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COVID-19

Covid19Several people have asked me for my opinion of where we are with regard to the coronavirus pandemic, and what the future might hold. Obviously I'm not an expert in epidemiology or, in fact, any relevant science. I do, however, have a good basic grounding in science generally, and I have a circle of friends and contacts who are experts in a variety of scientific and medical fields.

Firstly some basics 

The virus causing the pandemic belongs to a family of similar viruses named coronaviruses.  It is NOT a type of flu - that is a different family of viruses. The current virus is closely related to the SARSvirus from a few years ago - it is, in fact, officially called Severe Acute Respiratory Syndrome Coronavirus 2.   

There are many coronaviruses, and they are all zoonotic(they can pass between animals and humans). Other coronaviruses include several found in common cold infections.  The current pandemic is named COVID-19(CoronaVirus Disease 2019). The outbreak started in Wutan,  China, and has now spread to all continents and most countries.  The virus attacks the respiratory system - particularly the lungs. It is quite infectious but, fortunately, not extremely deadly.  The mortality rate is a measure of how many people per hundred cases die from the disease or from complications. In this pandemic most mortality will result from pneumonia or other severe lung damage. Mortality varies with age and location, and it currently looks like this:

Case fatality rates (%) by age and country
Age80+70–7960–6950–5940–4930–3920–2910–190–9
China as of 11 February14.88.03.61.30.40.20.20.20.0
Italy as of 12 March16.99.62.70.60.10.10.00.00.0
South Korea as of 15 March9.55.31.40.40.10.10.00.00.0


Current predictions for global morality rates look something like this :

COVID Mortality

 

The predicted overall mortality rate, across all cases, is between 1% and 2%. To put this in some further context, seasonal flu has a typical mortality rate of around 0.1%, meaning that this virus is more than ten times as lethal as seasonal flu. 

As regards infectiousness - the agreed measure for this is called the reproduction number, which is given by the symbol R0.  It is a measure of the number of new cases, on average, each infected person causes whilst they are infectious.  The basic reproduction number is the maximum theoretical epidemic potential of the pathogen.  It describes what would happen if an infectious person were to enter a fully susceptible community.  The effective reproduction number is the actual value observed, and takes account of counter-measures and other factors.  Once the value of R0 is below 1, the disease will begin dying out.  Both basic and effective R0 will vary, depending on the virus itself, the measures being taken against the virus, the local population density, the amounts and kinds of social interaction amongst the population, and several other factors.  The R0 for a typical seasonal flu is 2-3. The value for measles is 12-18.  Current estimates for the COVID-19 virus are somewhere between 2 and 8. 

 

How it has spread, and how it will spread

This seems to be the area of most confusion. With no special measures or controls, the virus will spread in an exponential manner. This means that the cases will double (or more), over a fixed period. For example, cases might double every 24 hours, or every 30 days.  This will continue until there are sufficient people with immunity to reduce the chance of spreading, .  It is really important to understand a few things about exponential growth because it leads to results which are, for many, counter-intuitive.  I recommend watching the video below to give some insight into exponential growth in this case.


For example, consider a scenario where cases double every 48 hours.

Starting from a single case on day 1, the entire global population would be infected by day 33. 

Q. At what point would half the population be infected?

A. Day 32. 

Q. At what point would only 1% be infected?

A. Day 26. 

The important point is that exponential growth means that numbers will start to get very big only at the later stages of the pandemic.

A graph of the pandemic would look like this:

COVID Progress

The large spike would result in hospitals and other services being overwhelmed.  Most countries are implementing measures in order to try to shift the graph to look more like the hatched version above, illustrated more clearly below.  The disease lasts longer, but with a much smaller peak value of infections, allowing medical and other services to cope.

 

COVID Progress

 

How long will the pandemic last and how many will be infected?

Estimates vary for both. The pandemic could last a minimum of another 3 months, or could re-emerge several times over a period of 18 months or longer.   The virus now exists in sufficient numbers to mean that it is effectively part of our environment for the foreseeable future - like seasonal flu or the common cold - and cases will flare up periodically. A large proportion of the global population is likely to be infected this year, with the worst-case estimates between 70% and 90%, and the most optimistic estimates between 20% and 40%. 

The nightmare scenario is that the virus will mutate to become either more infectious, more lethal, or both. In such an event all current bets are off  (this is what happened with the 1918 'Spanish' flu pandemic, which killed between 60 and 100 million globally).

 

How many will die?

It is impossible to give a useful estimate yet.  For the UK, the death toll could be as low as tens of thousands, or as high as several hundred thousand. 

For example, if we assume that 80% become infected, and that the overall mortality rate is 1.5%,  that would mean over 500,000 deaths. However, if we assume that 20% are infected and the mortality rate is held to 1%, that would mean 130,000 deaths. 

Below is a very simplistic model which you can edit to produce some very speculative mortality predictions.  The only parameter you should play with is the Assumed Penetration - set it to a value between 0 and 1, where 0 means that almost nobody is infected and 1 means that the whole population is infected. Click on the B2 cell, enter your own value and either click away or press enter to update the model predictions.



To provide some context, the typical UK mortality from seasonal flu is between 3,000 and 18,000 deaths per year (see below). 

UK Flu Mortality 

What should I be doing?

The most effective thing is to wash hands regularly. At this stage the objective is to slow the spread in order to reduce the peak infection numbers, as discussed above.  The most effective measure is isolation. Obviously if there is no contact between the population then there is no opportunity for new infection.  In reality complete isolation is impossible.  China managed to impose a very harsh regime of quarantine and isolation, which was very effective in reducing the spread, but it is difficult to see how similar measures could be imposed on the populations of western democracies.  The best advice I can give is to keep up to date on a daily (but not hourly) basis, using reliable sources (government and the BBC for those in the UK) and follow the advice given.

 

 

 

Understanding Wolfram
The BBC and other media
 

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