This is a Phase 1, randomized, controlled, dose-ranging clinical trial to assess the safety and immunogenicity of novel influenza A Group 1 and influenza A Group 2 mRNA chimeric hemagglutinin (HA) vaccine candidates given as intramuscular injections alone and in combination. A total of 60 healthy men and non-pregnant, non-breastfeeding women aged 18 through 59 years will be enrolled in one of 6 study arms. The 6 arms will consist of: 1) Sequential influenza A Group 1 mRNA chimeric hemagglutinin: cH8/1 (25 µg) followed by cH5/1 (25 µg), 2) Sequential influenza A Group 2 mRNA chimeric hemagglutinin: cH15/3 (25 µg) followed by cH4/3 (25 µg), 3) Sequential combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (25 µg) followed by cH5/1 + cH4/3 (25 µg), 4) A single dose of combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (50 µg) followed by placebo, 5) A single dose of combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH5/1 + cH4/3 (50 µg) followed by placebo, 6) Sequential combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (50 µg) followed by cH5/1 + cH4/3 (50 µg). The primary objectives are to evaluate safety and immunogenicity: 1) To assess the safety and reactogenicity of one or two doses of monovalent or bivalent Group 1 and 2 study products and 2) To describe the Group 1 and 2 anti-HA stalk IgG antibody responses of one or two doses of monovalent or bivalent Group 1 and 2 study products by ELISA.

This is a Phase 1, randomized, controlled, dose-ranging clinical trial to assess the safety and immunogenicity of novel influenza A Group 1 and influenza A Group 2 mRNA chimeric hemagglutinin (HA) vaccine candidates given as intramuscular injections alone and in combination. A total of 60 healthy men and non-pregnant, non-breastfeeding women aged 18 through 59 years will be enrolled in one of 6 study arms. The 6 arms will consist of: 1) Sequential influenza A Group 1 mRNA chimeric hemagglutinin: cH8/1 (25 µg) followed by cH5/1 (25 µg), 2) Sequential influenza A Group 2 mRNA chimeric hemagglutinin: cH15/3 (25 µg) followed by cH4/3 (25 µg), 3) Sequential combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (25 µg) followed by cH5/1 + cH4/3 (25 µg), 4) A single dose of combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (50 µg) followed by placebo, 5) A single dose of combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH5/1 + cH4/3 (50 µg) followed by placebo, 6) Sequential combined influenza A Group 1 and Group 2 mRNA chimeric hemagglutinin: cH8/1 + cH15/3 (50 µg) followed by cH5/1 + cH4/3 (50 µg). The primary objectives are to evaluate safety and immunogenicity: 1) To assess the safety and reactogenicity of one or two doses of monovalent or bivalent Group 1 and 2 study products and 2) To describe the Group 1 and 2 anti-HA stalk IgG antibody responses of one or two doses of monovalent or bivalent Group 1 and 2 study products by ELISA. The secondary objective is to describe the Group 1 and Group 2 anti-hemagglutinin stalk Immunoglobulin G (IgG) antibody response of one or two doses of monovalent or bivalent Group 1 and 2 study products by ELISA at a later timepoints.

Inclusion Criteria:

1. Provision of signed and dated informed consent form before the initiation of any study procedures.

2. Stated willingness to comply with all study procedures and availability for the duration of the study.

3. Non-pregnant healthy adults, aged 18 to 59 years of age, inclusive, at the time of enrollment.

4. In good general health as evidenced by medical history or diagnosed with stable chronic medical or psychiatric diagnoses or conditions.*

   *As determined by medical history, medications use, and physical examination to evaluate ongoing chronic medical or psychiatric diagnoses or conditions, defined as those that have been present for at least 90 days, which would not affect the assessment of the safety of participants or the immunogenicity of study products. These medical diagnoses or conditions should be stable for the last 60 days (no hospitalizations, emergency room [ER] or urgent care for the condition [excluding musculoskeletal conditions], or invasive medical procedure and no adverse symptoms that need medical intervention such as medication change/supplemental oxygen). This includes no change in chronic prescription medication or dose as a result of new symptoms or deterioration of the condition or disease being treated in the 30 days prior to enrollment. Any prescription change that is due to a change of health care provider, insurance company, etc., or that is done for financial reasons, as long as in the same class of medication, will not be considered a deviation of this inclusion criterion. Participants may be on chronic or as-needed (prn) medications if, in the opinion of the site PI or appropriate sub-investigator, they pose no additional risk to participant safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening or treatment of continued symptoms of medical diagnosis or condition. Note: Low-dose topical corticosteroids as outlined in the Exclusion Criteria as well as herbals, vitamins, and supplements are permitted.

5. Oral temperature is less than 100.4 degrees Fahrenheit*.

   *Inclusion requirement at enrollment and prior to study product administration.

6. Heart rate (HR) is 55 to 100 beats per minute, inclusive*. *Screening heart rate values in the normal range or with grade 1 abnormalities deemed not clinically significant by the site PI or designated clinician licensed to make medical diagnoses and listed on Form FDA 1572 are considered acceptable.

7. Systolic blood pressure is 90 to 140 mmHg, inclusive*.

   *Screening systolic blood pressure values in the normal range or with grade 1 abnormalities deemed not clinically significant by the site PI or designated clinician licensed to make medical diagnoses and listed on Form FDA 1572 are considered acceptable.

8. Diastolic blood pressure is 55 to 90 mmHg, inclusive*. *Screening diastolic blood pressure values in the normal range or with grade 1 abnormalities deemed not clinically significant by the site PI or designated clinician licensed to make medical diagnoses and listed on Form FDA 1572 are considered acceptable.

9. BMI between 18 kilograms/square meter (kg/m^2) (inclusive) and <35 kg/m^2 at screening

10. Females of childbearing potential* must agree to true abstinence** or use at least 1 acceptable primary form of contraception***,****

    • Not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, salpingectomy, or Essure® placement with history of documented radiological confirmation test at least 90 days after the procedure).

      **True abstinence is 100% of the time, no sexual intercourse (male's penis enters the female's vagina). (Periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception).

      ***Acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject receiving the study product, tubal ligation, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products

      ****Must use at least one acceptable primary form of contraception for at least 30 days before screening and agreement to use such a method during study participation and for an additional 30 days after the end of last study product administration.

11. Females of reproductive potential must have a negative serum pregnancy test at screening and a negative urine pregnancy test within 24 hours before the study product administration.

12. Must agree to have samples collected for secondary research.

Exclusion Criteria:

1. A history of any medical disease or condition that, in the opinion of the site PI or appropriate sub-investigator, is a contraindication to study participation*.

   *Including acute, subacute, intermittent, or chronic medical disease or condition that would place the participant at an unacceptable risk of injury, render the participant unable to meet the requirements of the protocol, or may interfere with the evaluation of responses or the participant's successful completion of this trial.

2. A history of any known or suspected immunosuppressive condition, acquired or congenital, or autoimmune conditions as determined by history and/or physical examination.

3. A history of known active or recently active (12 months) neoplastic disease or a history of any hematologic malignancy. (Non-melanoma, treated, skin cancers are permitted.)

4. A history of myocarditis or pericarditis.

5. A history of Guillain-Barré Syndrome.

6. Receipt of anticancer chemotherapy or radiation therapy (cytotoxic) within 3 years before study product administration.

7. Current pregnancy or breastfeeding.

8. Known allergic reactions to components of the investigational products or seasonal influenza vaccine.

9. Febrile illness or other acute illness* within 72 hours before the first study product administration.

   *An acute illness that is nearly resolved with only minor residual symptoms remaining is allowable if, in the opinion of the site PI or appropriate sub-investigator, the residual symptoms will not interfere with the ability to assess safety parameters as required by the protocol.

10. Receipt of another experimental agent* or other intervention within 60 days before the first study product administration**.

    *Including vaccine, drug, biologic, device, blood product, or medication.

    **Other than from participation in this trial

11. Any condition that, in the judgment of the investigator, precludes participation because it could affect participant safety.

12. Has a positive test result for hepatitis B surface antigen, hepatitis C virus antibody*, or HIV type 1 or 2 antibodies at screening.

    *Persons testing positive for hepatitis C virus antibody with a history of treatment and a negative HCV viral load may be acceptable in the opinion of the PI or appropriate Sub-Investigator.

13. Currently enrolled in or plans to participate in another clinical trial with an investigational agent*that will be received during the study-reporting period**.

    *Including licensed or unlicensed vaccine, drug, biologic, device, blood product, or medication.

    **Approximately 10 months after the first study product administration.

14. Has a history of hypersensitivity or severe allergic reaction* to any previous licensed or unlicensed influenza, mRNA, or LNP-containing vaccines.

    *For example, anaphylaxis, generalized urticaria, angioedema, or other significant reactions.

15. Chronic use (more than 14 continuous days) of any medications that may be associated with impaired immune responsiveness*.

    *Including, but not limited to, systemic corticosteroids exceeding 10 mg/day of prednisone equivalent, allergy injections, immunoglobulin, interferon, immunomodulators, cytotoxic drugs, or other similar or toxic drugs during the preceding 6-month period before study product administration (Day 1). The use of low-dose topical, ophthalmic, inhaled, and intranasal steroid preparations will be permitted.

16. Anticipating the need for immunosuppressive treatment within the next 6 months.

17. Received immunoglobulins and/or any blood or blood products (except Rho D immunoglobulin) within the 4 months before the first study product administration.

18. Plans to donate blood or blood products from screening through 90 days after dosing.

19. Has any blood dyscrasias or significant disorder of coagulation.

20. Has any chronic liver disease, including fatty liver.

21. Has a history of alcohol abuse or other recreational drug (excluding cannabis) use within 6 months before the first study product administration.

22. Has any abnormality or permanent body art (e.g., tattoo) that would interfere with the ability to observe local reactions at the injection site.

23. Received or plans to receive a licensed, live vaccine within 4 weeks before first study product administration through 4 weeks after last study product administration.

24. Received or plans to receive a licensed, inactivated vaccine within 2 weeks before first study product administration through 2 weeks after last study product administration.

25. Received or plans to receive a seasonal influenza vaccine within 90 days before first study product administration through 90 days after last study product administration.

26. Have a known history of confirmed influenza virus infection within the past 90 days.

27. Have screening clinical laboratory examinations of grade 1 or above*.

    *Grade 1 criteria for clinical laboratory (WBC count, hemoglobin, platelet count, creatinine, ALT, total bilirubin) may be acceptable if deemed not clinically significant by the site principal investigator (PI) or the study clinician listed on the delegation log.

28. Has had a positive SARS-CoV-2 test (home or laboratory-based) within 7 days before the screening visit or during the period between screening and enrollment visits.