Probiotics did nothing to ward off COVID in five trials

Through the pandemic, cheap and safe add-ons that might lower the odds of catching COVID-19 drew intense interest, and probiotics were a natural candidate. They are widely available, inexpensive, generally safe, and often marketed as supporting immune function through the gut. Several research groups therefore ran randomised trials asking a direct question: does taking probiotics, prebiotics or synbiotics actually prevent infection or boost the antibody response to the virus? This review set out to gather and weigh all of that trial evidence in one place.

The authors registered their protocol on PROSPERO and systematically searched Scopus, the Cochrane Library, Web of Science and PubMed for randomised trials published in English up to November 2022. Eligible studies compared probiotic, prebiotic or synbiotic supplementation against placebo and reported either the incidence of COVID-19 or levels of specific antibodies against SARS-CoV-2. Five trials qualified, covering a genuine spread of populations: high-risk staff exposed to COVID-19, elderly nursing home residents, healthy adults and household contacts of infected people.

Four of the five trials reported on how many people caught COVID-19, and none found a significant difference between the supplement and placebo groups over follow-up periods of one, two or three months. Three trials measured antibodies against the virus, including IgM, IgG and IgA, and again there were no significant differences between groups. The consistency of the null result across quite different settings is the notable part: whether in a nursing home, a household with an infected member or a high-exposure workplace, the supplements did not move the outcomes measured.

Probiotics have shown benefits in some other settings, which is partly why hopes were high here, but this review suggests preventing COVID-19 is not among them. The evidence base is small, at five trials, and the interventions ran for only one to three months, so longer regimens or different strains have not been ruled out. The review also addresses prevention only; it says nothing about whether these supplements affect the course of an existing infection. For now, the trial evidence that exists points the same way: no protective effect.

If a probiotic sits in your basket as general infection insurance, this is a reason to lower your expectations of it, at least where COVID-19 is concerned. Across the trials gathered here, supplementation changed neither the chance of catching the virus nor the antibody levels that were measured. That does not make probiotics useless for every purpose, but the specific claim of COVID-19 protection has now been tested and has not held up. Anyone taking them for this reason has a clear answer from the evidence so far.