Decreased complement C3 levels are associated with poor prognosis in patients with COVID-19: A retrospective cohort study

OBJECTIVES: To describe the humoral immune feature of patients with coronavirus disease 2019 (COVID-19). METHODS: The levels of total immunoglobulins (IgG, IgM, IgA, and IgE), complement (C3, C4) results were retrospectively analyzed in COVID-19 patients. Univariable and multivariable logistic regression were performed to explore the risk factors associated with the in-hospital death. RESULT: A total of 236 patients were enrolled in this study, of which 169 were transferred to another institution or discharged (survival group) and 67 died in hospital (non-survival group). Compared with survivors, the levels of IgA and IgE in non-survivors increased significantly, and level of complement C3 decreased. Non-survivors also showed higher incidence of chest tightness, breath shortness and dyspnoea; higher levels of inflammatory indicators, leukocytes and neutrophils; and low levels of lymphocyte subsets. Multivariable regression showed increasing odds of in-hospital death associated with older age (HR: 1.099; 95%CI: 1.057–1.143; p < 0.0001), d-dimer greater (HR: 1.294; 95%CI: 1.138–1.473; p < 0.0001) and decreased complement C3 level (HR: 0.073; 95%CI: 0.007–0.722; p = 0.025) on admission. Finally, in survival COVID-19 patients whose humoral immunity was re-examined, C3 levels tended to increase, while in non-survivors it decreased. CONCLUSION: Low level of complement C3 may be an alert to the admitted COVID-19 patients with additional management. Inhibition of the complement pathway might be an effective therapeutic to COVID-19 patients.