Lymphopenia is associated with severe coronavirus disease 2019 (COVID-19) infections: A systemic review and meta-analysis

•COVID-19 is a new respiratory and systemic disease which needs quick identification of potential critical patients.•There is a significant reduction of lymphocyte count in the severe COVID-19 group compared to the non-severe group.•Those with lymphopenia have a 3-fold higher risk of developing severe COVID-19.•Lymphopenia is a prominent feature of COVID-19 and lymphocyte counts may be a useful, easily available biomarker in predicting the severity and clinical outcomes. Coronavirus Disease 2019 (COVID-19) is a new respiratory and systemic disease which needs quick identification of potential critical patients. This meta-analysis aimed to explore the relationship between lymphocyte count and the severity of COVID-19. A comprehensive systematic literature search was carried out to find studies published from December 2019 to 22 March 2020 from five databases. The language of literatures included English and Chinese. Mean difference (MD) of lymphocyte count in COVID-19 patients with or without severe disease and odds ratio (OR) of lymphopenia for severe form of COVID-19 was evaluated with this meta-analysis. Overall 13 case-series with a total of 2282 cases were included in the study. The pooled analysis showed that lymphocyte count was significantly lower in severe COVID-19 patients (MD -0.31×109/L; 95%CI: -0.42 to -0.19×109/L). The presence of lymphopenia was associated with nearly threefold increased risk of severe COVID-19 (Random effects model, OR=2.99, 95% CI: 1.31-6.82). Lymphopenia is a prominent part of severe COVID-19 and a lymphocyte count of less than 1.5×109/L may be useful in predicting the severity clinical outcomes.