Today we switch to a new model, away from the simple Exponential Growth (EG) model and move to a new logistic or sigmoidal one. We have called this model Mitigation+Suppression (M+S). We will also change from plotting the Y axis linearly, to exponentially (base 10).
This new model presents (and utilises) our full history of total case numbers, detects what phase we are currently in, and then makes projections into the future based on a small number of conditions and rates that we can adjust for each phase. Our aim is not to try and nail a prediction of the future, we simply want to illustrate how varying these rates and conditions can influence our future case load.
The new model also estimates “active cases” based on the sum of our new cases in the last 10 days (directly, no “distribution about a mean” of these cases in time). We also plot a selected percentage of active cases; presently we have chosen to display 5% of active cases. This is a conservative estimate of the proportion that might require ICU treatment. Estimates on this vary and the local data is still emerging, so interpret these numbers with care. We have refrained from providing a line representing our available ICU resources, as this is also changing with new beds and vents being brought online. For reference, in previous models and updates we have used a number of 2,000 beds (available for COVID-19) as an estimate.
If we can reduce and maintain our confirmed case growth below 3% for 3 out of 4 consecutive days, this will signal we have entered Phase 3: Suppression. We have selected a case growth rate of 1% assumed for this phase, based on an analysis of South Korea’s data (likely our best case scenario).
While we’ve made some encouraging progress this week, we want to caution everyone against overconfidence or misrepresenting these results. A sharp increase in community transmission, increased testing rates or a broadening of the criteria to be eligible for testing could all result in a break-out from the current Mitigation trend, and a return to Exponential Growth phase (see below).
We have provided 4 separate Scenarios for your consideration. It should be noted that due to the (thankfully) reducing number of cases in recent days, the differences between these scenarios are relatively minor. That would not be the case were we still in a strong growth phase (i.e. a week ago). This data should not be interpreted as an excuse to “take our foot off the pedal”. We currently have the opportunity to suppress this disease locally, we should make the very most of it. Every fraction of a percent in this rate helps us and potentially shortens the crisis.