Gut Microbiome Dysbiosis Drives Long COVID — and Fixing It May Offer Relief

Long COVID, formally called post-acute COVID-19 syndrome (PACS), has affected an estimated 409 million people globally since 2020. Patients experience debilitating, multi-system symptoms — fatigue, cognitive impairment, gastrointestinal upset, anxiety, sleep disturbances, and more — for months or years after SARS-CoV-2 infection. Despite its massive burden, effective treatments remain elusive. This comprehensive 2025 review by Lau, Su, and Ng from The Chinese University of Hong Kong examines the growing body of evidence linking gut microbiome dysbiosis to PACS pathogenesis and evaluates microbiome-targeted therapeutic strategies.

The authors systematically reviewed studies published between December 2019 and December 2024, drawing on observational studies, animal research, clinical trials, and meta-analyses. Across 14 observational studies conducted in China, Brazil, Hong Kong, Japan, Latvia, Norway, Russia, and the United States, a consistent pattern emerged: PACS patients exhibit significantly reduced gut bacterial diversity and richness compared to healthy controls. This dysbiosis was detectable as early as 20 days post-infection and persisted up to 14 months after viral clearance in some cohorts.

At the bacterial genus level, Bacteroides and Flavonifractor were consistently enriched in PACS patients, while Bifidobacterium and Dorea were consistently depleted. At the species level, Ruminococcus gnavus — associated with inflammation and conditions like IBD, IBS, and colorectal cancer — was repeatedly elevated in PACS, whereas Faecalibacterium prausnitzii — a key butyrate producer with anti-inflammatory properties — was consistently reduced. These microbiome alterations appear to contribute to PACS via multiple mechanisms: impaired short-chain fatty acid (particularly butyrate) production, increased intestinal permeability ('leaky gut'), systemic immune dysregulation via the gut-lung axis, neuroinflammation affecting the gut-brain axis, and disrupted tryptophan/serotonin metabolism.

On the therapeutic side, the review highlights promising clinical trial data. Probiotic and synbiotic formulations (combining probiotics with prebiotics) significantly reduced fatigue, GI symptoms, and cognitive complaints in PACS patients. FMT — transplanting stool from healthy donors to PACS patients — showed particularly broad efficacy, improving fatigue, memory loss, concentration, GI function, sleep, and mood in randomized controlled trials. Dietary interventions emphasizing fiber and plant-based foods to support microbiome restoration are also discussed as complementary strategies.

The authors acknowledge important caveats: most observational studies had small sample sizes and heterogeneous patient populations; the specific SARS-CoV-2 variants, disease severity, and antibiotic exposure varied across studies, complicating comparisons. Causality between dysbiosis and specific PACS symptoms has not been fully established. Nevertheless, the convergence of mechanistic and clinical data positions the gut microbiome as one of the most actionable therapeutic targets in Long COVID — and potentially in other post-infectious chronic conditions.