Effects of Probiotics on Respiratory Tract Infections in Children: A Randomized, Placebo-Controlled Interventional Study

Inclusion Criteria: * Children aged 3 months to 6 years, regardless of sex. * Meet the clinical diagnostic criteria for upper respiratory tract infection, including but not limited to cough, rhinorrhea, nasal congestion, sore throat, and fever. * Assessed by a clinician as having mild or moderate URTI, not requiring antibiotics or other systemic prescription medications, and suitable for observation and/or symptomatic management. * Willing to discontinue other probiotics or supplements during the study period, except for the study product provided by the investigators. * Agree to complete questionnaires and provide fecal biological samples during the study period. * The child's parent(s) or legal guardian(s) are fully informed about the trial and voluntarily sign the written informed consent form before any study procedures begin. Exclusion Criteria: * Severe URTI as judged by the clinician, or symptoms severe enough to require antibiotics, corticosteroids, nebulized steroids/bronchodilators, or other systemic prescription medications, including suspected or confirmed bacterial infections such as bacterial tonsillitis, bacterial sinusitis, or acute suppurative otitis media. * Acute lower respiratory tract infections, such as acute bronchitis, acute laryngitis/laryngospasm, or pneumonia. * Respiratory tract infections secondary to underlying diseases, including primary immunodeficiency, acquired immunodeficiency syndrome, congenital airway malformation, congenital heart dis…