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.