Nosocomial COVID-19 in a tertiary care center - incidence and secondary attack rates after exposure

Introduction: Patients not suspected to be infected with SARS-CoV2 but suffering from COVID-19 or patients hospitalized during the incubation period of COVID-19 pose a risk for transmitting SARS-CoV2 to fellow patients. Objectives: We aimed to describe the incidence of patients hospitalized in our tertiary care center with community- and healthcareassociated COVID-19, calculate the number of patients who exposed other patients to SARS-CoV2 and the secondary attack rate of exposed patients, and investigate risk factors for SARS-CoV2 transmission. Methods: In this retrospective study, all patients admitted to the University Hospital Zurich, Switzerland, with a positive SARS-CoV2 PCR were included. Community- or healthcare-associated COVID-19 were defined according to the European Centre for Disease Prevention and Control (ECDC) criteria. Patients receiving standard care during their period of contagiousness were defined as potential index patients. Exposed patients were patients sharing a room with a potential index patient 1) on the general ward for any time, 2) on the intermediate or intensive care unit (IMC/ICU) for ≥ 6 h, or 3) on the IMC/ICU for any time with an index undergoing aerosol-generating procedures. Results: Of 1131patients with a positive SARS-CoV2 PCR or labeled to be a patient with COVID-19 between October 2020 and April 2021, 90.6% (n = 1025) had community-associated illness, 3.5% (n = 40) had probable and 3.8% (n = 43) had definite healthcare-associated COVID- 19, and 2.0% (n = 23) had indeterminate illness. Of all patients, 19.0% (n = 215) were potential index patients, and 56.3% (n = 121) of these exposed at least one other patient. Of 292 exposed patients, 47 were later diagnosed with COVID-19. The mean secondary attack rate per index patient was 14.4%. Transmission of SARS-CoV2 was higher if contact time was longer (adjusted Odds Ratio (aOR): 1.03, 95%CI: 1.02- 1.04) and if CT-value of index was lower (aOR: 0.92, 95%CI: 0.87-0.97). Conclusion: The secondary attack rate after exposure to SARS-CoV2 of patients in the hospital setting is comparable to rates reported from the household setting. The risk of an incident SARS-CoV2 infection increases with longer duration of exposure and higher viral load of index patient.