Probiotic Supplementation Reduces RRTIs and Enhances Gut Microbial and Immunity in Children: A Randomized Controlled Trial

Recurrent respiratory tract infections (RRTIs) are a major cause of morbidity in children, and strain-defined probiotics have been proposed as a supportive preventive strategy, although clinical evidence remains limited. In this randomized, double-blind, placebo-controlled trial, 120 children diagnosed with RRTIs received either <i>Bifidobacterium animalis</i> subsp. <i>lactis</i> XLTG11 and <i>Lactiplantibacillus plantarum</i> CCFM8661 or a matched placebo daily for 180 days. The probiotic group demonstrated significantly reduced duration and frequency of fever, cough, upper respiratory tract infections, trachea/bronchitis, pneumonia, and overall RRTI recurrence compared with the placebo group (all <i>p</i> < 0.05). Gut microbiota profiling showed clear community differences between groups at day 180, with the probiotic group exhibiting greater abundance of beneficial commensal taxa and the placebo group showing higher representation of opportunistic genera. Functional pathway analysis indicated shifts consistent with enhanced metabolic stability in probiotic recipients. Immune biomarker patterns further supported a more regulated humoral response in the probiotic group, reflected by comparatively stable IgG, IgM, and complement C3 levels over the intervention period. Growth trajectories remained normal in both groups, and no treatment-related adverse events were reported, confirming a favorable safety profile. These findings indicate that long-term supplementation with XLTG11 and CCFM8661 is safe, well tolerated, and effective in reducing RRTI burden in children, while also supporting healthier microbiota and immune patterns. This trial provides evidence for the use of strain-defined probiotics as a complementary approach within pediatric respiratory infection prevention strategies.