Which preventive control measure initiated the "flattening of the curve" A comparison of Austria and Victoria during the 2nd wave of the COVID-19 pandemic

Background When a country introduces different COVID-19 control measures over time, it is important to identify the specific measure that was effective and therefore responsible for "flattening the curve". This information helps policymakers find the right decision and saves the economy by avoiding severe yet ineffective measures. Objective This paper aims to fill the literature gap by investigating two regions that introduced two or three consecutive measures during the second COVID-19 wave, namely Austria and Victoria. Method We calculated the first derivative (acceleration) of the filtered daily case data and identified the date of the start and end of the acceleration's major downturn (effective phase) relative to the date when the control measures were introduced (Austria: soft/hard lockdowns; Victoria: stages 3/4 lockdowns, mask order). Results In Austria, the effective phase started 5 days after the introduction of the soft lockdown and ended at the start of the hard lockdown. In Victoria, the effective phase started 19 days after the introduction of the stage 3 lockdown, 5 days after the introduction of the mask order, and ended 6 days after the start of the stage 4 lockdown. Conclusion Considering that the effect of control measures is expected the earliest one serial interval after their introduction, the control measure responsible for "flattening the curve" was the soft lockdown in Austria and the mask mandate in Victoria. The severe lockdowns in both regions were ineffective.