Risk factors of nosocomial Covid-19 in a French university hospital: a case- control study

Abstract Background: During the Covid-19 pandemic, prevention strategies implemented by hospitals to reduce nosocomial transmission sometimes failed and determining transmission risk factors remains crucial. Our objective was to determine the risk factors of nosocomial Covid-19.Methods: A case-control study was conducted in a French hospital between 09/01/2020 and 01/31/2021. Adult patients hospitalized in medical or surgical units were included. Infants or patients hospitalized in ICU were excluded. Cases were patients with a nosocomial Covid-19 (clinical symptoms and RT-PCR positive for SARS-CoV-2 or RT-PCR positive for SARS-CoV-2 with Ct ≤28 more than five days after admission); controls were patients without infection (RT-PCR negative for SARS-CoV-2 more than 5 days after admission). They were matched according to length of stay before diagnosis and period of admission. Analyses were performed with a conditional logistic regression.Results: A total of 281 cases and 441 controls were included. In the bivariate analysis, cases were older (OR per 10 years: 1.22; CI95% [1.10; 1.36]), had more often shared a room (OR: 1.74; CI95% [1.25; 2.43]), had more often a risk factor of severe Covid-19 (OR: 1.94; CI95% [1.09; 3.45]), were more often hospitalized in medical units [OR: 1.59; CI95% [1.12; 2.25]), had a higher exposure to contagious health care workers (HCW; OR per 1 person.days: 1.12; CI95% [1.08; 1.17]) and contagious patients (OR per 1 person.days: 1.11; CI95% [1.08; 1.14]) than controls. In an adjusted model, risk factors of nosocomial Covid-19 were exposure to contagious HCW (aOR per 1 person.days: 1.08; CI95% [1.03; 1.14]) and exposure to contagious patients (aOR per 1 person.days: 1.10; CI95% [1.07; 1.13]).Conclusions: Exposure to contagious professionals and contagious patients are the main risk factors for nosocomial Covid-19, outweighing all other potential risk factors including the hospitalisation in double room. Prevention strategies need to be adjusted according to these results to decrease the risk of nosocomial COVID-19.Trial registrationStudy ethics approval was obtained retrospectively on 22 December 2021 (CECIC Rhône-Alpes-Auvergne, Clermont-Ferrand, IRB 5891).