Inclusion Criteria:

• * Age ≥18 years
• * Previously received ≥ 2-doses of US FDA-authorized mRNA vaccines
• * Comfortable reading and responding to text messages and emails sent in English or having an interpreter assist them
• * Plan to remain in the greater Salt Lake City area for the next 12 months
• * Daily access to the internet (via cell phone, laptop, desktop, or tablet) and a phone with text messaging capabilities
• * Willingness to complete weekly symptom and illness surveillance surveys sent via text and email
• * Willingness to complete an online survey at enrollment, mid-study, and end-of-study surveys
• * Willingness to be contacted periodically by study staff via text, email, and/or telephone as part of study activities
• * Willingness to self-collect rapid antigen tests (RAT; approved by FDA EUA for COVID-19 detection) weekly and when prompted for study purposes, and to send results via the study portal
• * Willingness to self-collect additional rapid antigen test (approved by FDA EUA for COVID-19 detection) if experiencing a qualifying symptomatic illness or upon RAT-confirmation of an asymptomatic infection
• * Willingness to attend in-person visit to receive supply of rapid antigen tests and training on their use (all participants) and to receive a COVID-19 booster (if in randomized group)

Exclusion Criteria:

• * Lives with another person who is already enrolled in this study as reported by the subject on the Eligibility Survey (Appendix C. Eligibility Survey)
• * Previous hypersensitivity reaction to the study vaccines as reported by the subject on the Eligibility Survey (Appendix C. Eligibility Survey)
• * Recent infection \[Real time Reverse Transcription Polymerase Chain Reaction assay (RT-PCR) and/or RAT confirmed infection ≤ 90 days of trial vaccine administration
• * Receipt of a COVID-19 vaccine within ≤ 90 days of trial vaccine administration
• * Participation in other vaccine trials
• * Medical history of immunosuppression
• * Receipt of J\&J vaccine prior to study enrollment
• * Receipt of any investigational prevention therapies for SARS-CoV-2 infections, such as prophylactic antiviral medications or other immune system modifying interventions within ≤ 90 days of trial vaccine administration
• * Unwillingness to provide electronic consent
• * Unwillingness to self-report occupation, work responsibilities, and prior COVID-19 illness.