Discussion
HCWs may be exposed to SARS-CoV-2 from contacts in the community, from
contacts with other HCWs, and from contacts with patients. In this
study, we developed a mathematical model to evaluate the relative
magnitudes of these risks, based on data collected during the second
wave of the SARS-CoV-2 pandemic in the UK (Nov 2020–Jan 2021).
Whilst using FRSMs, the majority of infections among HCWs working on red
wards could be attributed to direct exposure to patients with COVID-19.
In contrast, the majority of infections among HCWs working on green
wards arose from the community. After the change in RPE, cases
attributed to ward-based exposure fell significantly, with FFP3
respirators providing 31-100% protection (and most likely 100%)
against infection from patients with COVID-19. In keeping with previous
observations [9-11], our findings therefore suggest that the use of
FRSMs as RPE was insufficient to protect HCWs against infection from
patients with COVID-19. Conversely, excess infections amongst HCWs
caring for patients with COVID-19 may be prevented by the use of FFP3
respirators, in combination with other PPE and infection control
measures.
During the study period, the incidence of SARS-CoV-2 in England
increased [17], with spread of the more transmissible B.1.1.7
variant [18]. By the ninth week of the study, 79% of cases in
Cambridgeshire were caused by this variant [19]. Our observations on
the use of FFP3 respirators (weeks 9-11) were therefore made at a time
when the B.1.1.7 variant predominated, suggesting that they are robust
to any associated increase in SARS-CoV-2 transmissibility in a hospital
setting attributable to this variant.
Potential confounders of our observations, should they have changed over
the course of the study, include: (a) rates of natural immunity amongst
HCWs on red and green wards; however, the frequency of prior SARS-CoV-2
infections was low within CUHNFT [11]; (b) rates of vaccination of
HCWs on red and green wards; however, the proportion of high-risk HCWs
at CUHNFT offered vaccination prior to 08/01/21 was very low; (c)
frequency of screening amongst HCWs on red and green wards; however, the
proportion of cases ascertained by symptomatic testing and asymptomatic
screening was similar in both settings; (d) compliance with infection
control measures on red and green wards.
This observational study includes a small number of cases in a single
Trust, and there may be alternative explanations for the different
patterns of infection observed before and after the change in RPE.
Nonetheless, our data highlight a need for further study into the
appropriate level of RPE for HCWs caring for patients with COVID-19, as
well as other respiratory viruses. In accordance with the precautionary
principle, we propose a revision of RPE recommendations until more
definitive information is available.