Immune Signatures Distinguish Pure and Mixed Sepsis in Critical COVID-19: A Retrospective Cohort Study

Li Xiong,* Huan Tang,* Qian Xie, Hong Fang, Da Jing, Lin Chen Department of Pulmonary and Critical Care Medicine. Sichuan Provincial People’s Hospital, School of Medicine, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China*These authors contributed equally to this workCorrespondence: Lin Chen, Department of Pulmonary and Critical Care Medicine, Sichuan Provincial People’s Hospital, School of Medicine, University of Electronic Science and Technology of China, No. 32, West 2nd Section, 1st Ring Road, Qingyang District, Chengdu, Sichuan Province, 610072, People’s Republic of China, Tel +8618111585286, Email [email protected]: COVID-19-associated sepsis poses unique challenges in intensive care units (ICUs), where patients are often immunocompromised and prone to secondary bacterial infections. Differentiating immune phenotypes between pure viral and viral-bacterial sepsis is essential for timely diagnosis and personalized treatment.Objective: To compare clinical characteristics, immune profiles, and outcomes between patients with pure COVID-19 sepsis and those complicated by confirmed secondary bacterial infection, and to identify immune markers capable of differentiating sepsis phenotypes.Methods: This retrospective cohort study enrolled ICU patients with severe COVID-19 pneumonia between July 2021 and December 2023. Patients were classified into viral sepsis (V group, n=53) and viral-bacterial sepsis (VB group, n=28) based on microbiological confirmation of secondary infection. Clinical data, inflammatory markers, cytokine levels, and neutrophil CD64 expression (via flow cytometry) were analyzed. Logistic regression, ROC curves, and Kaplan–Meier survival analysis were used to evaluate phenotypes, identify discriminative markers, and assess outcomes.Results: The VB group exhibited significantly higher SOFA scores (median 9.5 vs 7, P< 0.001), serum creatinine (103.2 vs 80.85 μmol/L, P=0.001), and LDH levels (531 vs 392 U/L, P=0.009), indicating more severe organ dysfunction. ICU stay was longer in the VB group (median 16 vs 13 days, P=0.027), and ICU mortality was slightly higher (85.7% vs 83.0%, P=0.785). Immune profiling showed significantly higher IL-2 (36.8 vs 22.1 pg/mL, P< 0.001), IL-10, IFN-γ, and MYD88 in the V group, whereas TNF-α (62.3 vs 43.7 pg/mL, P=0.002) and PCT (2.51 vs 1.12 ng/mL, P=0.001) were higher in the VB group. IL-2, MYD88, and PCT independently discriminated phenotypes. Neutrophil CD64 showed strong predictive value for superinfection (AUC=0.969).Conclusion: COVID-19 patients with secondary bacterial sepsis exhibit distinct immune and clinical profiles. Immune phenotyping may enable early recognition of bacterial superinfection and guide phenotype-driven therapy in severe viral sepsis.Trial Registration: ClinicalTrials.gov, NCT06491966. Registered 2 April 2024, https://clinicaltrials.gov/ct2/show/NCT06491966.Keywords: COVID-19 sepsis, secondary bacterial infection, cytokines, immune profiling, ICU, organ dysfunction