Nosocomial transmission of SARS-COV-2: experience from the contact tracing activity of the infection control unit for the university hospital of lausanne

Introduction: The high transmissibility of SARS-CoV-2 is of particular concern for hospitals as hospitalized patients are at risk of severe COVID-19 and related death. Objectives: Describe the contact tracing activity related to nosocomial SARS-CoV-2 cases in our hospital. Methods: The Infection Control Unit received alerts for positive PCR results performed by the Microbiology laboratory in hospitalized patients. Nosocomial infections, occurring after at least 5 days of hospitalization, were further investigated. Contact tracing was performed via an institutional software allowing tracing index patients' movements and their roommates. Patients were considered “contacts” at risk and were placed in quarantine if they shared the same room with the index-case up to 72 h before index's first positive PCR or first symptoms. Contacts systematically had nasopharyngeal SARS-CoV-2 PCR testing at days 0, 10 counting from last contact with the index or at symptom onset. Results: Between November 1st 2020 until March 31st 2021, 322 nosocomial SARS-CoV-2 cases were identified, of whom 195 (61%) had previously been in contact with another known case while for 127 (39%) source of infection was unknown. Median time from admission to positive PCR was 13 days (IQR 8 - 25). Symptoms where present in 67% of cases. Median age was 76 years (IQR 64 - 84). Of all episodes, 75% were diagnosed in medical units (including 50 cases, 16% in geriatric sections),24% in surgical sectors and 1% in the ICU. Contact tracing activity identified 605 contacts with a median of 2 contacts per index case (range 0-19). Of these, 32% had a positive PCR result during follow-up. Conclusion: A thorough contact tracing with systematic PCR screening is necessary after detection of a nosocomial SARS-CoV-2 case as transmissibility is high and more than 1/3 of cases are asymptomatic. The non-identification of a source for more than a third of cases raises concerns of potential implication of healthcare workers in transmission.