“With most people, disbelief in a thing is founded on a blind belief in something else"
G. C. Lichtenberg
The Cochrane Collaboration found low-quality evidence that probiotics prevent UTRIs, and moderate-quality evidence that probiotics reduce antibiotic prescriptions for UTRI.
Because the antibiotic finding is most robust (aside from bearing the higher-quality GRADE score, it's also not a self-reported outcome), we'll take this as our baseline and see if it is strengthened or weakened by a Bradford Hill analysis.
1) Strength of association: for antibiotic use, RR 0.65 (0.45 to 0.94), n=1184.[1]
The true association is subject to type 2 confounding by two factors - by intention-to-treat analysis, and potentially by the random consumption of yogurt and other fermented foods supplying similar effects.
2) Consistency of association: the association is consistent, with similar (but slightly larger) effect sizes for all other measures of URTI. The association is reasonably consistent between different trials (there is no major contradiction). The association is strongly consistent with the effects of probiotics on vaccination immunity (RCTs using independently measured serum makers so more robust than the UTRI trials). The association is consistent with results for sambucus (effect size 1.717) (see 9, "analogy").[1,2,3,4]
3) Specificity: Probiotics have had no consistent association with several other outcomes predicted for them. Probiotic treatment of mothers during pregnancy only results in better immunity after vaccine for mother, not infant, consistent with dendrite cell pathway for effect.[2]
4) Temporality: Implicit in trial design
5) Biological gradient: dose-reponse is seldom tested in probiotic experiments perhaps because of assumption that a living organism can replicate, however a dose-response for duration of treatment is seen in the vaccine studies.[3] The analogous effect of echinacae purpurea in zebrafish is strongly dose-dependent.[5]
6) Plausibility: effect of probiotic on immune tone is well-studied, insofar as the immune system is currently understood the effect is plausible. Increased innate immunity around infection improves the acquired immune response. "A significant property of these bacteria is their ability to mimic natural infections, while intrinsically possessing mucosal adjuvant properties".[6] Dendrite cell presentation provides plausible pathway.[7]
7) Coherence - laboratory and field work are strongly coherent, animal experiments support human, herbs with similar immune effects to probiotics in experiments tend to have similar associations with URTIs in RCTs.[4]
8) Experiment - the association is experimental, the mechanism holds across a wide range of experiment types, including those with lowest risk of confounding or placebo effect.
9) Analogy - herbal effects are analogous, as when plant polysaccharides mimic bacterial lipopolysaccharides. Vaccine adjuncts are also analogous. Probiotics and herbal antivirals are researched as adjuvants.
Bradford Hill analysis allows us to see an association within its complete scientific context, to test whether it is causal.
It was originally designed to test the low-quality evidence that arises from purely observational, non-interventional studies, but is also useful to test the results of experiments where one considers these inadequate by themselves.
Here's what I think is the parsimonious way to explain the association: In the vaccine research, probiotics double the odds of lasting immunity after vaccination. Many infections are "repeats" of infections we have had before but lost immunity to. If probiotics prevent "repeat" infections by maintaining antibody responses, this can account for the effect, even without a direct effect on immunity to any new pathogen (although this is also plausible).
Immunologists are currently worried that many exposed to COVID-19 have not had a sufficient or lasting antibody response and are at risk of re-infection. The less re-infectious COVID-19 is, the sooner we can safely end our current economic restrictions, which will also take a toll on human life if maintained indefinitely.
1] Cochrane Database of Systematic Reviews. Q Hao, RB Dong, T Wu.
Probiotics for preventing acute upper respiratory tract infections
https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD006895.pub3/full
2] Zimmermann P, Curtis N. The influence of probiotics on vaccine responses - A systematic review.
Vaccine. 2018 Jan 4;36(2):207-213. doi: 10.1016/j.vaccine.2017.08.069. Epub 2017 Sep 18.
https://www.ncbi.nlm.nih.gov/pubmed/28923425
3] Lei WT, Shih PC, Liu SJ, Lin CY, Yeh TL. Effect of Probiotics and Prebiotics on Immune Response to Influenza Vaccination in Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2017;9(11):1175. Published 2017 Oct 27. doi:10.3390/nu9111175
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5707647/
4] Hawkins J, Baker C, Cherry L, Dunne E. Black elderberry (Sambucus nigra) supplementation effectively treats upper respiratory symptoms: A meta-analysis of randomized, controlled clinical trials. Complement Ther Med. 2019 Feb;42:361-365. doi: 10.1016/j.ctim.2018.12.004. Epub 2018 Dec 18.
5] Guz L, Puk K, Walczak N, Oniszczuk T, Oniszczuk A.
Effect of dietary supplementation with Echinacea purpurea on vaccine efficacy against infection with Flavobacterium columnare in zebrafish (Danio rerio). Pol J Vet Sci. 2014;17(4):583-6.
6] Benef Microbes. 2020 Mar 27:1-14. doi: 10.3920/BM2019.0121. [Epub ahead of print]
Immune modulatory capacity of probiotic lactic acid bacteria and applications in vaccine development.
Mojgani N, Shahali Y, Dadar M.
7] Gallo PM, Gallucci S. The dendritic cell response to classic, emerging, and homeostatic danger signals. Implications for autoimmunity. Front Immunol. 2013;4:138. Published 2013 Jun 10. doi:10.3389/fimmu.2013.00138
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