Solicited local AEs were defined as injection site erythema, injection site pain, injection site induration, and injection site tenderness. Solicited systemic AEs were defined as arthralgia, chills, diarrhea, dizziness, fatigue, fever (categorized by measured body temperature), headache, myalgia, nausea and vomiting. Data are reported for the number of participants with solicited local and solicited systemic AEs. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Unsolicited AEs were defined as any spontaneously reported or discovered AE. Data are reported for the number of participants with unsolicited AEs. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

An MAAE was an AE that led to an unscheduled visit (including a telemedicine visit) with a health care provider (HCP) (e.g., nurse, nurse practitioner, physician's assistant, physician), including visits to a study site for unscheduled assessments (e.g., rash assessment, abnormal laboratory follow up, coronavirus disease 2019 \[COVID-19\]) and visits to HCPs external to the study site (e.g., urgent care, primary care physician). An SAE was defined as any event that resulted in death, was immediately life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability/incapacity, or was a congenital anomaly/birth defect. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Blood samples were collected to assess participants' immune response. GMC data are reported for the pseudoviral D614G variant. Data are reported in international units per milliliter (IU/mL).

Blood samples were collected to assess participants' immune response. GMC data are reported for the pseudoviral B.1.351 (beta) variant. Data are reported in arbitrary units per milliliter (AU/mL).

Blood samples were collected to assess participants' immune response. GMFR data are reported for the pseudoviral D614G variant.

Blood samples were collected to assess participants' immune response. GMFR data are reported for the pseudoviral B.1.351 (beta) variant.

Blood samples were collected to assess participants' immune response. Data are reported for the pseudoviral D614G variant.

Blood samples were collected to assess participants' immune response. Data are reported for the pseudoviral B.1.351 (beta) variant.

Blood samples were collected to assess participants' immune response. Data are reported for the pseudoviral D614G Variant, B.1.351 Variant, B.1.617.2 Variant, BA.1 Variant for the GMC ratio of ARCT-165 vs ARCT-154 and ARCT-021 vs ARCT-154. Data are reported for the ARCT-165 and ARCT-021 arms (vs ARCT-154 values).

Blood samples were collected to assess participants' immune response. Data are reported for the pseudoviral D614G Variant, B.1.351 Variant, B.1.617.2 Variant, BA.1 Variant for the GMC ratio of ARCT-021 vs ARCT-165. Data are reported for the ARCT-021 arm (vs ARCT-165 values).

Spike binding antibody levels expressed as GMCs are reported for ancestral, D614G, and B.1.351 variants and RBD binding antibodies for ancestral strain.

Spike binding antibody levels expressed as GMFRs are reported for ancestral, D614G, and B.1.351 variants and RBD binding antibody levels expressed as GMFR for ancestral strain.

Data are reported for ancestral, D614G, and B.1.351 variants for Spike binding antibodies and ancestral for RBD binding antibodies.