Diversity and Dosage Determine the Efficacy of the Probiotic SYN-53 in Allergic Rhinoconjunctivitis: A Randomized, Double-Blind, Placebo-Controlled Trial.

SYN-53, a multi-strain probiotic food supplement, was recently shown to significantly alleviate allergic rhinoconjunctivitis (ARC) and its symptoms. The diversity and dosage of bacterial strains administered via SYN-53 have been proposed as key drivers of its efficacy. The aim of this study was to assess the role of bacterial diversity and dosage by comparing SYN-53 to a low dose variant (SYN-53-LD), a low diversity variant (SYN-4), and a placebo in the management of ARC. This double-blind, parallel-group, placebo-controlled clinical trial included subjects with moderate-to-severe grass pollen allergy. Following baseline exposure in an allergen exposure chamber (AEC), 166 subjects were randomized to undergo three weekly 3-day intake cycles of SYN-53, SYN-53-LD, SYN-4, or placebo, followed by a final allergen exposure. During AEC exposure, symptoms were continuously measured via assessment of the Total Symptom Score (TSS). SYN-53 was significantly superior in reducing TSSMAX compared with its low dose variant SYN-53-LD (∆TSSMAX [Mean ± SE]: -5.19 ± 0.80 vs. -2.27 ± 0.65; p = 0.0372), its low diversity variant SYN-4 (-3.41 ± 0.52; p = 0.0482), and placebo (-2.82 ± 0.78; p = 0.0329). No significant differences to placebo were seen for either SYN-53-LD (p = 0.7377) or SYN-4 (p = 0.5152). SYN-53 and its variants were well tolerated, and adverse events were not different from placebo. Our findings reaffirm the efficacy of SYN-53 in the management of ARC and demonstrate that the effectiveness of this multi-strain probiotic is intricately linked to the diversity of bacterial strains and dosage administered.