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 **

Country	Deaths	Total cases	Mild Disease	Hospital	Critical 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	5,867,657	1,100,186	366,729

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