Thread by @john_actuary

We've recently seen a fall in COVID beds occupied in London where COVID is the primary cause of occupation, from around 85% to 75%. Is this a sign of Omicron being less serious? Or is it a result of a rapidly growing variant, and the lag before "With COVID" admissions rise?

1/

Here's a very simple example, which starts at Day 0 with 100 From COVID adm'ns, and 20 With COVID (so 83% primary diagnosis).

I've modelled this assuming infections double every 3 days (up 25% per day) for just 10 days, level either side.

2/

Now, people admitted who just happen to test positive ("With" COVID) should increase exactly in line with this curve. (Big simplification - I've not modelled age differences here.)

So the sharp-eyed will notice this graph is exactly the same, except for axes and titles.

2/

However, for those admitted because of COVID, the upward curve is pushed to the right - I've assumed 7 days here, just for the purposes of this example.

3/

The total admissions is the sum of the two, and we can see here how the "with COVID" starts building up first, followed by a much sharper increase once the "From COVID" adm'ns start rising, 7 days later.

4/

In terms of the proportion of "From COVID" cases, the % drops as the "With COVID" cases start to increase first, only levels off once the "From COVID cases start to rise, and in this model, as I assume level from day 10, eventually it reverts to the original figure.

5/

Thus, in a period of very rapid growth, it's not at all surprising that the proportion of "With COVID" cases has increased - indeed, I'd be more surprised if it hadn't.

But what, if seems the case, the driver of the rapid increase is a variant less likely to cause adm'n?

6/

We then decouple the change in "with COVID" adm'ns, which are still based on infection levels, and "From COVID" adm'ns, which will now be growing at a slower rate. I've assumed a rate which means by the end of 10 days there are only half as many From COVID adm'ns as before.

7/

We then see a much lower total number by the end of the period (as expected), but the absolute contribution of the "With COVID" cases is as before.

7/

And that means, that whilst the movement in the % falls in the same way at first, it doesn't recover to the original level and settles at a lower figure, as With COVID cases take a higher proportion than if severity was unchanged.

8/

As noted before, this is a very simple example, and other factors, notably how any rise ripples through the age groups will add to the complexity.

But I hope it helps give some context to what we are seeing and might see in admission levels and the With/From split.

9/

And finally, it doesn't cover the important aspect of hospital derived infections - partly because I can't think of an obvious way to model it, and partly because this thread is long enough for Boxing Day!

10/10