Immunogenicity and Safety Study of Self-amplifying MRNA COVID-19 Vaccine Administered with Influenza Vaccines in Adults

This is a multicenter, observer-blind, randomized, controlled phase 3 study to evaluate the immunogenicity, reactogenicity, and safety of an investigational self-amplifying RNA COVID-19 vaccine (ARCT-2303) administered concomitantly with quadrivalent influenza vaccines or standalone in adults who previously received authorized COVID-19 vaccine.

Approximately 1680 participants previously vaccinated with authorized COVID-19 vaccine will be enrolled in this study in two age cohorts (younger adults and older adults). Within each cohort, participants will be randomly assigned in a ratio of 1:1:1 to receive the ARCT-2303 vaccine concomitantly with a quadrivalent influenza vaccine, the ARCT-2303 vaccine and placebo, or the quadrivalent influenza vaccine and placebo. The assessment of immunogenicity will be performed 28 days after vaccination. To provide equal benefit from the participation in the study and complete seasonal vaccination against COVID-19 and influenza, a switchover vaccine dose (influenza, ARCT-2303 or placebo) will be administered 28 days after initial vaccination. All participants will be followed up for safety assessment until the end of the study.

A historical control group vaccinated on a similar schedule (ARCT-154 vaccine) from a previous study (ARCT-154-J01) will be used to compare with the immunogenicity of the ARCT-2303 vaccine.

Cohort A (younger adults; approximately 1200 participants):

* Group 1a (ARCT-2303/Influenza vaccine): participants will receive one dose of ARCT-2303 and one dose of Influenza vaccine (opposite arms) on Day 1, and one dose of placebo on Day 29.
* Group 2a (ARCT-2303): participants will receive one dose of ARCT-2303 and one dose of placebo (opposite arms) on Day 1, and one dose of Influenza vaccine on Day 29.
* Group 3a (Influenza vaccine): participants will receive one dose of Influenza vaccine and one dose of placebo (opposite arms) on Day 1, and one dose of ARCT-2303 on Day 29.

Cohort B (older adults; approximately 480 participants):

* Group 1b (ARCT-2303/Influenza vaccine): participants will receive one dose of ARCT-2303 and one dose of Influenza vaccine (opposite arms) on Day 1, and one dose of placebo on Day 29.
* Group 2b (ARCT-2303): participants will receive one dose of ARCT-2303 and one dose of placebo (opposite arms) on Day 1, and one dose of Influenza vaccine on Day 29.
* Group 3b (Influenza vaccine): participants will receive one dose of Influenza vaccine and one dose of placebo (opposite arms) on Day 1, and one dose of ARCT-2303 on Day 29.

Eligibility Criteria:
Inclusion Criteria:

1, Individuals are male, female, or transgender adults ≥18 years of age.

2. Healthy participants or participants with pre-existing stable medical conditions.

3. Participant or legally authorized representatives must freely provide documented informed consent prior to study procedures being performed.

4. Individuals must have been previously vaccinated with COVID-19 vaccines.

5. Individuals of childbearing potential must be willing to adhere to contraceptive requirements.

Exclusion Criteria:

1. Individuals with acute medical illness or febrile illness.
2. Individuals with a positive SARS-CoV-2 rapid antigen test at Screening.
3. Individuals with a history of COVID-19 or virologically confirmed SARS-CoV-2 infection within the past 5 months or history of COVID-19 with ongoing sequelae.
4. Individuals with a known history of severe hypersensitivity reactions, including anaphylaxis, or other significant adverse reactions to any components of mRNA vaccine, or influenza vaccine, including egg protein.
5. Individuals who have a positive pregnancy test at the Screening visit or who intend to become pregnant or breastfeed during the study.
6. Individuals with a history of myocarditis, pericarditis, myopericarditis or cardiomyopathy.
7. Individuals with a history of Guillain-Barré syndrome, encephalomyelitis, or transverse myelitis.
8. Individuals with a history of congenital or acquired immunodeficiency.
9. Individuals who have received immunomodulatory, immunostimulatory, or immunosuppressant drugs within 3 months of Screening; or individuals requiring systemic corticosteroids exceeding 10 mg/day of prednisone equivalent for ≥10 days within 30 days of Screening.
10. Individuals who have received immunoglobulins and/or any blood or blood products within the 3 months before the first vaccine administration or plan to receive such products at any time during the study.
11. Individuals with a documented history of HIV infection, or who are currently known to have active tuberculosis.
12. Individuals receiving treatment with another investigational drug, biological agent, or device.
13. Individuals who have received any investigational COVID-19 vaccines.
14. Individuals who received any influenza vaccine within 6 months prior to enrollment or plan to receive an influenza vaccine during the study period.
15. Individuals who have received any other licensed vaccines within 14 days prior to enrollment in this study or who are planning to receive any vaccine up to 14 days after the study vaccination.
16. Individuals who are investigator site staff members, employees of the Sponsor or the Clinical Research Organization directly involved in the conduct of the study, or site staff members otherwise supervised by the investigator or immediate family members of any of the previously mentioned individuals.