A Study of mRNA-based Influenza and SARS-CoV-2 (COVID-19) Multi-component Vaccines in Healthy Adults

The study is divided into 2 parts: Part 1 and Part 2. The purpose of Part 1 of this study is to generate sufficient safety, reactogenicity, and immunogenicity data to enable selection of an mRNA-1083 vaccine composition and dose level to evaluate in a subsequent Phase 3 clinical trial in adults.

The purpose of Part 2 of this study is to generate safety and immunogenicity data for additional mRNA-1083 compositions and dose levels in young adults ≥18 years and \<50 years of age.

Part 1: Participants will be enrolled into 1 of 2 age cohorts: Cohort A for adults ≥65 to \<80 years of age or Cohort B for adults ≥18 to \<65 year of age. In Cohort A, approximately 600 participants will be randomized (in equal allocation ratio) into the investigational treatment arms, stratified by influenza vaccine status in the most recent influenza season (received or not received since September 2022). In Cohort B, approximately 624 participants will be randomized (in equal allocation ratio) into the investigational treatment arms, stratified by 2 age groups: ≥18 to \<50 years and ≥50 to \<65 years of age and by influenza vaccine status in the most recent influenza season (received or not received since September 2022).

Part 2: Approximately, 520 participants between ≥18 to \<50 years of age will be randomized (in equal allocation ratio) into the investigational treatment arms, stratified by influenza vaccine status in the most recent influenza season (received or not received since Sept 2023).

Eligibility Criteria:
Key Inclusion Criteria:

Part 1 (Phase 1/2)

* Adults ≥18 to \<80 years of age at the time of consent.
* Body mass index (BMI) of 18 kilograms (kg)/square meter (m\^2) to 35 kg/m\^2 (inclusive) at the Screening Visit.
* Healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests.
* For female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to Day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding.
* Fully vaccinated for COVID-19 primary series according to the locally authorized or approved regimen, and their last COVID-19 vaccine (primary series or booster) was ≥120 days prior to Day 1.

Part 2 (Phase 2 Extension)

* Adults ≥18 to \<50 years of age at the time of consent.
* BMI of 18 kg/m\^2 to 35 kg/m\^2 (inclusive) at the Screening Visit.
* Healthy as determined by medical evaluation, including medical history, and physical examination.
* For female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to Day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding.
* Have received at least 2 doses of locally authorized or approved COVID-19 vaccines and last dose was ≥90 days prior to Day 1.

Key Exclusion Criteria:

Part 1 (Phase 1/2) and Part 2 (Phase 2 Extension)

* Participant is acutely ill or febrile (temperature ≥38.0 degrees Celsius \[°C\]/100.4 degrees Fahrenheit \[°F\]) 72 hours prior to or at the Screening Visit or Day 1.
* Any medical, psychiatric, or occupational condition, including reported history of drug or alcohol abuse, that, in the opinion of the Investigator, might pose additional risk due to participation in the clinical trial or could interfere with the interpretation of study results.
* Participant has received systemic immunosuppressants for \>14 days in total within 180 days prior to Screening Visit (for glucocorticoids ≥10 milligrams \[mg\]/day of prednisone or equivalent) or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the clinical trial (including intra-articular steroid injections). Inhaled, nasal, and topical steroids are allowed.
* Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤28 days prior to study intervention administration or plans to receive a vaccine authorized or approved by a local health agency within 28 days after study intervention administration.
* Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine within 150 days prior to Day 1.
* Participant tested positive for influenza by local health authority-approved testing methods ≤150 days prior to Day 1.
* Participant has had close contact to someone with COVID-19 as defined by the Centers for Disease Control and Prevention (CDC) in the past 10 days prior to Day 1.
* Participant has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit or plans to donate blood products during the clinical trial.
* Working or has worked as study personnel, is an immediate family member or household member of study personnel, study site staff, or Sponsor personnel, or resides in a nursing home.

Part 2 (Phase 2 Extension) Only

* Participants who enrolled in Part 1 of the mRNA-1083-P101 (Phase 1/2) study.

Note: Other inclusion/exclusion criteria may apply.