Abstract:Objective To explore characteristics of co-infection of Chlamydia pneumoniae (Cpn) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and identify their effect on SARS-CoV-2-induced inflammatory response. Methods Patients with coronavirus disease 2019 (COVID -19) who received treatment in a hospital in Chenzhou City from December 20, 2022 to February 20, 2023 were selected. According to the severity of COVID -19, severe and critical cases were classified as the severe symptom group, while mild and moderate cases were classified as the mild symptom group. Meanwhile, according to the age of patients (≥18 years old as adults, <18 years old as juveniles), they were divided into the adult severe symptom group, adult mild symptom group, juvenile severe symptom group, and juvenile mild symptom group. Propensity score was adopted to match age, gender, and underlying diseases of patients in severe symptom and mild symptom group in a 1 ∶1 ratio. Bronchoalveolar lavage fluid (BALF), throat swabs, and serum specimens of patients were collected. Cpn IgG/IgM antibody was detected by enzyme-linked immunosorbent assay (ELISA), levels of 12 common cytokines (including interleukin-8 ［IL-8］) in BALF were detected by flow cytometry, differences among groups were compared. Results A total of 102 patients were included, with 61 severe and critical (severe symptom) patients, as well as 41 mild and moderate (mild symptom) patients. There were 71 patients aged ≥18 years and 31 juvenile patients aged <18 years. There were 39 patients in the adult severe symptom group and 32 in the adult mild symptom group, and 30 pairs were successfully matched through propensity score analysis. There were 22 patients in the juvenile severe symptom group and 9 in the juvenile mild symptom group, and 8 pairs were successfully matched through propensity score analysis. Among COVID -19 patients, the positive rates of Cpn IgG and IgM were 36.27% (n=37) and 8.82% (n=9), respectively, with 1 case positive for both Cpn IgG and IgM. The level of interferon (IFN) - α in serum specimens from adult patients with severe symptom combined with positive Cpn IgG was higher than that of IgG negative patients (P=0.037). There was no statistically significant difference in the levels of other cytokines in BALF and serum specimens between the two groups of patients (all P>0.05). The levels of IL-8 and IL-17 in serum specimens of patients with positive Cpn IgG in the adult mild symptom group were both higher than those in Cpn IgG negative patients (both P<0.05). The levels of IL-8 in both BALF and serum specimens from Cpn IgM positivity patients in the juvenile mild symptom group were higher than those from patients with negative Cpn IgM (both P<0.05). Logistic regression analysis results showed that Cpn IgG and IgM positivity were not risk factors for the development of severe COVID -19. Conclusion Combined Cpn infection is not a risk factor for the development of severe symptom in COVID -19 patients, and Cpn infection has limited impact on the secretion of inflammatory factors caused by SARS-CoV-2.