ABSTRACT
Introduction: LTCFs are at risk of COVID-19 outbreaks but evidence regarding SARS-CoV-2 acquisition and transmission routes among their employees remains weak. Objectives: We investigated the relative contribution of occupational (vs. community) exposure for COVID-19 acquisition among employees of a university affiliated LTCF in Switzerland, from March to June 2020. Methods: This is a prospective cohort study with a nested analysis of a COVID-19 seroprevalence study among LTCF staff. We performed Poisson regression to determine risk factors for seropositivity and to measure the influence of community vs. nosocomial exposure to COVID-19 on SARS-CoV-2 seropositivity using adjusted prevalence ratios (aPR). In addition, we conducted a COVID-19 outbreak investigation in a LTCF ward using both epidemiological and genetic sequencing data. We constructed a maximum likelihood phylogenetic tree and evaluated strain relatedness to discriminate between community- vs. hospital-acquired infections among employees. Results: Among 285 LTCF employees, we included 176 participants in the seroprevalence study, of whom 30 (17%) became seropositive for SARS-CoV-2. The majority (141/176, 80%) were healthcare workers and had ≥ 1 symptom compatible with COVID-19 (127/167, 76%). Risk factors for seropositivity included exposure to a COVID- 19 patient in the LTCF (aPR 2.6;95%CI 0.9-8.1) and exposure to a SARS-CoV-2 positive person in the community (aPR 1.7;95%CI 0.8- 3.5). Among 18 employees included in the outbreak investigation, phylogenetic analysis suggests that 8 (44%) acquired their infection in the community. Conclusion: During the first pandemic wave, there was a high burden of COVID-19 among LTCF employees. Both occupational and community exposures contributed to seropositivity and infection risk. These data may allow to better assess occupational health hazards and related legal implications during the COVID-19 pandemic. (Figure Presented).
ABSTRACT
BACKGROUND: Nosocomial outbreaks of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) are frequent despite implementation of conventional infection control measures. An outbreak investigation was undertaken using advanced genomic and statistical techniques to reconstruct likely transmission chains and assess the role of healthcare workers (HCWs) in SARS-CoV-2 transmission. METHODS: A nosocomial SARS-CoV-2 outbreak in a university-affiliated rehabilitation clinic was investigated, involving patients and HCWs, with high coverage of pathogen whole-genome sequences (WGS). The time-varying reproduction number from epidemiological data (Rt) was estimated, and maximum likelihood phylogeny was used to assess genetic diversity of the pathogen. Genomic and epidemiological data were combined into a Bayesian framework to model the directionality of transmission, and a case-control study was performed to investigate risk factors for nosocomial SARS-CoV-2 acquisition in patients. FINDINGS: The outbreak lasted from 14th March to 12th April 2020, and involved 37 patients (31 with WGS) and 39 employees (31 with WGS), 37 of whom were HCWs. Peak Rt was estimated to be between 2.2 and 3.6. The phylogenetic tree showed very limited genetic diversity, with 60 of 62 (96.7%) isolates forming one large cluster of identical genomes. Despite the resulting uncertainty in reconstructed transmission events, the analyses suggest that HCWs (one of whom was the index case) played an essential role in cross-transmission, with a significantly greater fraction of infections (P<2.2e-16) attributable to HCWs (70.7%) than expected given the number of HCW cases (46.7%). The excess of transmission from HCWs was higher when considering infection of patients [79.0%; 95% confidence interval (CI) 78.5-79.5%] and frail patients (Clinical Frailty Scale score >5; 82.3%; 95% CI 81.8-83.4%). Furthermore, frail patients were found to be at greater risk for nosocomial COVID-19 than other patients (adjusted odds ratio 6.94, 95% CI 2.13-22.57). INTERPRETATION: This outbreak report highlights the essential role of HCWs in SARS-CoV-2 transmission dynamics in healthcare settings. Limited genetic diversity in pathogen genomes hampered the reconstruction of individual transmission events, resulting in substantial uncertainty in who infected whom. However, this study shows that despite such uncertainty, significant transmission patterns can be observed.