Cases per ward day amongst HCWs on green wards (A ) were strongly correlated with the number of community cases identified the previous week (p-value < 5x10^-5, Pearson correlation test), suggesting that infection in the community explains cases amongst HCWs on these wards. Conversely, cases per ward day amongst HCWs on red wards (B) did not correlate with the community incidence (p-value > 0.7, Pearson correlation test). R² values shown in the figures are coefficients of determination arising from linear regression calculations performed using the Mathematica software package (version 12.1.0.0).