A Study of mRNA-1010 Seasonal Influenza Vaccine in Healthy Adults

Key Inclusion Criteria: Phase 1/2: * Participant has a body mass index (BMI) of 18 kilograms (kg)/square meter (m\^2) to 35 kg/m\^2 (inclusive) at the Screening Visit. * Participant is in good health, in the opinion of the Investigator, based on review of medical history and physical examination performed at screening. * 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 3 months following vaccine administration, and not currently breastfeeding. Phase 2 NH and Phase 2 Extension: * Participant is medically stable, in the opinion of the Investigator, based on review of medical history and physical examination performed at screening. * 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 3 months following vaccine administration, and not currently breastfeeding. Key Exclusion Criteria: Phase 1/2: * Participant has had significant exposure to someone with laboratory-confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, COVID-19, or influenza-like illness (ILI) in the past 14 days prior to the Screening Visit, as defined by the United States Centers for Disease Control and Prevention (CDC) as close contact with someone who has COVID-19. * Participant has a positive SARS-CoV-2 reverse transcriptase polymerase chain reaction (RT-PCR) or antigen test in the past 10 days prior to the Screening Visit. * Participant is acutely ill or febrile (body temperature ≥ 38.0 degrees Celsius \[°C\]/100.4 degrees Fahrenheit \[°F\]) 72 hours prior to or at the Screening Visit or Day 1. * Participant has a pre-existing medical condition that is not stable, at the discretion of the Investigator. * Participant has a medical, psychiatric, or occupational condition that may pose additional risk as a result of participation or that could interfere with safety assessments or interpretation of results according to the Investigator's judgment. * Participant has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to Screening Visit (for corticosteroids ≥10 milligrams \[mg\]/day of prednisone equivalent) or is anticipating the need for immunosuppressive treatment at any time during participation in the study. * Participant has received or plans to receive any licensed vaccine ≤28 days prior to the investigational product (IP) injection (Day 1) or plans to receive a licensed vaccine within 28 days after the IP injection, with the exception of vaccines authorized or approved for the prevention of COVID-19 (regardless of type of vaccine) that become available to participants during the study. Efforts should be made to space study vaccination and COVID-19 vaccination by at least 7 and preferably 14 days, but COVID-19 vaccination should not be delayed. * Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine after 01 January 2021. Phase 2 NH: * Participant has had close contact to someone with SARS-CoV-2 infection or COVID-19 as defined by the CDC (CDC 2021a) in the past 14 days prior to the Screening Visit, unless the participant has been fully vaccinated for COVID-19. * Participant is acutely ill or febrile (body temperature ≥ 38.0°C/100.4°F) 72 hours prior to or at the Screening Visit or Day 1. Participants meeting this criterion may be rescheduled within the 28-day screening window and will retain their initially assigned participant number. * Participant has a medical, psychiatric, occupational condition, or history of substance abuse that may pose additional risk as a result of participation or that could interfere with safety assessments or interpretation of results according to the Investigator's judgment. * Participant has a current or previous diagnosis of immunocompromising/immunosuppressive condition, immune mediated disease requiring immune-modifying therapy, asplenia, recurrent severe infections (human immunodeficiency virus \[HIV\] positive participants on antiretroviral therapy with cluster of differentiation \[CD\] 4 count ≥ 350 cells/mm3 and HIV RNA ≤ 500 copies/mL within the past 12 months are permitted). * Participant has received systemic immunosuppressants or immune-modifying drugs for \> 14 days in total within 6 months prior to Screening Visit (for corticosteroids ≥ 10 mg/day of prednisone or equivalent) or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the study. * Participant has a history of anaphylaxis, urticaria, or other significant AR requiring medical intervention after receipt of a vaccine or any of the components contained in the mRNA 1010 or the comparator vaccine, which is an egg-based influenza vaccine. * Participant has received or plans to receive any licensed vaccine ≤ 28 days prior to the IP injection (Day 1) or plans to receive a licensed vaccine within 28 days after the IP injection, with the exception of vaccines authorized or approved for the prevention of COVID-19 (regardless of type of vaccine) that become available to participants during the study. Efforts should be made to space study vaccination and COVID-19 vaccination by at least 7 and preferably 14 days, but COVID-19 vaccination should not be delayed. * Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine within 6 months prior to the Screening Visit. * Participant has tested positive for influenza by CDC-recommended testing methods within 6 months prior to the Screening Visit. * Participant has participated in an interventional clinical study within 28 days prior to the Screening Visit based on the medical history interview or plans to do so while participating in this study. Phase 2 Extension: * Participant has had close contact to someone with SARS-CoV-2 infection or COVID-19 as defined by the CDC in the past 10 days prior to the Screening Visit. * Participant is acutely ill or febrile (body temperature ≥ 38.0°C/100.4°F) 72 hours prior to or at the Screening Visit or Day 1. Participants meeting this criterion may be rescheduled within the 28-day screening window and will retain their initially assigned participant number. * Participant has a medical, psychiatric, occupational condition, or history of substance abuse that may pose additional risk as a result of participation or that could interfere with safety assessments or interpretation of results according to the Investigator's judgment. * Participant has a current or previous diagnosis of immunocompromising/immunosuppressive condition, immune-mediated disease requiring immune-modifying therapy, asplenia, recurrent severe infections (HIV-positive participants on antiretroviral therapy with CD 4 count ≥ 350 cells/mm\^3 and HIV-RNA ≤ 500 copies/mL within the past 365 days are permitted). * Participant has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 180 days prior to Screening Visit (for corticosteroids ≥ 10 mg/day of prednisone or equivalent) or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the study. * Participant has a history of anaphylaxis, urticaria, or other significant AR requiring medical intervention after receipt of a vaccine or any of the components contained in the mRNA-1010 or the comparator vaccine, which is an egg-based influenza vaccine. * Participant has received or plans to receive any licensed vaccine ≤ 28 days prior to the IP injection (Day 1) or plans to receive a licensed vaccine within 28 days after the IP injection, with the exception of vaccines authorized or approved for the prevention of COVID-19 (regardless of type of vaccine) that become available to participants during the study. Efforts should be made to space study vaccination and COVID-19 vaccination by at least 7 and preferably 14 days, but COVID-19 vaccination should not be delayed. * Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine within 180 days prior to the randomization visit. * Participant had tested positive for influenza by CDC-recommended testing methods within 180 days prior to the Screening Visit. * Participant has participated in an interventional clinical study within 28 days prior to the Screening Visit based on the medical history interview or plans to do so while participating in this study. Other inclusion/exclusion criteria may apply.