Inclusion Criteria:
1. Age 18-65 years inclusive at the time of consent.
2. Generally healthy at enrolment, with no active upper respiratory tract infection (URTI) or other acute infection requiring treatment at baseline, as determined by protocol-defined screening procedures and investigator judgement.
3. Able and willing to provide valid written informed consent before undertaking any study-specific procedures.
4. Willing and able to take the allocated study product daily for 12 weeks, complete online/mobile-based daily URTI surveillance for 16 weeks, complete episode-specific symptom assessments when required, and attend the scheduled baseline, Week 12, and Week 16 assessments.
5. Able to read and understand the approved Greek-language study materials, complete the required study questionnaires and monitoring procedures, and provide valid written informed consent.
Exclusion Criteria:
1. Known or suspected clinically significant immunodeficiency or current use of systemic immunosuppressive therapy that, in the investigator's judgement, could affect participant safety, susceptibility to infection, or interpretation of study outcomes.
2. Active URTI or other clinically relevant acute infection at baseline; use of systemic antibacterial antibiotics within 14 days before baseline; or another acute illness that could confound baseline clinical or biomarker assessments.
3. Receipt of any influenza, COVID-19, or other systemic vaccination within 14 days before baseline, or ongoing clinically relevant vaccine-related symptoms at baseline.
4. Use of prebiotic or probiotic supplements or yeast-based probiotic products within 1 month before baseline.
5. Current pregnancy or breastfeeding, or intention to become pregnant during the 16-week study period.
6. Known allergy, intolerance, or hypersensitivity to any component or excipient of the study products.
7. Acute, unstable, or uncontrolled chronic disease, or any other clinically significant medical condition that, in the investigator's judgement, could compromise participant safety, materially influence immune or inflammatory outcomes, interfere with assessment of URTI outcomes, or affect adherence to study procedures.
8. Current participation in another interventional clinical trial or other research study that could interfere with the intervention or outcome assessment.
9. Any medical, cognitive, practical, or other circumstance that, in the investigator's judgement, would prevent reliable completion of study-product administration, mobile monitoring, questionnaires, scheduled visits, or other study procedures, or would preclude provision of valid informed consent.
