Validation of a risk stratification tool for SARS-CoV-2 Delta community transmission in the Australian Capital Territory

During the SARS-CoV-2 Delta (B.1.617.2) variant outbreak, from August to October 2021 in the Australian Capital Territory (ACT), the number of new cases 'in the community for part of their infectious period' was publicly reported daily. We describe the stratification tool used during the outbreak to determine presumptive risk of community transmission from cases, and present the results of a contemporaneous validation of each case's risk against their onward transmission detected by routine surveillance. After case interview, epidemiologists identified the most likely source of infection for each new case and used the stratification tool to classify the case as either no, low, or high risk of community transmission. Each case notified between 12 August and 14 September 2021 was matched to its recipient case(s) to determine how well the tool predicted transmission risk. Household transmissions were excluded. Of the 530 notified cases stratified, 159 (29.3%) were cases who transmitted to a recipient case. Of the 59 cases who were the source of community transmission, 66% (38/59) were undertaking high-risk activities not associated with permitted essential work at the time. Only six source cases stratified as low risk or no risk transmitted SARS-CoV-2 to those outside their own household. The tool was essential in the rapid determination of community transmission risk in the ACT, and validation of the tool against detected onward transmission provided evidence for the effectiveness of public health restrictions. In the early stages of outbreaks of diseases for which transmissibility has not yet been established, the validation of such a stratification tool relies on high quality case investigation data, but may help to understand transmission dynamics and to inform interventions.