Thursday 30 January 2020

How deadly is the coronavirus really?

I'm don't have any medical training (unless you count a first aid certificate), and I'm certainly not a medical expert, but I do have post-grad training in industrial mathematics, and something seems very wrong with the current media (and even medical professional) discussions regarding how lethal the coronavirus actually is.

It is often reported that the death rate is "around 2%", which is, on the face of it, lower than the 6.5% fatality rate for SARS back in 2002-2003 (when there were 348 deaths and 5327 people diagnosed as having SARS).

However, given the incubation period of the new coronavirus is apparently 1-14 days (before symptoms appear), surely the current death toll (as deaths are known/reported almost immediately) needs to be compared to the number of cases that existed when the victims were infected (on average about 7 days ago?). Even allowing for a delay of a couple of days from when symptoms appear to when a case is detected/reported, the current death toll needs to be compared to the number of people that were infected when those that just died were actually infected (ie. not today, but several days previously).

Hence, rather than divide the death toll at any particular date with the reported cases at that same time, the death toll needs to be compared with the number of cases reported several days ago?

Looking at how the death rate changes if you compare figures with the number of cases reported at that time, vs. the number of cases known 1, 2 or 3 days previously seems to suggest that the death rate may be a lot higher than 2%. I suppose we won't know the true figure until the spread of the disease ends and we can get final figures for the total number of people that were infected and how many of them eventually died. That rate may also change over time if the virus mutates (often ease of transmission increases while virulence declines - I'm not sure if there is any theoretical reason for that, or if it is just a 'rule of thumb'?). Fortunately the spread of the disease appears to be linear rather than geometric at this time - possibly due to isolation/containment efforts?

JAN   CASES   DEATHS   T0     T1     T2     T3
24     651      18    2.8%
25     941      26    2.8%   4.0%
26    1438      42    2.9%   4.5%   6.5%
27    2116      56    2.7%   3.9%   6.0%   8.6%
28    2794      80    2.9%   3.8%   5.6%   8.5%
29    4474     107    2.4%   3.8%   5.1%   7.4%

T0 denotes death tally compared to known cases as at that date, T1 denotes deaths compared to the number of cases known as at the previous date etc.

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