Effect of past Chlamydophila pneumoniae infection on the clinical course of COVID-19: A retrospective cohort study

Abstract Background Although Chlamydophila pneumoniae (CP) is known to play a role in atherosclerosis and endothelial injury, its effect on the clinical course of coronavirus disease 2019 (COVID-19), which was also reported to be a vascular disease, remains unknown. Methods In this retrospective cohort study, we examined 76 COVID-19 patients and 44 bacterial pneumonia patients who visited a tertiary emergency center in Japan between April 1, 2021, and March 31, 2022. CP antibody levels, including IgM, IgG, and IgA, were measured. Results Among all patients, the CP IgA-positive rate was significantly associated with age (P = 0.002). Between the COVID-19 and non-COVID-19 groups, no difference in the positive rate for both CP IgG and IgA was observed (P = 1.00, and 0.51, respectively). However, in univariate analysis, age, proportion of males, and mortality were significantly higher in the IgA-positive group than in the IgA-negative group (60.35 vs. 76.33, P = 0.001; 59.6 vs. 95.2, P = 0.002; 11.5 vs. 33.3, P = 0.042, respectively). Age-adjusted and sex-adjusted logistic regression analyses revealed that IgA positivity was not significantly associated with higher mortality (odds ratio [OR], 0.36; 95% confidence interval [CI], 0.10–1.29; P = 0.12, and OR, 0.38; 95% CI, 0.11–1.30; P = 0.12, respectively). Conclusion Persistent infection with CP may be a contributing factor to the higher mortality of elderly male patients with COVID-19.