In a 16-week double-blind RCT of 104 healthcare workers, daily Lab4 probiotic supplementation produced consistently favourable but statistically non-significant reductions across all illness-related endpoints compared with placebo. URTI occurrence risk was 40% lower (RR 0.60, 95% CI 0.30–1.20), GP visit risk was 52% lower (RR 0.48, 95% CI 0.21–1.07), antibiotic use risk was 52% lower (RR 0.48, 95% CI 0.20–1.18), and probiotic recipients reported better overall wellbeing. No significant gut microbiota differences emerged in the faecal-sample subset.
The consistent directional signal across multiple outcomes is biologically plausible—Lab4 is a multi-strain formulation whose components have previously shown immunomodulatory effects in smaller trials—but the wide confidence intervals throughout reflect a study powered for exploration, not confirmation. None of the primary illness endpoints crossed the p<0.05 threshold, meaning chance cannot be ruled out. The 104-participant sample is simply too small to distinguish a real moderate effect from noise, a limitation the authors themselves acknowledge by calling for a larger, prospectively powered trial.
For practical purposes, the 40–58% relative risk reductions, if real, would be clinically meaningful in high-exposure occupational settings, but adults should not interpret directional trends as proof of efficacy. Probiotic research remains complicated by strain specificity, dose dependency, and individual microbiome variation. Crucially, this is a preprint posted on medRxiv and has not yet been peer-reviewed; findings may change substantially after independent scrutiny.