This phase 2a is designed to evaluate the anamnestic response of a booster dose (either 180 µg or 480 µg) of OVX836 influenza vaccine administered intramuscularly to participants vaccinated three to five years ago in OVX836-002 or OVX836-003 studies. Participants who had received a placebo or Influvac Tetra® in previous studies would serve as controls and would receive the highest dose of OVX836 vaccine (480 µg) as a primary vaccination.

This is a phase 2a, single center, randomized, double-blind study designed to evaluate the immunogenicity and safety of one single administration of OVX836 influenza vaccine at two dose levels (180μg or 480μg) given intramuscularly, either as a booster or a primary vaccination in approximately 160 to 220 healthy participants (aged 20-64 years) previously administered with OVX836, Influvac Tetra® or placebo in the OVX836-002 (EudraCT number: 2019-002939-28) and OVX836-003 (EudraCT number: 2021-002535-39) studies. This includes OVX836-002 participants who had received 180μg OVX836 during the influenza season 2019-2020 and OVX836-003 participants who had received 180μg, 300μg or 480μg OVX836 before the influenza season 2021-2022. Volunteers who received either Influvac Tetra® in the OVX836-002 study or a placebo in the OVX836-003 study will serve as controls and will receive a primary single dose of OVX836 480μg.

Inclusion Criteria:

1. Subject who voluntarily provides written informed consent to participate in the study.
2. Healthy male or female subjects, as determined by medical history and medical examination
3. Subject compliant with the reproductive criteria for female participants
4. Subjects who participated in the OVX836-002 or OVX836-003 studies.
5. Subjects aged 20 and 64 years, inclusive.
6. Reliable and willing to commit to participating for the duration of the study, and capable of following study procedures diligently.
7. Ability and technical capability to complete an eDiary.

Exclusion Criteria:

1. Subjects with a body mass index (BMI) ≤18 kg/m² or ≥35 kg/m² on the day of vaccination.
2. Previous influenza vaccination within 6 months before the day of vaccination or planned to receive during the whole study period.
3. vaccination with an mRNA-based influenza vaccine including NP in its composition.
4. Any known or suspected immunodeficient conditions.
5. Past or current history of significant autoimmune diseases, as judged by the Investigator.
6. Current history of uncontrolled medical illness such as diabetes, hypertension, heart, renal or hepatic diseases.
7. Known or suspected infection with human immunodeficiency virus (HIV), hepatitis C virus (HCV), or hepatitis B virus (HBV).
8. Pregnant or lactating woman.
9. Female planning to become pregnant or planning to discontinue contraceptive precautions until the end of the trial.
10. Participants with extensive tattoos covering deltoid regions on both arms that would preclude the assessment of local reactogenicity.
11. Other vaccination within 3 months prior to the day of study vaccination for live attenuated vaccines, or within 1 month prior to the day of study vaccination for non-live vaccines, at the exception of COVID vaccines which can be administered within 14 days before or after study vaccine administration.
12. Planning to receive other vaccines during the first 28 days following the study vaccine administration.
13. Administration of any investigational or non-registered drug or vaccine within 3 months prior to the administration of study vaccines, or planned administration of any such product during the entire study period.
14. History of receiving blood, blood components or immunoglobulins within 3 months prior to the day of vaccination or planned to receive such product during the whole study period.
15. Presence of an acute febrile illness on the day of vaccination (oral temperature >38.0°C, temporary exclusion criterion).
16. Past or current history of any progressive or severe neurological disorder, seizure disorder or Guillain-Barré syndrome.
17. Behavioral or cognitive impairment, or psychiatric disease that, in the opinion of the Investigator, may interfere with the subject's ability to participate in the study.
18. History of alcohol or drug abuse that ceased less than 6 months before enrolment, current alcohol or drug abuse according to the Investigator's judgement), smoking habit of more than 10 cigarettes/day, or current vaping (nicotine consumption corresponding to more than 10 cigarettes/day).
19. Chronic administration (defined as more than 14 days in total) of immunosuppressants or other immune-modifying drugs during the period starting 3 months before the study vaccine administration. For corticosteroids, this will mean prednisone equivalent ≥20 mg/day. Inhaled, topical and intra-articular steroids are allowed.
20. Current or past malignancy, unless completely resolved for >5 years (except non-metastatic basal cell carcinoma which has been completely resected).
21. History of severe allergic reactions and/or anaphylaxis, or serious adverse reactions to vaccines or allergy to kanamycin or any other component of the vaccine.
22. Any contraindication to IM administration, as judged by the Investigator.
23. Individuals with a history of any illness that, in the opinion of the Investigator, could potentially interfere with the results of the study or pose additional risk to the subjects by participating in the study.
24. Sponsor employees or Investigator site personnel directly affiliated with this study and their immediate families. Immediate family is defined as a spouse, parent, child or sibling, whether biological or legally adopted, including children of newly composed families.