A framework for counterfactual analysis, strategy evaluation, and control of epidemics using reproduction number estimates
Fig 4
A) Daily confirmed cases at Purdue during Fall 2020 and Spring 2021. We mark five shaded areas for five major events across the two semesters at Purdue, corresponding to the entry screening of Fall 2020 (I), the start of the Big Ten football season (II), Thanksgiving Break (III), the entry screening of Spring 2021 (IV), and Spring Break (V). Purdue allowed students to stay home after Thanksgiving break, leading to a significant decrease in confirmed cases, as shown in Shaded Area III. The reconstructed spreading process (dotted solid line) matches the confirmed cases observed on the Purdue campus during Fall 2020 and Spring 2021. Unlike UIUC, where we reset the initial condition at the start of Spring 2021, not resetting it for Purdue results in overestimating daily cases during Spring 2021. B) Estimated effective reproduction number for Purdue from Fall 2020 to Spring 2021. The estimated effective reproduction number (95% confidence interval) was around one for most of the time, reflecting that Purdue’s testing-for-isolation strategy avoided potential large outbreaks. Two major spikes were observed in the estimated effective reproduction number (95% confidence interval) at Purdue: one around the beginning of August (I) and the other around the beginning of January (IV). As shown in Fig 4A, these two spikes correspond to the infection process during the Summer and Christmas breaks. Unlike UIUC, Purdue provided sufficient data prior to 2020-08-18, allowing us to estimate the effective reproduction number from 2020-08-18 to 2020-08-24, as shown in Fig 4B. To match the dates in both figures, we did not include data prior to 2020-08-18 in Fig 4A.