study entry. For participants who are in, or who have completed, the ACTIV-2/A5401 trial and will be receiving community-provided mRNA-based COVID-19 vaccine: Receipt of the last dose of investigational therapy or active comparator/placebo for that therapy ≥60 and ≤240 days prior to planned receipt of the first dose of community-provided vaccine.  Exclusion Criteria:  For participants who are in, or who have completed, the ACTIV-2/A5401 trial: Self-report of prior receipt of a non-mRNA-based COVID-19 vaccine For participants who are in, or who have completed, the ACTIV-2/A5401 trial: Self-report of receipt of the second dose of an mRNA-based COVID-19 vaccine. For participants who are in, or who have completed, the ACTIV-2/A5401 trial: Self-report of a second SARS-CoV-2 infection after the infection that qualified the participant for ACTIV-2/A5401. For non-A5401/ACTIV-2 participants: Self-report of receipt of any prior COVID-19 vaccine. For non-A5401/ACTIV-2 participants: Known prior history of any SARS-CoV-2-positive test (e.g., PCR test, Nucleic Acid Amplification Test (NAAT), antigen test, serology test). For participants who will receive study-provided Moderna mRNA-1273 COVID-19 vaccine: Known allergy to any component of the Moderna COVID-19 vaccine.