Asymptomatic health-care worker screening during the COVID-19 pandemic - Authors' reply.

Kevin Fennelly and Christopher Whalen emphasise that health-care workers (HCWs) are at a higher risk of severe acute respiratory syndrome coronavirus 2 infection than are the general population. Angela Chow and colleagues describe their experience in Singapore of very low rates of HCW infections and nosocomial transmission when effective personal protective equipment is implemented. We agree with both perspectives, and our Correspondence did not contradict either of these viewpoints.

Front-line HCWs have a reported hazard ratio of more than 3 compared with the general community. This risk is variable between studies, with reported seropositivity rates in the UK ranging from 6% to 43% across different hospital settings.3, 4 Explanations for this variation include confounding by sampling timepoints during an emerging epidemic wave, participant selection (random vs symptomatic), and rates of self-isolation, and differences in the nature of exposures, policies for infection control, and use of personal protective equipment.

Our study was done when symptomatic HCWs were already required to quarantine. We sought to address the need for repeated mass screening of staff without disease-defining symptoms to help to reduce transmission associated with health care. Therefore, we focused on asymptomatic or pauci-symptomatic infection in HCWs at sequential timepoints during the first epidemic wave in London, UK, sampling only HCWs who attended work because they did not meet the symptomatic criteria to self-isolate. PCR-positive results peaked one week before the PCR-positive peak in London (which was at that time reflected mainly by symptomatic patients presenting to hospitals). We inferred from this that the peak of asymptomatic infection in our HCW cohort coincided with the peak of virus circulation in the community. Thereafter, the rates of prevalent asymptomatic infection in our cohort reduced in line with the decline in community cases, despite a persistent number of patients with COVID-19 within the hospital. Further serial swabbing of HCWs over 16 weeks to mid-August, 2020, showed no new cases (appendix p 1) and neither did extension to two further hospitals and a total of 731 participants who were studied longitudinally (data not shown). The number of HCWs who were self-isolating fell to nearly zero over this time period. Despite some persistent hospitalised cases, zero cases were identified by PCR and nearly zero HCWs were self-quarantining by approximately 4 weeks after the peak, suggesting that nosocomial transmission had ceased. A key contributor to the absence of ongoing nosocomial transmission was likely to be the effective implementation of infection control practices.

Our approach to focus on asymptomatic infections underestimates the absolute incident rate of infections among HCWs, but it identifies the scale of infection missed by case-definition criteria and is likely to be a fair surrogate for the trend of incident infections. These data suggest that tracking community prevalence to trigger asymptomatic screening of HCWs is more informative than monitoring hospital caseloads.