Fig. 4

Analyses of the potential predictors of spatial heterogeneity in hospitalisation incidence (HI). A Principal component analysis (PCA) based on all spatial covariates, each dot corresponding to a distinct hospital catchment area (HCA; see also Additional file 2: Figure S2 for an alternative PCA that also includes HI variables). B Map of HCAs coloured by HI value computed for the entire epidemic period under consideration. C Correlogram reporting Spearman correlations among all spatial covariates and HI values for the three considered periods; only significant correlation values (p-values < 0.05) are reported. D Selected result from the boosted regression trees (BRT) analysis performed with all spatial covariates and HI values computed for the entire epidemic period as response variable: partial responses for HI values for the ratio of nursing home (NH) beds divided by the population in each HCA; i.e., the spatial covariate associated with the highest relative influence in the BRT model (~ 57%; Table 1). (*) indicates a potential HCA outlier discarded for the statistical analyses reported in Additional file 4: Table S2