Effectiveness of probiotics on COVID-19 prevention and treatment against mild COVID-19 in outpatient care: A systematic review

Background In previous research, probiotics have shown to be beneficial in preventing and limiting the progress of upper respiratory infections. Their effectiveness in relation to coronavirus disease 2019 (COVID-19) has been investigated mainly in hospitalized patients, and less so among outpatients who constitute majority of COVID-19 cases. Aim This systematic review evaluates the available evidence regarding the effectiveness of probiotic use on prevention and treatment of COVID-19 among patients with mild symptoms in outpatient settings. Methods PubMed, Embase and Cochrane Library were searched for studies from their inception to May 2024, restricting to randomized controlled trials and before-and-after studies. The primary outcomes were infection incidence and complete remission rate. Cochrane risk-of-bias tool (RoB 2.0) and risk of bias in non-randomized studies of interventions tool (ROBINS-I) were used to assess the risk of bias. The Grading of Recommendations, Assessment, Development, and Evaluations approach was performed to assess the certainty of the evidence. Results Eight randomized controlled trials and one pre-post study on 1235 participants were included. Four studies had low risk of bias. Probiotics were effective in reducing the incidence of COVID-19 upon exposure and accelerating the symptomatic remission of mild COVID-19 with less systemic symptoms. Overall, the certainty of evidence on both primary outcomes was moderate. Comorbidities and old ages were found to be significant confounders. Probiotics demonstrated significant immunomodulatory and humoral effects in the nasopharyngeal cavity. Conclusion These results suggest that probiotics are effective at preventing COVID-19 and support faster recovery from mild COVID-19 among individuals seeking for outpatient care. People with comorbidities, that is, metabolic disorder and elderly benefit the most from probiotics supplements.