Study aim: Investigation of the effect of Lactocare® synbiotic on the prevention of COVID-19 infection in the staff of emergency department Design: Clinical trial with control group and parallel groups, randomized and double blind Settings and conduct: This clinical trial is conducted on 80 members of the personnel of the emergency department of Imam Reza hospital including physicians and nurses who work where patients infected by COVID-19 are hospitalized. These participants are randomly assigned to two groups of control and treatment. At the beginning of the study, patients are examined by a specialist and take antibody test and their data including their personal and professional information are recorded. Patients and outcome assessors are unaware of the groupings. Participants/Inclusion and exclusion criteria: Inclusion criteria: being a member of the personnel of the emergency department of Imam Reza hospital; active presence in the wards where patients with COVID-19 are hospitalized. Exclusion criteria: history of clinical or laboratory diagnosis with COVID-19 Intervention groups: Intervention group: In this group, patients will receive a Lactocare® synbiotic capsule daily for a month. Control group: Patients in this group will receive a capsule as placebo (with the same shape, smell, design and color) containing no probiotic daily for one month. Main outcome variables: Fever, findings of CT scan and CXR, number of lymphocytes, WBC and infection by COVID-19, cough and sore throat, nausea, vomiting and diarrhea