Background: The Northern Sydney Local Health District was one of the first health regions to be affected by COVID-19 in Australia.Aims: To describe the clinical characteristics, risk factors and outcomes in our lowprevalence Australian population.Methods: This is a retrospective analysis of 517 laboratory-confirmed COVID-19 cases between January and June 2020. Patient information was collected as part of routine care within the COVID-19 Virtual Hospital system. Outcomes examined were death, recovery at 30 days and intensive care unit (ICU) admission. Results:The case fatality rate was 1.8%. Multivariate analysis showed factors independently associated with death, composite outcome of death/ICU admission or incomplete recovery at 30 days were age >80 years and presence of two or more comorbidities. Most cases acquired COVID-19 through international (50.9%) or cruise ship travel (9.1%). Healthcare workers comprised 12.8% of the cohort and represented a disproportionately high percentage of the 'unknown' source group (27.6%). The median incubation period was 5 days (interquartile range 3-8); one patient had an incubation period of 15 days. Hospitalisation was required in 11.8%, ICU admission in 2.1% and ventilation in 1.4%. A Radiographic Assessment of Lung Oedema score on chest X-ray of >10 was independently associated with death. Conclusions:In this low prevalence, well resourced Australian setting, we report an overall low mortality. Factors associated with adverse patient outcomes on multivariate analysis were age greater than 80 and the presence of two or more comorbidities. These data can assist in early risk stratification of COVID-19 patients, and in surge capacity planning for hospitals.

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Early COVID-19 experience in Australia involved clusters in northern Sydney, including hospital and aged-care facility (ACF) outbreaks. We explore transmission dynamics, drivers and outcomes of a metropolitan hospital COVID-19 outbreak that occurred in the context of established local community transmission. A retrospective cohort analysis is presented, with integration of viral genome sequencing, clinical and epidemiological data. We demonstrate using genomic epidemiology that the hospital outbreak (n=23) was linked to a concurrent outbreak at a local aged care facility, but was phylogenetically distinct from other community clusters. Thirty day survival was 50% for hospitalised patients (an elderly cohort with significant comorbidities) and 100% for staff. Staff who acquired infection were unable to attend work for a median of 26.5 days (range 14-191); an additional 140 staff were furloughed for quarantine. Transmission from index cases showed a wide dispersion (mean 3.5 persons infected for every patient case and 0.6 persons infected for every staff case). One patient, who received regular nebulised medication prior to their diagnosis being known, acted as an apparent superspreader. No secondary transmissions occurred from isolated cases or contacts who were quarantined prior to becoming infectious. This analysis elaborates the wide-ranging impacts on patients and staff of nosocomial COVID-19 transmission and highlights the utility of genomic analysis as an adjunct to traditional epidemiological investigations. Delayed case recognition resulted in nosocomial transmission but once recognised, prompt action by the outbreak management team and isolation with contact and droplet (without airborne) precautions were sufficient to prevent transmission within this cohort. Our findings support current PPE recommendations in Australia but demonstrate the risk of administering nebulised medications when COVID-19 is circulating locally.