2.3. Study selection criteria, data extraction and quality assessment
Title, abstract and then full-text of all articles were screened to select & extract studies that investigated the effect of probiotics on viral infections (with emphasis on SARS-CoV-2) in English language (all research types with both human and animal origin). We excluded HIV papers, due to not being directly related to respiratory viral infections. Data extraction from primary articles was performed by one author (AR.M) by using a standardized form. A second author checked the accuracy of the data extracted (M.P). Data collected from the studies included authors’ family name, type of study, probiotic strains and dose, sample size, and overall outcomes. The Cochrane Collaboration criteria for assessing risk of bias were applied to assess the quality of the studies included in the review 17.