[SystemSafety] NHS COVID-19 Contact Tracing App

Peter Bishop pgb at adelard.com
Thu May 7 11:56:59 CEST 2020

A contagion dies out when the replication factor R drops below 1, i.e.:

R = Ro U < 1

Ro is the "normal" number of people a person can infect.
U is the proportion of the population that can still be infected

The UK Covid figure for Ro was around 3 (before social distancing)

So in a "do nothing" scenario

R <= 1, when U= 1/Ro= 1/3

i.e. when (1-U)=67% of the population  has been infected.

If interventions (like distancing, testing, quarantine) can make Ro < 1,
the contagion will die out without infecting the whole population.
(as seen in New Zealand).

On 06/05/2020 17:41, Peter Bernard Ladkin wrote:
> On 2020-05-06 18:14 , RICQUE Bertrand (SAFRAN ELECTRONICS & DEFENSE) wrote:
>> May be that, as in safety for accidents, it is a question of time?
> Very unlikely. Even the Plague, which was almost invariably fatal, only killed off 1/3 of the
> population of Europe (and brought an end to feudalism). Given living conditions at the time, we can
> presume most people in cities were exposed to Yersinia. But only a third got the disease.
> Prof. Peter Bernard Ladkin, Bielefeld, Germany
> Styelfy Bleibgsnd
> Tel+msg +49 (0)521 880 7319  www.rvs-bi.de
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Peter Bishop
Chief Scientist
Adelard LLP
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