We revisit the spread of COVID-19 in New Zealand in early 2020.
The object is to apply the exercise below to the current 2021 Delta outbreak in New Zealand.
If you have not done this exercise, please do it now:
To estimate the total number of cases, we first have to consider when we may have reached the midpoint.
Even if the midpoint is assessed accurately, it is not just a matter of doubling the number of cases to estimate the total number of cases:
- The number of cases usually declines far slower than they have risen
- There is often a very long tail- at least add 10% for the tail alone
Let’s look again at the spread in NZ in 2020. You may like to use a sheet of A4 paper to cover at least the bottom half of your screen (and scroll down the screen) to cover future cases as you look at the graphs (so that you are seeing the data day by day).
The object is to apply this exercise to the current outbreak. How many cases do you think there will be in this 2021 outbreak?
Try not to look ahead- Try to look at one day at a time as if you were there at the time.
First lets look at this graph:
Do you think we have reached the midpoint? How many cases do you think there will be?
This pdf has one more day:
Now look at this version:
Look at the highlighted numbers in the Pairs column. This PDF may be easier to read:
We have highlighted 819 to 965 and 1355 to 1505.
The total number of cases in the initial 2020 outbreak was 1504.
Do you think 1355 to 1505 is a good range estimate?
We may need to add on 10% to the higher end of the range if it appears the outbreak may have a very long tail.
Alternatively maybe just make the top end the midpoint by adding on the amount of the difference. i.e. make the range 1355 to 1655 so that the midpoint is 1505.
What do you think?
The midpoint may be in a trough if there is more than one peak.
In the pairs column the numbers are the sum of the two numbers in the cases column. e.g. 819 = 368 + 451.
We will not know if we may have reached a midpoint for some days later.
Therefore we will have these numbers by this time.
We highlight the Pairs for all potential midpoints as the spread progresses.
We would have done the same again if the second peak had become larger.
We can do the same for the 2021 Delta variant outbreak once we may have reached a peak.
Here are some PDFs:
For all 2020 data in this outbreak see: