There are a lot of statistics around Covid-19 and its likely impact over the next few weeks and months. There have been many charts online from expert as well as people who are playing with the numbers. Predicting the future is always difficult and epidemiology seems to mostly be the study of confounding factors. It can be easy to produce a simple model - and much more complicated to implement it.

I am certainly not an epidemiologist and so I have not published any numbers so far. I have played with a few simple models largely to see how they worked but nothing that had not been done to a much higher standard by other people.

Recently I had need to estimate some figures for my practice. I am making no predictions about how the pandemic will play out. There are no predictions in here. I have taken predictions from other people to work out the effect on my practice. In fact I have done this for every practice and PCN in England and it really is not much more work. It does make the spreadsheet work harder though!

There are various estimates of the total numbers of deaths and, whilst they influence the result we can model that fairly late. A quick way to get a ball park figure is to simply divide the deaths by the number of practices. There are almost exactly 7000 practices in England and, at the time of writing, 14,399 deaths. That is pretty close to 2 deaths per practice. Every death is a bad thing but we are clearly not seeing huge numbers in individual practices.

There are numerous other estimates. I have seen 40,000 deaths as an estimated UK total which would work out at about 5.5 deaths per practice in total. I will use this total, but it is pretty easy to convert to other numbers if see a figure which appears more reliable.

I have not make any allowance for practice size. There are a shade over 60 million patients registered with practices in England and so a quick bit of division suggests that we would expect .66 deaths per thousand patients. Thus a practice with 10,000 patients would expect around 6-7 deaths from Covid-19. By this stage we are getting to something that practices can use to estimate workload. It is unlikely that the figure of 40,000 is spot on but you can say, for instance, that you could plan for double that whilst hoping for a lower figure.

Can we refine this any more? There are many risk factors for death from Covid-19. As the disease has not been around for very long there have not been many good studies. One of the best was a look at mortality in China by Imperial College. This looked at age as a risk factor and have published this in ten year bands. Helpfully the age and sex makeup of the population is also published. This can come down to the year by year level but the five yearly bands are quite enough and still run to more than a third of a million lines on a spreadsheet.

There is also some information about disease risk factors such as diabetes and heart disease. We do have some of that information at practice level from the QOF. Could that be used to refine the risk level? Unfortunately probably not. The data for age related risk and the risk from co-morbidities has been calculated separate and not as independent factors. In reality the increasing age is a risk factor for diabetes and heart disease and so if we corrected for both we would likely be correcting twice. The risks are not independent. In the future there may be studies which look at these as individual variable and this would allow us to use the QOF information on top of the age related risk.

The process I used was to multiply the population in each year group by the mortality risk. So if a practice had 100 patients in a group and the risk was 1% I would count 100. If the risk was 15% I would count 1500. I add all of these together and then scale back to the national population to produce "Covid adjusted" list size. This is the list size of completely average people you would have to produce the same total mortality. This works a bit like the Carr-Hill formula.

The major assumption here is that all ages will have a similar rate of developing the disease. This has not been shown in the paper and hopefully shielding and social distancing will give a lower rate of disease in the elderly. On the other side the risk in care homes seems, at least from media reports, to be particularly high. I have also assumed that the infection rate is the same across England. That is certainly not the case at the moment but I think that it is probable that it will become more similar as we get towards the end of the pandemic.

With the adjusted list size you can then do what we did above to allocate the deaths in proportion. You can adjust the national deaths and the others will change, although this is a linear relationship. Increasing to 80,000 will just double the deaths for each practice and you could probably do that in your head.

I hope that you find this data to be useful. We are using this at our practice as a basis for planning services. Whilst the number will not be precise they give a rough estimate of what we should be providing. Other workload is likely to be proportional to mortality and so can get some guide to likely volume of work that we will be seeing. There is likely to be a lot of local variation. The final figures for a practice may be double or half of what is shown here but equally it would be surprising if they were out by a factor of ten. We can at least approximate what our response should be.

You can either see the list on Google Docs or download the spreadsheet. You can also see the full workings out on a very large (24Mb) spreadsheet which runs very slowly on my computer.

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