Nosocomial COVID-19 Incidence and Secondary Attack Rates among Patients of Tertiary Care Center, Zurich, Switzerland

Abstract

Of 1,118 patients with COVID-19 at a university hospital in Switzerland during October 2020-June 2021, we found 83 (7.4%) had probable or definite healthcare-associated COVID-19. After in-hospital exposure, we estimated secondary attack rate at 23.3%. Transmission was associated with longer contact times and with lower cycle threshold values among index patients.

Keywords: COVID-19; SARS-COV-2; Switzerland; coronavirus disease; cross-infection; infection control; infectious disease transmission; nosocomial infections; risk factors; severe acute respiratory syndrome coronavirus 2; viruses; zoonoses.

Figure 1

Incidence of admitted patients with positive SARS-CoV-2-PCR per week, including categorization hospital-associated versus community-associated, temporal trend of incidence of SARS-CoV-2 positive patients from week 40 of 2020 through week 25 of 2021. Incident cases were stratified according to European Centre for Disease Prevention and Control definitions of healthcare-associated COVID-19.

Figure 2

Transmission clusters of patients after exclusion of 5 exposed patients in whom phylogenetic data did not support transmission. Circles are index patients, squares are infected contact patients. Green arrows represent phylogenetically confirmed transmissions, with the labels “0 SNP” and “1 SNP” indicating 0 or 1 SNP difference between index and exposed patient. Green dashed arrows represent phylogenetic proof of second-generation transmission. Black arrows represent assumed transmissions without phylogenetic proof. i, index patient; a–d, exposed