Regional Trajectories

I've spent some time looking at some regional trajectories in the UK to get a better feel on where we are in the infection lifecycle at the moment. In my previous post it appeared that London was probably experiencing a decreasing rate of new cases and that the North East and North West were still in growth.

However, a simple snapshot of that type is not always that useful, what we need is a method of visualisation that allows a clearer comparison over time that combines several elements: rate of growth, level of infection (new cases per 100K population) and trajectory over time. This is one way we can represent this data. (You may want to click on the chart to see it full size).

The line shows the trajectory of new cases in the London area based on this government data. On the x axis we have the average daily growth rate of new cases, this is based on a rolling weekly value centred on the date shown compared to the same figure one week earlier. On the y axis we have the average weekly rate of new cases per 100K of the population and the line shows the evolution of these variables over time (date in dd/yy format as data labels). The data displayed cuts off a week ago, because the data feed is not fully up to date and has incremental cases added retrospectively. Based on the last week or so, the time periods shown are now unlikely to change significantly so I feel comfortable in publishing the data.

So, for London we need to start at the lower right hand side where we had relatively low levels of new cases (9 per 100K) but very high growth (33% per day is equivalent to about 600% a week). As time progresses the rate of growth slows (curve moves to the left) but the rate of infection increases (curve moves upwards), before peaking at 67 cases per 100K population in early April. Zero growth was achieved around 4th April and the rate of new cases has been declining ever since. As time moves on I now expect the curve to carry on downwards towards the lower left hand corner.

It's worth noting that this is new cases, and doesn't reflect the potential loading on the NHS at any point - this would give a flatter lagged curve that would not provide any insight into changes in the rate of new cases.

So, going back to my earlier point, how does this compare to the North East and North West regions, especially in regards to lag, loading and growth? Superimposing on those two regions gives the chart below. (I could have added all regions but I think we are rapidly approaching the point when you cannot see anything clearly).

Despite slightly different starting dynamics (growth was slower for the first few weeks, but still significant) all three regions show similar shapes. However, the North East shows considerably higher levels of infection and growth during the earlier weeks which probably led to a higher per capita case load at peak. 

Both the North East and North West start to plateau around April 4th - about 5 days after London, and a similar lag time can be seen before cases start to decline. 

The good news is that all three regions appear to have passed the peak of new cases and providing the local health services have coped so far, they should be able to manage going forward. This is one reason why looking at cases per 100K is so useful - not only does it allow a direct comparison of values, but if we assume that hospital bed numbers are roughly related to local population numbers, these comparisons should give a good proxy for hospital bed loading. 

One interesting note is how the shape of the London curve has a period of near constant (exponential) growth from 19/3 to 24/3, (seen as the curve being vertical), you could also argue there are similar vertical segments on the corresponding curves for the other regions around the same point in time. This time period was just before the formal lockdown began and when most people had started to work at home where possible - the change from vertical to sloping may indicate the impact of that first wave of working at home (curve starts to flatten on 25/3 for London) and the second impact of the formal lockdown about a week later (curve plateau starts 1/4 for London). We can only speculate as to what might have happened had these measures been in place a week or so earlier. 

Addendum:

By special request, please find below the equivalent charts for the South East and South West again compared with London. As expected we see much lower levels of growth and infection rates, with both regions just starting to decline (as of 13/4 data).

And for completeness: