This is a randomized, double-blind, placebo-controlled Phase 1b study to evaluate safety and immunogenicity of the investigational Sing2016 M2SR H3N2 influenza vaccine delivered intranasally to a healthy adult population age 50 to 85 years.

This is a randomized, double-blind, placebo-controlled Phase 1b study to evaluate safety and immunogenicity of the investigational Sing2016 M2SR H3N2 influenza vaccine delivered intranasally to a healthy adult population age 50 to 85 years. Eligible subjects will be randomized 2:1 to receive one administration of Sing2016 M2SR or placebo followed by a dose of age-appropriate licensed inactivated influenza vaccine (IIV) at least 28 days later. Two lead-in groups of adults ages 50-64 years (Cohort 1) and 65 - 85 years (Cohort 2) will be followed by enrollment of an expansion group, ages 65 - 85 (Cohort 3). Safety data will be reviewed between enrollment of successive cohorts.

Inclusion Criteria:

1. Subjects must be willing and able to provide written informed consent.
2. Cohort 1: age 50-64 years at time of enrollment; Cohorts 2 and 3: age 65 to 85 years at time of enrollment (signing of informed consent form).
3. Subject willing to adhere to the requirements of the study and willing and able to communicate with the Investigator and understand the requirements of the study
4. Healthy adults and those with stable chronic conditions as determined by medical history, physical examination, vital signs, clinical safety laboratory examinations and clinical judgment of the investigator to be eligible for study inclusion.
5. Women of child-bearing potential must agree to abstain from sexual intercourse or to correctly use an acceptable method of contraception from 30 days prior to vaccination until 30 days after the last study vaccination.
6. Women of childbearing potential must have a negative urine pregnancy test within 24 hours prior to vaccination with investigational product.

Exclusion Criteria:

1. Any acute or chronic physical or mental condition that would limit the subject's ability to complete the study, increase risk of study participation or participant, or may interfere with interpretation of study results as based on the assessment by the Investigator.
2. Acute or chronic medical condition or history of a medical condition that, in the opinion of the Investigator, would render the study procedures unsafe or would interfere with the evaluation of the responses.
3. Abnormal screening hematology or chemistry value per the FDA Guidance: Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventative Vaccine Clinical Trials.
4. Currently receiving, or planned to receive during the study, any immunosuppressive therapy.
5. Had a flu-like illness, influenza treatment, influenza vaccination, or prophylactic influenza viral drug administered in the previous 6 months before investigational product administration.
6. Females who are pregnant or nursing; non-vasectomized males, with female partners of child-bearing potential, who are unwilling to use a highly effective form of contraception from the time of enrollment through at least 28 after administration of the investigational product.
7. History of receipt of any investigational vaccine within 28 days of visit 1 or investigational drug within the past six months. An exception is made for receipt of a Covid-19 vaccine whether licensed or under emergency use authorization (EUA) as long as the second dose was given at least 28 days prior to dosing with investigational vaccine.
8. Acute febrile illness within 72 hours prior to investigational product vaccination
9. Receipt of blood/plasma products or immunoglobulin within 6 months before administration of the investigational product or planned for during the period of study participation.
10. Planned donation of blood or blood product of at least approximately 1 pint within 3 months after investigational product administration.

Within each cohort, subjects were to be stratified based on a prescreening antibody titer to HA (by hemagglutinin inhibition [HAI] assay) from Sing2016 H3N2 influenza virus. Subjects were stratified into groups that had a pre-dose HAI titer of <40 and an HAI titer from 40 to <320.

IP administration until 28 days post-IIV (where interval between IP and IIV was up to 91 days with an average span of 49 days)

Includes treatment emergent AEs