key: cord-1056630-5uh9v8kj authors: Chow, Angela; Htun, Htet Lin; Kyaw, Win Mar; Lee, Lay Tin; Ang, Brenda title: Asymptomatic health-care worker screening during the COVID-19 pandemic date: 2020-10-29 journal: Lancet DOI: 10.1016/s0140-6736(20)32208-x sha: 6fe2496221499a3caa613afdcf94660390af7b71 doc_id: 1056630 cord_uid: 5uh9v8kj nan Moreover, the figure compares symptomatic inpatients who were tested in hospital with asymptomatic HCWs. It is possible that HCWs were exposed to asymptomatic patients who were infectious in the hospital or to patients with false-negative tests. The number of infections among HCWs was also most likely underestimated, as HCWs with symptoms or who were selfisolating were excluded. The role of hospital exposures versus community exposures has been problematic for assessments of the occupational risk of other infectious diseases (eg, multidrug-resistant tuberculosis). Valid risks for disease were not appreciated until HCWs were compared with controls with similar educational and economic status, such as medical students versus chemicalengineering students. 2 Finally, evidence exists that the risk to HCWs from severe acute respiratory syndrome coronavirus 2 is not only from community exposures but also from other types of exposures. Among 1423 HCWs in the USA with COVID-19, 780 (55%) HCWs reported contact with a patient with confirmed COVID-19 in the 14 days before the onset of their symptoms, whereas 384 (27%) reported contact only with a household member, 187 (13%) in a community setting, and 72 (5%) in more than one of these settings. 3 Some HCW exposures confer a higher risk than do others, and personal protective equipment and infection control training are associated with a decreased risk of infection. 4 We declare no competing interests. We applaud the establishment of the COVIDsortium by Thomas Treibel and colleagues 1 as a bioresource focusing on asymptomatic health-care workers (HCWs). 1 However, we disagree with the authors' conclusion that "the rate of asymptomatic infection among HCWs more likely reflects general community transmission than in-hospital exposure". This report was an ecological study subject to the ecological fallacy. 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 1 did not contradict either of these viewpoints. Front-line HCWs have a reported hazard ratio of more than 3 compared with the general community. 2 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 time points 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 selfisolate. 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. 1 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 selfquarantining by approximately 4 weeks after the peak, suggesting that nosocomial transmission had ceased. A key contributor to the absence of ongoing nosocomial transmis sion 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 casedefinition 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. patient contacts of HCWs who were confirmed to have COVID-19 also screened negative. No in-hospital clusters resulted from the HCWs with COVID-19. During this period, 35 acute respiratory illness clusters in HCWs were identified, but SARS-CoV-2 was not detected. One-time screening of 1378 asymptomatic housekeeping, facilities, information-technology, and security staff identified one (0ยท07%) person with COVID-19 infection. Since May 6, 2020, all HCWs with acute respiratory illness have been tested for SARS-CoV-2, further reducing the risk of nosocomial transmission. In hospitals that have staff who are well trained and supplied with personal protective equipment, have comprehensive sickness-surveillance systems, and have a universal mask policy, testing of asymptomatic HCWs would not be indicated. COVID-19: the case for health-care worker screening to prevent hospital transmission First experience of COVID-19 screening of health-care workers in England COVID-19: PCR screening of asymptomatic health-care workers at London hospital Responding to the COVID-19 outbreak in Singapore: staff protection and staff temperature and sickness surveillance systems Evaluation of the effectiveness of surveillance and containment measures for the first 100 patients with COVID-19 in Singapore We declare no competing interests.