Here, we discuss our group’s most recent Ontario COVID-19 forecasts, and elaborate on a key modelling decision that enabled us to predict Ontario’s third wave accurately, weeks in advance, while infection reports were still declining.

Context

Every two weeks, our group provides COVID-19 infection report forecasts for the province of Ontario to the Ontario Modelling Consensus Table, a partner of the Ontario Science Advisory Table, which presents this information to the Health Coordination Table of the Ontario Ministry of Health.1

Our forecasts are based on a compartmental epidemic model implemented in our publicly available McMasterPandemic R package, and involve statistical fits to the province’s latest infection report data.

An early warning

We made a forecast on 21 Feb 2021, when infection reports were still declining steadily from a peak in early January. At the time, we were asked to consider what would happen if public health measures were to relax in early March.

The forecast made on 21 Feb 2021 (curve framed by a 95% confidence band). Observed infection reports (known at the time of the forecast) are denoted with solid black points. The forecast date is indicated with the dotted black vertical line. The forecast assumed that public health (PH) measures would be reduced on 8 Mar 2021, indicated with the dashed black vertical line.

Figure 1: The forecast made on 21 Feb 2021 (curve framed by a 95% confidence band). Observed infection reports (known at the time of the forecast) are denoted with solid black points. The forecast date is indicated with the dotted black vertical line. The forecast assumed that public health (PH) measures would be reduced on 8 Mar 2021, indicated with the dashed black vertical line.

Our model predicted exponential growth in infection reports very soon after an early-March reduction in public health measures. This prediction was rather startling at the time, especially given that infection reports appeared to still be declining.

The forecast made on 21 Feb 2021, with infection reports observed after the forecast date denoted with hollow black points. The actual lifting of the stay-at-home order on 8 Mar 2021 is indicated with the solid black vertical line.

Figure 2: The forecast made on 21 Feb 2021, with infection reports observed after the forecast date denoted with hollow black points. The actual lifting of the stay-at-home order on 8 Mar 2021 is indicated with the solid black vertical line.

Public health measures did indeed relax further in early March: on March 8th, the province lifted the January stay-at-home order in the last few regions where it was still in effect (Toronto, Peel, and North Bay Parry Sound) and loosened restrictions in seven other regions.

Infection reports began growing exponentially soon after our forecast was made, propelling Ontario into its third, and largest, wave of infection as of early April 2021.

An updated trajectory

When we made a forecast on 20 Mar 2021, infection reports had been increasing for a few weeks, so we were able to calibrate the growth rate of the third wave more precisely than in the 21 Feb forecast. We presented a “status quo” forecast, where we assumed that there were no changes to public health policy after the forecast date.

The forecast made on 20 Mar 2021 (curve framed by a 95% confidence band). Observed infection reports (known at the time of the forecast) are denoted with solid black points. The forecast date is indicated with the dotted black vertical line.

Figure 3: The forecast made on 20 Mar 2021 (curve framed by a 95% confidence band). Observed infection reports (known at the time of the forecast) are denoted with solid black points. The forecast date is indicated with the dotted black vertical line.

Our model once again predicted exponential growth in infection reports into April 2021.