UK update with Pillar 1 data only

For the past few days I've been wrestling with the model for the UK, in particular why are does the top line data for the number of new cases still stay in a plateau when the daily death rate falling?

I think I have the answer... it's the Pillar 2 numbers (thanks to MH for being my sounding board on this). By including Pillar 2 we are muddying the water considerably as we mix apples and oranges. Let me explain...

Pillar 1 are (give or take) a reflection of positive Covid-19 cases on admission to hospital, so they are serious cases and as we have observed, typically, a lag from this data to a fatality rate of 15% in 6-7 days time.

Pillar 2 are swab-based tests on key workers. These are nothing like Pillar 1 for a number of reasons:

1. Pillar 2 cases should include many proactive diagnoses - pre-symptom / early symptoms and general screening etc. that are not the same as Pillar 1
2. Most Pillar 2 cases will not lead to hospitalisation - because they happen earlier, most tests are before severe symptoms and only a small proportion will probably be hospitalised. 
3. Even on hospitalisation, this is the working population, so most likely under 65 years of age and therefore have a lower fatality rate anyway. 
4. Because of the earlier testing, the lag time will be different, maybe 2-3 weeks as opposed to 7 days. 

Net impact: Pillar 2 cases are very much different from Pillar 1 and adding them together for reporting or modelling purposes is a big mistake. This is especially true when the proportion of Pillar 2 case are rising to a point where they represent a significant proportion of all cases.

So, what does it look like when we model with only Pillar 1 cases? Hint: see chart below.

Suddenly the model seems to make more sense - I could kick myself for missing this earlier. So, although the total reported cases are flat, Pillar 1 cases (which predict the death rate) are in decline and Pillar 1 provides the best indication of the future death rate. 

Pillar 2, although interesting has little part to play in the model. That said Pillar 2 cases do represent a risk of future disease transmission, so can definitely not be ignored for those in hands-on roles trying to reduce the spread of Covid-19 in the UK. 

Going forward I will be amending the UK model to mostly focus on Pillar 1, with Pillar 2 reported as a secondary item - given the desire to extend testing of Key Workers, expect this value to increase in the coming days as a result of increase testing rather than because of increased cases. Furthermore we can well expect parts of the media to go into complete meltdown if they continue to publish the data as a total when comparing with the historical Pillar 1 data.

If we apply only the UK Pillar 1 cases to the post-plateau analysis, we can just about start to see a decline in the UK comparable to Italy. It's too early to start celebrating (we need about another week for that) but I am quietly optimistic that we are now showing signs of improvement.