2.3.2. Quality assessment
Four reviewers (YJ.G, YX.Y, ML, HB. Y) independently extracted the following features of the literatures listed in study characteristics: first author, published time, study date, the number and age of patients and the number of severe case or death, fever, cough, lymphopenia, positive CT findings, coexisting disorders, preterm labor, cesarean section, fetal distress, neonatal asphyxia or neonatal death or stillbirth, neonatal infection, virus in breast milk. They also evaluated the quality of the literature using the Institute of Health Economics (IHE) case series methodological quality evaluation tool(8), which is evaluated in 8 areas: (1) Research purpose, (2) Research population, (3) Intervention and joint intervention, (4) Outcome measures, (5) Statistical analysis, (6) Results and conclusions, (7) Conflict of interest and funding sources, (8) New entry. There is a total of 20 items above, satisfying 14 (70%) or more is considered acceptable quality.