Model update

We have had some higher quality data recently which we can use to compare to my model assumptions and projections

Update on Mortality Rate

I used 1% in my models from early April. This still looks a good estimate but now I think rather than the range being 0.5-1.0% it is perhaps 1.0-1.5%

The reason is that we now have had 3 high quality pieces of data from large samples of people from areas of high infection to see how many people have been infected. I have already written about the New York results which suggested something a little over 1% mortality rate. Spain found that 5% of its population have had it and France 4.5% In Spain this result means that if we just take the raw COVID death data we have a mortality rate of 1.1%. If we take a more reasonable assumption that this data is underreported and use the excess death data instead then the true Infection Mortality Rate would be perhaps 1.4%.

I used the updated mortality rate of 1.1% to relook at the path of deaths in Spain and from that estimate what R0 must have been.

To get this fit we need an R0 of close to 3 early in the infection, dropping to 2 as people became aware of the pandemic and then to 0.8 after the lockdown on March 13th. This is the same as my models from last month.

Update on R0

Public Health England has recently suggested that the overall R0 in the UK is currently just below 1 which is a combination of being far below this in London (perhaps 0.4) and likely above 1 in other parts of the country such as the North East.

Source: FT

Since we are relaxing lockdown it seems reasonable to project that the R0 will rise from here. In the projection below I have put the future R0 at just 1.2 and if we want the next peak in deaths to be the same as the last one then we would need to go into lockdown in early July i.e. about 6 weeks’ time. Then we would come out of lockdown again in early September i.e. we spend more time in lockdown than we do in periods of minor easing and we never reach Boris’ Phase 2 of further easing of lockdown restrictions.

Is there a more optimistic projection?

To be much more optimistic we need to find a way to limit transmission without going back into lockdown. We must find more targeted ways to change behaviour rather than the very blunt tool of making us all stay inside our homes. This could be Test, Trace and Isolate or better public information on how to limit transmission.

How else could the projection be changed?

The simplest way is that policy is different. If lockdown becomes more severe then the R0 never rises and deaths keep falling. If instead we choose to carry on along Boris’ path of further lockdown relaxation then the R0 would be higher again and the number of deaths would rise far more rapidly. If we choose not to go back into lockdown then the number of deaths just keeps rising exponentially.