The respiratory microbiome in patients with post-COVID-19 residual lung abnormalities resembles that of healthy individuals and is distinct from idiopathic pulmonary fibrosis.

Introduction: Up to 11% of patients are left with residual lung abnormalities following COVID-19 infection. It is unclear whether these changes resolve over time or progress to fibrosis. The airway microbiome is altered in interstitial lung disease, potentially contributing to pathogenesis and disease progression. We hypothesised that the airway microbiome in patients with post-COVID-19 residual lung abnormalities may be altered.

Methods: The POST COVID-19 interstitial lung DiseasE (POSTCODE) study recruited subjects with post-COVID-19 residual lung abnormalities for bronchoscopy. 16S ribosomal RNA gene amplicon sequencing was performed on DNA extracted from bronchoalveolar lavage fluid and compared with that from patients with idiopathic pulmonary fibrosis, fibrotic hypersensitivity pneumonitis and control subjects.

Results: 28 subjects with post-COVID-19 residual lung abnormalities were recruited an average of 11 months after infection. No significant associations were found between the lower airway microbiome or bacterial burden and disease severity or trajectory. There was no difference in bacterial burden between post-COVID-19 patients and interstitial lung disease or control subjects. Furthermore, no differences in microbial composition were observed between these patients and those with fibrotic hypersensitivity pneumonitis or controls. However, compared with idiopathic pulmonary fibrosis, there was an increased abundance of and higher α-diversity in subjects with post-COVID-19 residual lung abnormalities.

Conclusions: The microbiome and bacterial burden in the lower airways of subjects with post-COVID-19 residual lung abnormalities do not differ from those of controls. The microbiome differs from idiopathic pulmonary fibrosis. This, and the absence of associations between microbial features and disease severity or clinical outcomes, suggests that the microbiome is unlikely to contribute to residual lung abnormalities in patients recovering from COVID-19 infection.