Estimating temporal variation in transmission of COVID-19 and adherence to social distancing measures in Australia: technical report 5th May 2020

Key messages: Estimating the effective reproduction number • Analyses were performed to identify temporal changes in the effective reproduction number (Reff) in each Australian state/territory (Figure 2). Due to very low case incidence, these estimates are becoming increasingly unstable, i.e., based primarily on model assumptions rather than actual case data. • We are currently developing new methods for assessing epidemic activity, which should be more informative when daily case incidence is very low. These new methods will incorporate data on population mobility and chains of transmission e.g., from localised outbreaks. Forecasts of the daily number of new confirmed cases nationally • Our most recent estimates of Reff which were deemed reliable — estimated up to 11 April, based on data up to 20 April — were input into a mathematical model of disease dynamics which was projected forward to forecast the daily number of new confirmed cases. • We report an Australia-wide forecast of the daily number of new confirmed cases up to 8 June (Figure 3), assuming that the effective reproduction number remains at its current estimated level (from 11 April). Assessment of adherence to social distancing measures • An analysis of trends in population mobility data streams up to 3 May was performed to assess adherence to social distancing policy. Relating changes in these data streams to changes in Reff may enable the development of early indicators of increased transmission. • This analysis suggests that adherence to social distancing measures may have decreased marginally in the past two to three weeks (Figure 4), although we note that the strongest signal of reduce adherence is for activities considered to be relatively low risk (driving and visiting parks).

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