COVID-19: Day 142: Population Prevalence Projections

Actual (black) and Projected (red) UK daily deaths

How will the population prevalence of COVID-19 develop?

This is a question about the future and so – of course – the answer is “We don’t know“. But we can make some estimates based on our understanding of viral transmission.

I approached the question of the population prevalence of COVID-19 using a projection from Worldometer. Downloading the data, I mapped out how we might expect the rate of daily deaths to decline.

I feel bad that I don’t know the basis of the Worldometer model, but then I am only looking at the results semi-quantitatively. They will help to guide my expectations as the summer progresses.

The graph at the head of the article shows the 7-day rolling average of daily deaths as a black line, and the projection as a dotted red line. There are two features to notice:

  • The current death rate is still high: more than 300 deaths every day.
  • As we proceed into the summer the death rate reduces, falling below 100 a day in mid-June. The uncertainty in the projection is shown shaded between two finely-dotted lines.

However it is difficult to see both the large numbers and the small numbers on the same graph. So, time to use a logarithmic vertical axis! The graph below shows the same data as the previous graph, but plotted on a logarithmic axis.

Actual (black) and Projected (red) UK daily deaths plotted on a logarithmic vertical scale.

Now we can see the behaviour in the tail of the graph.

  • We expect the death rate to fall to 30 deaths per day, a factor 10 lower than at present, in 6 to 7 weeks – around 45 days. If events proceed closer to the lower projection, this could happen in as little as 35 days.
  • Projecting further it will fall to around 3 deaths per day, a factor 100 lower than at present, in around 90 days – this is around the start of September and the new school term. If events proceed closer to the lower projection, this could happen in as little as 70 days.

However, the rate at which people die does not tell us about the hazard that we personally face.

A better indicator of personal hazard is the prevalence of ill people in the population.

Population Prevalence Projection

As shown on both figures in blue, a survey between 4th and 17th of May found a population prevalence of ill people of 1 in 400 – or 2500 people in every million people were ill with COVID-19.

Assuming that the population prevalence changes at the same rate as deaths, the graph below shows how the ill population might be expected to decline with time.

Estimated population prevalence of people actively ill with COVID-19

The coarsely dotted red line is based on the central projection from the first two graphs. The lower dotted red line is based on the more optimistic projection in the graphs above. Based on these slightly optimistic projections we expect:

  • Around the start of June, the population prevalence should be just less than 1000 per million.
  • Sometime in August we can expect the population prevalence to have fallen by a further factor 100 to around 10 per million.
  • At the start of the school term in September, the population prevalence might possibly be as low as 1 in a million.

These very low levels of population prevalence still hold the possibility for viral growth and so social distancing measures would still be required.

Additionally, as international travel resumes, new sources of viral transmission will fly into the country

But at these very low levels, the severity of restrictions on schools and large gatherings could be much more relaxed, especially if a strong contact tracing service was available at that time.

In the next article I will look at where the virus will go “In the end”!


5 Responses to “COVID-19: Day 142: Population Prevalence Projections”

  1. Simon Duane Says:

    Hi Michael
    I just caught up with the independent SAGE group’s zoom call today, responding to questions from public, journalists, etc.
    I was very impressed with their explanations, including of why even delaying the reopening of schools to mid June would be a great help.
    There was even a good, if brief, explanation of how we might reasonably look forward to some relaxation of the current social distancing regime we’re under.
    Available here in case anyone missed it.
    Best wishes

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