Friday, April 24, 2020

Intra-Country Spread of COVID and the role of Lockdowns - II

Joint Blog with Dr Surjit Bhalla also available at,

    In a previous blog we examined the progress made against COVID in the four large democratic countries India, USA, Japan, and UK. We had examined the progress thru the prism of W0, an index we have developed to track phases of virus transmission (expansionary or in recession) and to identify, and highlight, the magnitude of W0. Like R0, the index used by epidemiologists to evaluate virus transmission, W0 also has the property that when it is greater than unity, it is indicative of the virus in expansion; when less than one, and persistently less than one, it is indicative of the beginning of the end of the virus.  We have modelled the pattern of diffusion of the virus for every country the virus is present.
 In this blog we analyze the progress of the five largest countries in Western Europe by population plus Sweden. Table 1 summarizes the pace of transmission in Italy, France, Germany, Spain, Netherlands, Belgium, and Sweden, in terms of the latest value of W0 for each country and the value of the index 7, 14, 21 and 28 days earlier. This puts the current index into perspective, so we can judge whether the current value is part of a trend, or merely a spike or temporary occurrence. All five countries have gone below one, the critical threshold of transmission.
Just four weeks ago, on 26th march, all seven countries had a W0 of 1 or higher. A week later one country, Italy had moved rapidly downwards. Italy had gone through an explosive phase earlier, with the number of COVID cases tripling in one week – from 10149 on March 10 to 31506 Match 17th. Over the next week, perhaps because of the lockdown, cases slowed and by March 28th, W0 had gone below 1 and continued to decline rapidly over the next 21 days.
Table 1: Index (W0) of Speed of Spread of COVID within Countries
Note: * Italy is for 19th April. Source Oxford university COVID Data Base.
     Sweden, one of the very few countries in Europe not to implement a lockdown, had the lowest W0 (equal to 1)  28 days ago, This despite Sweden having very cold weather - temperature is one of the factors we find is helpful towards lower transmission! Sweden went into a contractionary phase in the the first week of April, later than Italy and has improved (marginally) less quickly than Italy. But note that Sweden is improving faster (again marginally) than Spain, Belgium and Netherlands, all with extensive lockdowns. 
Pattern of W0: Figure 1 shows the evolution of W0 since the start of the epidemic in France, Germany and Sweden. Note that day represents the day of the virus in each country. These countries show a similar twin peak pattern for W0, with Sweden’s peaks being highest and furthest apart (8 days). Germany’s twin peaks were 4 days and France’s 2 days apart. The decline in W0 from the peak was also the smoothest for Sweden, with France and Germany showing a slower decline with larger fluctuations.  France’s W0 fell in a series of ups and downs to reach 1.9 on day 54 and then very slowly to 0.4 on day 71 of pandemic. Germany’s W0 reached the threshold on day 55 and reached 0.9 on day 59.  Since then it has declined steadily to 0.2 on the 71 day of the Pandemic. Sweden’s W0 declined rapidly from its peak to go below the threshold of 1 to reach 0.6 on day 41 of pandemic in Sweden.

Figure 1: Speed of Transmission of Corona Virus, Index (W0): France, Germany & Sweden

Source: Authors calculations based on Johns Hopkins University COVID data base 

       The order of peaking in terms of days from start of pandemic was therefore Sweden, followed by France and Germany, with actual dates as 3rd March 4th March and 11th March, respectively. Sweden was also the first to cross the threshold W0 value of 1 to reach the recessionary phase, but Germany was quicker than France in crossing the threshold. As noted in the previous blog (Time Will Tell – 2), Japan’s performance was the best among all the countries discussed. The sub-set of Sweden, Germany and UK were the next best taking about 67 days from the start of Pandemic, while the sub-set of France, USA and India, brought up the rear by taking over 77 days.
Figure 2 plots W0 for Italy and Spain. Italy’s peak was an order of magnitude higher than all other countries, while Spain’s was the second highest. This is one of the reasons for the attention focused on these two countries. Italy’s peak W0 value was an unprecedented 217 on day 21 of Pandemic (27th February), while Spain’s W0 was 59, on day 24 of the epidemic (2nd March).

Figure 2: Speed of Transmission of Corona Virus and Critical Threshold (Index W0): Spain & Italy

Source: Authors calculations based on Johns Hopkins University COVID data base

     Italy and Spain’s 7 day rise to their respective peaks was very similar and can only be described as meteoric. The decline follows a different pattern with Italy’s smoother fall to moderate values of W0 contrasting with the decline in Spain’s W0 which was marked by several setbacks. W0 of both countries, declined to 0.2 by the 66th day of the pandemic. 
  Thus Italy, followed by Spain, crossed the threshold from expansionary to recessionary stage before Sweden was firmly in the recessionary stage. However, if we take note of Sweden’s first W0 reduction to 0.8 on day 44, this was only six days before Italy’s W0 reached 0.8 on day 50.
As several countries are entering the declining phase of W0 after the peak has been passed, we depict this separately in Figure 3.

Figure 3: Pattern of Decline of Index W0 after Peak increase

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