My last post (here) on March 25th highlighted the importance of reducing the day-over-day growth rate in COVID-19 cases, and I lamented how the rate was remaining stubbornly high. Fortunately, it now looks like the isolation actions taken by states and major localities are bearing fruit.
Figure 1 shows the day-over-day growth rate in COVID-19 cases since early March along with the percentage of the USA’s population under stay-at-home or other significant restrictions. The day-over-day rate was trending up from March 6th to March 19th, but it shows a clear negative trend since the 19th. The first statewide action on March 17th closed nonessential businesses in Nevada, and on March 19th the California governor issued a stay-at-home order. By March 24th 50% of the country’s population was under an isolation order, and as of March 29th we are at 73%. Coupled with less severe state actions taken prior to March 19th, these orders appear to be working to drive down the daily growth rate.
Figure 1 Day-Over-Day COVID-19 Case Growth Rate and Percentage of USA Population Under Isolation Orders
Looking at the daily data from March 19th onward, 9% is a best estimate of the average reduction in daily growth rate. An uncertainty analysis suggests this rate could be as low as 6% and as high as 12%. As shown in Figure 2, the nature of exponential growth is such that the predicted number of new daily cases covers a wide band in April and May. The daily numbers are quite large, but the good news here is that the peak in daily new cases is likely to occur by mid April.
Figure 2 Historical and Projected Number of New Daily COVID-19 Cases
References
www.worldometers.info/coronavirus/country/us/
www.cnn.com/2020/03/23/us/coronavirus-which-states-stay-at-home-order-trnd/index.htm
www.nytimes.com/interactive/2020/us/coronavirus-stay-at-home-order.html