This is an open-label, first-in-human, dose-finding study to evaluate the safety and immunogenicity of a booster vaccination of Prime-2-CoV_Beta in healthy participants.

Eligible participants will undergo baseline assessments and will receive 1 injection of Prime-2-CoV_Beta at Day 1. Participants will be followed up through 6 months post-booster vaccination. Follow-up visits will be performed at Days 4, 8, 15, 29, and Months 3 and 6, to assess the safety, tolerability, and immunogenicity of Prime-2-CoV_Beta. Additional safety and tolerability data will be assessed 1 day and 2 days after booster vaccination (Days 2 and 3) by telephone. Initially, a total of 60 participants were planned to be vaccinated in 5 cohorts of 12 participants each. Dose ranging of Prime-2-CoV_Beta was planned to be done by dose escalation with doses ranging from 3x104 plaque forming units (PFUs) up to 3x10^7 PFUs.

With protocol Version 7.0, Cohort 6 with 12 participants was added with 6x10^7 PFU. With protocol Version 8.0, 1 additional cohort, Cohort 7 (1.2x10^8 PFU, stratified by previous COVID-19 vaccinations and previous SARS-CoV-2 infections) with 24 participants will be added (total number thus 96 participants).

Procedures for Cohorts 6 and 7 will be the same as for Cohorts 1 - 5, except that for some participants in Cohort 7 ORFV shedding in urine, saliva, blood, and stool will be evaluated.

Inclusion Criteria:

Inclusion criteria

1. Healthy adult men or women aged 18 to 55 years

2. Previous COVID-19 vaccination and previous SARS-CoV-2 infection as follows:

   1. Cohorts 1 to 7a: Full course of vaccination e.g., having received at least 3 doses of a licensed mRNA COVID-19 vaccine or having received at least 2 doses of a licensed mRNA COVID-19 vaccine and a prior SARS-CoV-2 infection, with the last dose of the vaccine being administered ≥ 10 weeks before Day 1 as documented in a respective vaccination certificate
   2. Cohort 7b: No more than 2 doses of any licensed COVID-19 vaccine. Those with fewer COVID-19 vaccinations, preferably none or just one, will be given preference. Non-mRNA COVID-19 vaccines are acceptable. The last vaccine dose must have been given ≥ 10 weeks before Day 1 as documented in a respective vaccination certificate

3. Able to understand the participant information and providing written informed consent

4. Body mass index of 18.5 to 30.0 kg/m² and weight > 50 kg at Screening

5. Women of childbearing potential must:

   1. have a negative pregnancy test at Screening (blood) and at Day 1 (urine)
   2. agree to use, and be able to comply with, highly effective measures of contraception without interruption, from 14 days before Prime-2-CoV_Beta booster vaccination until the end of the study.

   A highly effective method of contraception or birth control (failure rate less than 1% per year when used consistently and correctly) for this study: combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, transdermal, injectable), progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, implantable), intrauterine device, intrauterine hormone-releasing system, bilateral tubal occlusion, sexual abstinence or vasectomized sexual partner. Abstinence is only acceptable as true abstinence when this is in line with the preferred and usual lifestyle of the participant (abstinent on a long-term and persistent basis). The reliability of sexual abstinence needs to be evaluated in relation to the duration of the clinical study and the preferred and usual lifestyle of the participant. (Periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods and withdrawal] are not acceptable methods of contraception.) Postmenopausal (no menses for at least 1 year without alternative medical cause) or surgically sterile women (tubal ligation, hysterectomy or bilateral oophorectomy) may be enrolled.

6. Male participants must agree not to intend to father a child or to donate sperm starting at Screening, throughout the clinical study. Male participants must also

   1. abstain from sexual intercourse with a female partner (acceptable only if it is the participant's usual form of birth control/lifestyle choice: abstinent on a long-term and persistent basis). The reliability of sexual abstinence needs to be evaluated in relation to the duration of the clinical study and the preferred and usual lifestyle of the participant or
   2. use adequate barrier contraception (male condom) during treatment with the investigational product until the end of the study, and
   3. ensure that, if they have a female partner of childbearing potential, the partner uses a highly effective contraceptive method as outlined in inclusion criterion number 5
   4. use condoms during the entire study if they have a pregnant partner, to avoid exposure of the fetus to the investigational product

7. Willing and able to comply with all study procedures based on the investigator's judgment

Exclusion Criteria:

Previous and concomitant therapy:

1. Receipt of any vaccine (licensed or investigational) from 4 weeks before Prime-2-CoV_Beta booster vaccination or anticipated vaccination during the study until 6 weeks after the Prime-2-CoV_Beta booster vaccination

2. Previous vaccination against COVID-19 with vaccines (licensed or investigational) other than mRNA-based vaccines (licensed or investigational) only applicable for Cohorts 1 to 7a

3. Current or previous treatment with another investigational drug and/or medical device (within 30 days of enrollment or 5 half-lives of that investigational drug)

4. Administration of immunoglobulins or any blood products within 2 months of Prime-2-CoV_Beta booster vaccination

5. Chronic administration of medication associated with impaired immune responsiveness as judged by the investigator (including, but not limited to: immunosuppressive therapy, systemic corticosteroids exceeding 10 mg/day of prednisone equivalent, allergy shots for hypo-sensitization, immunoglobulin, interferon, immunomodulators, cytotoxic drugs, or other similar or toxic drugs) within 2 months before the Prime-2-CoV_Beta booster vaccination (Day 1). Inhaled/nebulized, intra-articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

   Previous and concomitant medical condition:

6. Active SARS-CoV-2 infection, confirmed by a commercially available SARS-CoV-2 rapid antigen test at Day 1, or currently on quarantine

7. {deleted}

8. Known history of severe adverse reactions to any vaccine and/or severe allergic reactions to any component of the study vaccine, to any drug, or to any other exposure

9. Known history of angioedema

10. Pregnant or lactating women

11. Any confirmed or suspected immunosuppressive or immunodeficient condition

12. Known history of Guillain-Barré Syndrome

13. Known infection with human immunodeficiency virus, hepatitis C virus or hepatitis B virus

14. Active cancer (malignancy) within 5 years before Day 1 (except for adequately treated non-melanomatous skin carcinoma, at the discretion of the investigator)

15. Moderate or severe illness and/or fever > 38.0 °C within 1 week before Prime-2-CoV_Beta booster vaccination

16. Any clinically significant health problem (medical history and physical examination) or clinically significantly abnormal finding in biochemistry and/or hematology blood tests, urinalysis, or electrocardiogram at Screening according to the investigator's opinion

17. Current or history of cardiovascular disease or structural cardiac disease (including chronic or congenital heart conditions, such as chronic hypertension, coronary heart disease, myocardial infarction and arrhythmias, hypertrophic cardiomyopathy, as well as a history of myocarditis after mRNA vaccinations)

18. History of mRNA vaccination-associated adverse events that were in nature and severity beyond the common AEs expected

19. Current or history of gastrointestinal disease, liver disease, renal disease or endocrine disorders, (including diabetes) and neurological illness (excluding migraine), when judged as clinically significant according to the investigator's opinion

20. Current or history of chronic respiratory diseases, including mild asthma treated by on-demand medication (resolved childhood asthma is allowed)

21. Current or history of alcohol and/or drug abuse within the last 6 months before Day 1

    Previous and concomitant clinical study experience

22. Current participation in another study or previous enrollment in this clinical study

    Other exclusion criteria

23. Investigator or employee of the study group or sponsor with direct involvement in the proposed study or relatives of the research staff with direct involvement in the proposed study

24. Prolonged exposure to sheep or goats (e.g., shepherds, sheep farmer)