In the PCA analysis, principal component 1 (PC1) mainly separated viral (CAP-flu and COVID-19) from bacterial pneumonia (CAP-strep) and was driven by IL-6, CD163, TREM-1 and procalcitonin (Figure 5c, ANOVA P<0·0001, Games-Howell P<0·0001 for COVID-19 versus CAP-strep, P<0·01 for CAP-flu versus CAP-strep, COVID-19 versus CAP-flu not significant). This evidence concerns the gene TREM1 and bacterial pneumonia.