Covid-19 Numbers – Extrapolation Methodology

In the absence of mass testing in the West (other than Germany and USA), this Blog also seeks to put forward an alternative methodology in calculating the size and spread of Covid-19,  using a proxy measure of a 1.4% death rate (** see Wuhan, China evidence below) , and working backwards to calculate the numbers of people infected with Covid-19, and trying to validate these extrapolations against WHO published statistics.

The methodology is simple. It follows that if you have a death rate of X%, then it is possible to calculate what 100% of the Covid-19 infected population is :  ie number of deaths divided by X% multiplied by 100. If that number is then split into 3 main  proportions** of 80% (mild infection) 15% (hospitalisation) and 5% (need for critical care), then one is able to calculated the estimated numbers of people/patients in each category. **Paper by the Chinese Centre for Disease Control and Protection stated that 80% cases were mild or asymptomatic, 15% had severe disease often requiring treatment in hospital, and 5% requiring critical care with a death rate of 2.3%

This method can be applied to any size of population, from Countries, Regions, Counties, Cities etc.. ie any population where you have reasonable accurate death rate figures. Whilst this is not perfect, and excludes deaths in the Community, it provides more accurate information on the size and spread of Covid-19, including the possible numbers of people with mild infection, and is better than saying “We just don’t know until we have widespread testing”, which is NOT anytime soon.

** Estimating clinical severity of COVID-19 from the transmission dynamics in Wuhan, China : Nature Medicine – full article reference : https://www.nature.com/articles/s41591-020-0822-7#citeas

“Using public and published information, we estimate that the overall symptomatic case fatality risk (the probability of dying after developing symptoms) of COVID-19 in Wuhan was 1.4% (0.9–2.1%). “

Just as the Government keeps saying they are advised by the science, I have tried to follow their example by using only fully referenced statistics from legitimate academic sources.

Extrapolation of Covid-19 Cases in selected Countries and the World

Data Reported April 10th 2020 (previous 24 hours) based on 1.4% death rate **

CountryDeathsTotal casesMild DiseaseHospitalCritical Care
   100% 80% 15%Inc Deaths 5%
      
UK  8,958  639,857  511,886  95,979  31,993
Italy 18,849 1,346,357 1,077,086 201,954  67,318
Spain 16,801 1,200,071  960,057 180,011  60,004
USA 18,747 1,339,071  1,071,257 200,861  66,954
France 13,197  942,643  754,114 141,396  47,132
Germany  2,736  195,429  156,343  29,314  9,771
Sweden  870  62,143  49,714  9,321  3,107
Norway  113  8,071  6,457  1,211  404
China  3,336  238,286  190,629  35,743  11,914
South Korea  208  14,857  11,886  2,229  743
      
World 102,684 7,334,571 5,867,6571,100,186   366,729
extrapolated numbers based on Wuhan study death rate of 1.4%

Using this methodology, this Table shows that there are 7.3 million cases of Covid-19 Worldwide as of 10th April 2020 rather than the 1.7 million (1,698.835 Total cases) published by the WHO on 10th April 2020 (www.worldometre.com).

The WHO numbers have been validated against the numbers from the extrapolated  1.4% death rate) using two methodologies, both for total hospital cases and also for the mild infection numbers. Although there is some variance, they are within the Wuhan variance of 0.9% – 2.1% at approximately 1.2%.

The benefit of these results is that a reasonably scientific estimate of the number of mild and asymptomatic infections is provided for a Country, which could be used to inform planning for any possible phased lifting of the Lockdown ie for example, there are just over half a million cases in the UK and over 1 million in Italy and the USA, with Spain approaching 1 million cases, as of April 10th 2020. Numbers have obviously increased since then.

These numbers could be broken down for the various hotspots in the UK, such as London, the Midlands and South Wales (Gwent in particular).

My Excel spreadsheets are available for scrutiny on request.

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