We present the results of a large and comprehensive Covid-19 staff testing programme across four teaching hospital sites in one UK county, attended by 71% of 13,800 staff. Using a composite outcome of either a positive PCR or serology result, by June 8^th we detected evidence of Covid-19 at some time in 11% of staff. Put in context, UK-wide seroprevalence was 6.8% on 28^th May 2020, with a higher incidence among healthcare workers than in the general population.(14)

We observed varying risk to our hospital staff associated with working location, occupational role and demographic factors. The greatest risk was associated with Covid-19 infected household contacts (although only 36.7% of staff with a contact became infected) and with working in Covid-19-facing areas (21.2% vs. 8.2% elsewhere) where there was 1 additional SARS-CoV-2 infection per ∼8 staff compared to elsewhere. On univariable analysis staff with most direct patient contact were at increased risk including porters, cleaners, nurses, healthcare-assistants, therapists and junior doctors. Adjusting for working in a Covid-19 area captured much of this risk, except for porters and cleaners who had the highest adjusted risk of any staff group, and who typically operate across the hospital.

A heterogenous pattern also emerged across different Covid-19-facing areas. Risk seen on acute medical wards was greater than in the emergency department which was often bypassed by Covid-19 patients, whilst working on a Covid-19 facing ICU was relatively protective. One key difference across these areas was the type of PPE worn and the time periods over which it was mandated. Level-2 PPE was mandatory on ICU and HDU throughout whereas policies changed over time on other wards (Table S1). Moreover, staff on ICU and HDU received extensive training in donning and doffing and had dedicated space and supervision for this whereas ward staff did not. Prior to 1^st April 2020, in line with national guidance, in acute medical areas outside of Covid-19 cohort wards level-1 PPE was only worn for contact with patients with known or suspected Covid-19, potentially leading to unprotected exposure to patients in whom Covid-19 was not suspected, such as afebrile elderly patients with delirium, functional decline or diarrhoea. This likely explains the greater number of staff infected in several acute medical wards (shown in green near the top of Figure 4B), compared to Covid-19 cohort wards (shown in red).

The reported rates of exposure without PPE were similar among medical and ICU staff (42% and 38%, Table S5), likely reflecting exposures to ICU staff visiting wards to assess critically ill patients. Universal admission testing was only introduced on 24th April 2020, and the limited availability and speed of testing in the early phase of the pandemic likely delayed identification of some Covid-19 cases.

It is difficult to say whether level-1 PPE was less protective than level-2. Increased Covid-19 in staff reporting exposure to a Covid-19 patient without PPE suggests surgical masks afford some protection, and protection from influenza has been reported to be similar using surgical versus FFP2 masks.(15) However, it is likely that a bundle of measures (level-2 PPE, training, supervision and space for donning and doffing, increased staffing levels) influenced the lower risk in ICU and HDU staff (Figures 3, 4). As with many infection control intervention-bundles it is difficult to distinguish which component was most important.

It is also likely that staff-to-staff transmission amplified incidence, based on high Covid-19 rates in several wards without large numbers of Covid-19 patients. Future viral genome sequencing studies may allow analysis of the relative contribution of patients and staff to transmission.

Increased risk of adverse outcomes has been widely reported in Black and Asian ethnic groups,(16) with evidence they are also at increased risk of infection.(3,17) Here we show Black and Asian staff were at greater risk of infection after controlling for age, gender, working location, role, and exposure at home. Job role can be thought of as a proxy for socio-economic background but we were not able to control directly for income levels, home circumstances, pre-morbid conditions or other potential structural inequalities. That staff working as porters or cleaners had the greatest adjusted risk of infection is consistent with economics playing a part in risk, potentially reflecting conditions outside of the hospital, e.g. dense occupancy of living space due to lower incomes.

To calculate a personalised Covid-19 risk score, all factors in the multivariable model need to be considered, e.g. an Asian Covid-19-facing medical nurse is 7.74 (95%CI 5.65-10.61) more likely to be infected than a white non-Covid-19-facing administrative worker. Notably, this exceeds the risk of living with someone with known Covid-19 (aOR 4.63, 95%CI 3.30-6.50).

Limitations of our study include its cross-sectional nature and that data gathered on particular exposures may be subject to recall bias. It is unknown what proportion of staff were infected who either mounted no detectable antibody response or in whom it had waned by the time of testing. Our data are also from a single setting and findings may vary by practice, geography and population-wide Covid-19 incidence.(5,6)

Our study suggests that an earlier move to universal level-1 PPE may have prevented some infections, and that a consistent bundle of level-2 PPE provision and use, training, and supervision and space for donning and doffing protected staff working in high-risk areas. Wider deployment of this bundle should be considered where staff are at increased risk. Our study provides data to inform risk assessments for staff, to ensure those staff most at risk are deployed appropriately. Given likely staff-to-staff transmission where COVID-19 patient pressure was low, there is a need to protect all staff regardless of role. This includes reinforcement of measures to support social distancing and raises questions about the role of social inequality in Covid-19 transmission. If some staff are already immune the impact of any future Covid-19 surge may be less marked for staff, although differential deployment or use of PPE based on immune status would require evidence it was safe and socially acceptable. Our testing programme has been highly popular with staff, ensured enhanced detection of those with Covid-19, and now also provides a large cohort to inform studies on the extent of antibody-mediated protection against future infection.