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

Background: 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.