Weeks following the change in RPE are highlighted in grey. Community
incidence (total cases per week) is shown for the East of England, UK,
with raw data shown in Figure 1–source data 1 .
To further quantify the risk of infection for HCWs working on red and
green wards, we generated a simple mathematical model. According to this
model, the total risk of infection is divided into a risk from
community-based exposure, and a risk from direct, ward-based exposure to
patients. The risk from direct exposure on red wards was allowed to vary
upon the introduction of FFP3 respirators, and was fitted to a maximum
likelihood model. Inferred parameters and their confidence intervals are
shown in Table 2 . Our model produced a qualitatively close fit
to the observed numbers of cases (Figures 3A-B ).
The inferred risk of direct infection from working on a green ward was
low throughout the study period, and consistently lower than the risk of
community-based exposure, which increased in proportion to rising levels
of community incidence (Figure 3C ). By contrast, the risk of
direct infection from working on a red ward before the change in RPE was
considerably higher than the risk of community-based exposure, and
approximately 47-fold greater than the corresponding risk from working
on a green ward (confidence interval [7.92, ∞]). Thus, whilst almost
all cases on green wards were likely caused by infection in the
community, cases on red wards at the beginning of the study period were
attributed mainly to direct, ward-based exposure (Figures
3D-E ). Critically, our model further suggests that the introduction of
FFP3 respirators provided 100% (confidence interval [31.3%,
100%]) protection against direct, ward-based COVID-19 infection
(Table 2 , r2/r1).