Population-based analysis of the epidemiological features of COVID-19 epidemics in Victoria, Australia, January 2020 - March 2021, and their suppression through comprehensive control strategies

Abstract

Background: Victoria experienced the greatest burden of COVID-19 in Australia in 2020. This report describes key epidemiological characteristics and corresponding control measures between 17 January 2020 and 26 March 2021.

Methods: COVID-19 notifications made to the State Government Department of Health were used in this analysis. Epidemiological features are described over 4 phases, including enhancements to testing, contact tracing and public health interventions. Demographic and clinical features of cases are described.

Findings: Victoria recorded 20,483 cases of COVID-19, of which 1073 (5•2%) were acquired overseas and 19,360 (95%) were locally acquired. The initial epidemic (Phase I) was well-contained through public health interventions and was followed by relaxation of restrictions and low-level community transmission (Phase II). However, an outbreak in a hotel used to quarantine returned travellers led to wide-scale community transmission accounting for a majority (91%) of cases (Phase III). Outbreaks occurred in vulnerable settings including aged care and hospitals, contributing to high hospitalisation (12%) and case fatality rates (3•7%). Aggressive restrictions ultimately led to local elimination, and subsequent outbreaks have been swiftly managed with improved processes (Phase IV). The demographic composition of cases evolved across phases from an older, wealthier population to a less advantaged younger population, with many from culturally and linguistically diverse backgrounds.

Interpretation: Over time, adaptations to the public health response have strengthened capacity to respond to new cases and outbreaks in a more effective manner. The Victorian experience underscores the importance of authentic engagement with diverse communities and balancing restrictions with livelihoods.

Keywords: SARS-CoV-2; hotel quarantine; non-pharmaceutical interventions; outbreak; travel restricitions.

Figure 1

Testing capacity by sample collection date, Victoria 17 Jan 2020 – 26 Mar 2021: A. Daily number of tests performed (green bars) and percent of samples positive for SARS-CoV-2 (black line). Annotations indicate key changes to testing criteria. B. Weekly turn-around times from sample collection to test (black boxplots; outliers not shown) and from sample collection to notification (grey boxplots). Shaded bar shows the number of laboratories performing testing. Notes: SARI: Severe Acute Respiratory Infection; ARI: Acute Respiratory Infection; ROK: Republic of Korea; HCW: Healthcare worker; LGA: Local government area.

Figure 2

Epidemic situation: A. Epidemic curve of daily new cases by symptoms onset date or diagnosis date if asymptomatic, and key events in the epidemic and response. The first case had symptoms onset on 19 Jan 2020, the last locally-acquired case had symptoms onset on 8 Mar 2021; B. Effective reproduction number (R_eff) estimates Mar-Oct 2020. R_eff not estimated for periods with sporadic cases due to uncertainty around estimates; C. Key mitigation measures implemented during each phase of the epidemic. Darker shading indicates more stringent restrictions. The full list of measures is shown in Supplementary Figure 1, which details the restrictions associated with the colours. Notes: WHO: World Health Organization; PUE: pneumonia of unknown aetiology; SARS-CoV-2: Severe Acute Respiratory Syndrome Coronavirus 2; PHEIC: Public Health Event of International Concern; COVID-19: Coronavirus Disease 2019; HCW: Healthcare worker; NYW: New Year's Eve. Refer to the Supplementary information for a description of the calculation of R_eff.

Figure 3

Geographic distribution of COVID-19 incidence for Metropolitan Melbourne (cases per 100,000 population per square kilometre), by epidemic phase. Note: Refer to the Supplementary information for a description of the calculation of incidence rates and mapping.

Figure 4

The distribution of cases by age, sex and socio-economic position, by epidemic phase. Note: Socio-economic position is based on postcode [8,9].