In the first weeks of the 2021 Auckland August Covid-19 outbreak, the contagion network team provided a number of reports to officials with estimates of the likely size of the outbreak and updates on the estimated effect of Alert Level interventions on curtailing spread.

The report ‘Modelling estimates of expected size: The August 2021 Covid-19 outbreak in Aotearoa’ describes the modelling approaches used in these estimates, including both a contagion network model and a branching process model. Output from both of these models was conditioned to real-world case data as it became available, using a gaussian process model.

The reports ‘Preliminary modelling of a new community case of Covid-19 as of 17 August 2021’ and ‘Impact of wastewater testing results on preliminary estimates of Covid-19 community transmission as of 18 August 2021’ were compiled in the first two days of the outbreak when very little was known about the details of the outbreak. The first of these was produced when all that was known was that a single case of Covid-19 had been discovered in the community, linked to a man living in Devonport. The second included consideration of the information of a negative wastewater test from the weeks before the detection of the first case. Both of these reports significantly underestimate the size of the outbreak at detection, though both did predict that the outbreak was likely to be significantly larger than was known at the time of writing.

As the outbreak progressed, daily updates were produced to incorporate the effect of new cases discovered over the previous 24-hour period. These semi-automated daily updates used the same methods as described in the report of 26 August, with both a contagion network model and a branching process model treated with gaussian process conditioning. As the outbreak progressed, and new daily cases decreased, detailed information from contact tracing became relatively more important for estimating near-term risk and outbreak progression. Hence, the example shown here from 7 September is the last daily update provided for the initial phase of the outbreak.

Read the reports