The treatment of the new coronavirus infection (COVID-19) and COVID-19-associated diarrhoea and liver injury remains challenging. Optimizing treatment approaches for COVID-19 remains an issue. It is assumed, that changes in composition of intestinal microbiota is closely related to a change in the regulation of the immune response in the lungs in patients with COVID-19. These gut microbiota changes in combination with antibiotic prescription during the treatment increase the risk of antibiotic-associated diarrhea and C. difficile infection as well as worse clinical outcomes in these patients. Probiotics are useful for restoring the human gut microbiome and increasing anti-inflammatory response also. Despite the variety of uses of probiotics, there is still insufficient data on the clinical efficacy of including probiotics in the treatment of patients with COVID-19 infection.

A randomized controlled open-label study approved by the local ethical committee. The study included patients with COVID-19 admitted to the T.M. Tareev Clinic of Internal Diseases of I.M. Sechenov First Moscow State Medical University in whom the disease was confirmed using polymerase chain reaction on nasopharyngeal and oropharyngeal swabs to detect severe acute respiratory syndrome-related coronavirus 2 (SARS-COV-2). The study was conducted from December 2020 to March 2021 and included participants in the age range from 18 to 75 years.

The exclusion criteria were age over 75 years or under 18 years, consumption of probiotics for 3 months prior to admission, history of intolerance to probiotics or their components, refusal to participate and sign informed consent, pregnancy or breastfeeding, cancer or mental illness, and severe renal (glomerular filtration rate less than 50 mL/min) or hepatic (equivalent to cirrhosis class B or C on the Child-Pugh scale) dysfunction at the time of admission.

Patients who prematurely discontinued the consumption of probiotics for reasons not related to the development of side effects were excluded from the study.

The patients were randomised to the probiotics group (PRO group) or the control group (CON group). Patients in the PRO group received probiotics containing ~109 colony forming units (CFU) of Lactobacillus rhamnosus PDV 1705, ~109 CFU of Bifidobacterium bifidum PDV 0903, ~109 CFU of B. longum subsp. infantis PDV 1911, and ~109 CFU of B. longum PDV 2301 three times a day during the hospital stay but for no more than 14 days (PDV is the commercial letter designation of the bacterial strains). The end point of the trial was day 14 of hospitalisation or the day of the patient's discharge or death, whichever occurred earlier.

The control group consisted of patients who did not receive probiotics. Patients in both the groups also received dexamethasone and antiviral (favipiravir and/or riamilovir), antibacterial, anticoagulant (enoxaparin in most cases; rivaroxaban and dabigatran were used much less frequently), and anticytokine (tocilizumab or/and olokizumab) drugs according to indications and contraindications.

Death from any cause was considered the primary outcome. Duration of hospitalisation, total duration of the disease, incidence of admission to intensive care unit, need for oxygen support or mechanical ventilation, and changes in the values of key biomarkers were considered the main secondary outcomes. The duration of diarrhoea [loose or watery stools or an increase in the frequency of bowel movements (more than three times per day)] and incidence of hospital-acquired diarrhoea as well as the progression of pre-existing liver injury and onset of liver injury were considered additional secondary outcomes. Liver injury was determined through the presence of abnormalities in any of the main liver test findings (serum alanine transaminase, aspartate transaminase, alkaline phosphatase, gamma-glutamyl transferase, total bilirubin, and albumin levels).

The volume of the affected lungs was measured using chest computed tomography (CT); it included the sum of ground glass and consolidation volumes.

Results are presented as the median [interquartile range]. The groups were compared using Mann-Whitney test for continuous data and chi-square test for categorical data. Wilcoxon test was used to assess the changes in continuous biomarker values. Mortality was assessed using the Kaplan-Meier estimator and Cox's test. A p value ≤0.05 was considered the criterion for significance. Statistical calculations were performed using "Statistica 10" soft (TIBCO Software inc, Palo Alto, CA)

Inclusion Criteria:

• patients with COVID-19 infection confirmed by polymerase chain reaction of nasopharyngeal and oropharyngeal swabs to the causative agent of this infection
• age from 18 to 75 years old from December 2020 to April 2021

Exclusion Criteria:

• the patient's decision to withdraw from the study
• refusing to take the probiotic for a reason not related to the development of side effects.

Non-inclusion criteria:

• age over 75 years old or under 18 years old,
• taking probiotics for 3 months prior to admission
• history of intolerance to probiotics or their components
• refusal to participate and sign informed consent
• pregnancy or breastfeeding
• presence of cancer or psychoemotional disorders
• renal failure at the time of admission (glomerular filtration rate less than 50 ml / min)
• hepatic failure at the time of admission (equivalent to cirrhosis class B or C on the Child-Pugh scale)

The provision of data is prohibited by Local Ethical Committee and is possible upon receipt of an official request addressed to the principal investigator