THe Blog

April 23, 2020

Sensitivity of COVID-19 case projections to CDGR

Recent data projections (as leaked to the The Hindu) for the number of confirmed COVID-19 cases by the Ministry of Health and Family Welfare (MoHFW) estimate that the city of Mumbai is likely to have more than 42,000 cases by the end of April and more than 650,000 cases by mid-May.


These projections use a Compounded Daily Growth Rate (CDGR) of 20% and the number of confirmed cases in the city as on 16th April: 1,896. The projections are taken over a period of 15 and 30 days from 16th April. There is no reasoning for the CDGR of 20%. 


If we look at confirmed cases, the number in Mumbai grew from 1,896 to 3,683 over the last eight days. Thus, the average growth rate during this period was around 10% and the CDGR was around 9%. 


We estimate projections starting from 23rd April (when the number of cases was 3,683) to 17th May using 4 different CDGRs: 10%, 12%, 14%, and 18% (see Figure 1). We find that with the lowest CDGR of 10% — which is the closest to the actual CDGR of the past week, 9% — the projected number of cases on 17th May is under 40,000. In the high CDGR scenario — which is closest to what MoHFW used, 20% — the projected number of cases on 17th May is just under 200,000. 


Figure 1: Projected scenarios for COVID-19 total cases in Mumbai



The high number of projected cases in Mumbai, therefore, is down to two key factors.


1. CDGR is highly sensitive to the time period used. The MoHFW estimations used a longer time period of 30 days as compared to our projections — based on a time period of 24 days — which resulted in a very high projection of 650,000. To illustrate, if we use a time period of 30 days starting from 23rd April and 3,683 cases, the projected number of cases will be 520,000 using a CDGR of 18%. 


2. The use of a very high CDGR of 20%. This is not borne out by the data on the actual number of confirmed cases over the last eight days in Mumbai: 9%.

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