COVID forecasting – Rear view mirrors and uncertainty

Photo by Lukas Kloeppel on Pexels.com

What kind of COVID numbers can we expect to be seeing over the next few weeks?  Knowing what we might expect to see is crucial for those involved in making the big decisions because  case numbers impact on hospital admissions and that in turn has an impact on mortality figures.  Those involved in leading churches also need to be aware of these things because

  1. The national picture will affect whether or not the recent roadmap is reversed and restrictions are reintroduced.
  2. The national picture in terms of cases, admissions and deaths affects the confidence of people making decisions about coming to church.

The problem is that as confident as people want to be about their models and predictions, we cannot be that confident. You see, we are working on the basis of two things.

  1. Assumptions about how people will behave and interact.
  2. Past data.

We can model and forecast based on what we’ve seen happen in the past but that doesn’t always tell us what is going to happen in the future.  It’s like relying on the rear view mirror to calculate what is coming up on the road ahead.

So at the moment, there are two big and linked uncertainties.  The first is about how people will behave after July 19th. Will everyone take off their face masks and pack into crowded venues. If so, that is likely to be an ideal climate for COVID to spread rapidly. Or will people be a little cautious as they look at the stats? Furthermore, have people already started to behave differently, hugging more, wearing masks less, visiting people in their homes. If so, when we also factor in things like good weather, school holidays etc then there may not be much change in behaviour post July 19th and any increase in socialisation in clubs and bars may be offset by the reduced contact between students as well as the added protection of more people being double jabbed.

The other uncertainty is that we are trying to forecast growth based on trends. But those trends can change dramatically.  Have a look at this chart which shows the week on week % growth rate in positive cases going back to the start of December 2020. You’ll notice that there have been periods of rapidly increasing growth in the prevalence of the virus as well as periods when infection rates have been declining.

Look back to June and you’ll see that weekly growth was around 60%. It peaked at 65% on the 9th June before falling back to 31.07% on the 20th June.  At that point people were hopeful that we were seeing the current wave begin to slow and perhaps peak. However, what we then saw was a further acceleration in growth to a high of 71.8% on the 1st July.  The consensus seems to be that this coincided with the European Championships.  Growth then slowed again right down to 27.28% on the 11th July and stayed at that level for several days. However, towards the back end of last week it began to increase again and by the weekend was at 40.6% leading to 54,674 new cases reported on the 17th July. If growth continues at that rate then we are looking at 77k daily cases by the 24th July and over 108k new cases on the 31st July.  We might then expect the wave to slow down over the early part of August.

However, the weekly figure is being masked by lower rates of increase early in the week and we may be seeing actual growth of up to 70%. That would lead to daily case numbers of about 92000 by the 24th July and 160k in two weeks time. Furthermore, this would then still require time for the wave to reach its peak meaning that we could be looking at a peak well over 200,000 cases by the end of August.

There again, with the Euros out of the way, we may begin to see the rate of increase slow again and so if cases are starting to peak then we might be looking at 68000 cases by the 24th July if growth slows to 25%. If it slows again the following week then we might see daily infection figures begin to peak and plateau at around the 75k mark.

On that basis, if I were making a forecast, I would do so on the basis of best, possible and worse.

Best Case Scenario = 75k cases with hospital admissions of 1.8k.

Possible (Middle) Scenario = 150k cases with 3.75k daily hospital admissions

Worst Case Scenario = 250k cases with over 6k daily admissions.

The historical data does suggest that the worst case scenario is extremely unlikely.  The high end 60% plus growth rate hasn’t been sustained in the past and it does seen to have been caused by specific circumstances. 

One of the reasons why I think we are less likely to see the worst case scenario is that we probably would do well not to think in terms of one single continuous third wave. Rather the wave will be made up of lots of smaller waves.  So, for example one wave started in Bolton and spread to the North West. That wave began to abate but then the Delta Variant spread to other parts of the country leading to mini exponential waves in those areas.

We might expect each wave to follow an S curve, starting slowly before rising rapidly, slowing down and plateauing off before a new wave takes over, as demonstrated below.

My personal hope and expectation is that we are looking at something between 80k daily cases and 150k daily cases.

However, the truth is that we cannot be certain.  That’s why it is important to keep a hawk like eye on the data and why a level of caution is necessary in any planning. It means that it is unwise to jump on the band wagon with either extreme. We cannot confidently declare that the UK is out of the woods yet but nor should we join those who are declaring that we are all doomed and that the Government are wickedly reckless and unethical for going ahead with July 19th. It is clear that we are going through some very uncertain days and weeks. Christians should be careful and cautious in their actions whilst remembering that our confidence is not in human decisions, vaccinations or circumstances but in the Lord.

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