COVID-19 NZ: How many cases and deaths may we expect?

We look at the number of cases and deaths for New Zealand, Australia, and Norway to estimate the possible total number of cases and deaths in New Zealand.

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Update: Proof that 5 April was NZ COVID-19 midpoint.
NZ should not come out of Level 4 Lockdown for 4 extra weeks! See:
https://aaamazingphoenix.wordpress.com/2020/04/07/covid-19-nz-ii-how-many-cases-and-deaths-may-we-expect-when-should-nz-come-out-of-level-4-lockdown/
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In this post we make the following assumptions:

1. Linearity. We assume (as recently) the cumulative number of cases in New Zealand will remain (close to) linear.
2. Midpoint. We assume that New Zealand has reached or is near the midpoint for case numbers.
3. Lockdown. We assume that people in New Zealand will conform to Lockdown Level 4 requirements (and requirements for other levels) and will stay within their bubble with only one designated shopper per bubble. We also assume that New Zealand will not leave Level 4 (or other levels) at an early stage when future case numbers show COVID-19 is not under control.
4. Tail. We assume that the curve will not have a very long tail once case numbers fall to low numbers (but add on 10% just in case to allow for a long tail).
5. Demographical. We assume that for the purpose of providing limits for New Zealand cases, case results for Australia and Norway are appropriate.
6. Immunity. We assume that people who have recovered from COVID-19 will not get re-infected.
7. Asymptomatic cases. We acknowledge that up to 50% of new cases may be asymptomatic but do not have any information regarding the infection rates of COVID-19 from these cases or the length of time they may generate infections. We assume that the Government will have measures (e.g. randomised testing) to identify and control asymptomatic cases.
8. Secondary Outbreak. These estimations apply for the current outbreak only and do not apply to any secondary outbreak even if it overlaps with the existing outbreak.
9. Border Control. We assume New Zealand’s arguably lax border control isolation measures for arrivals from overseas will not cause case numbers to rise significantly or cause a secondary outbreak.
10. Clusters. We assume that there will not be a large increase in the number of clusters (groups where case numbers are greater than ten) and that infections will be contained (remain within the clusters).

We choose Australia for comparison because it is our nearest neighbour, and Norway because it has a similar population to New Zealand (less than 10% greater) and a land size less than 50% greater than that of New Zealand.

Firstly we consider the bar graphs for the three countries:

We note that the above bar chart for New Zealand appears to be bimodal (two peaks)  with a trough appearing on 31 Mar and 1 April.

This may be the midpoint (see later in this post).

We also note that both Australia and Norway appear to have peaked; both graphs now seem to have a downward trend (except maybe for today).

Just before Midday today NZ time, Australia and Norway had the same number of cases.

Country,	Total	Total	Tot Cases/	Deaths/
Other	Cases	Deaths	1M pop	1M pop
Norway	5,687	71	1,049	13
Australia	5,687	37	223	1

We estimate that Australia’s total number of cases may be around 6,000 (excluding 10% for a long tail) and scale the data accordingly.

Country,	Total	Total	Tot Cases/	Deaths/
Other	Cases	Deaths	1M pop	1M pop
Norway	6,000	75	1,107	14
Australia	6,000	39	235	1

We note that the number of cases (per 1 million population) for Australia is 235 cases.

New Zealand has a population of around 5 million people.

Therefore we now scale (x 5) the last two columns and put the results in the first two columns for Australia.

The first row in the table below shows the results (estimate for NZ):

Based	Total	Total	Tot Cases/	Deaths/
on	Cases	Deaths	1M pop	1M pop
Australia	1,176	5	235	1
Australia	1,353	6	271	1

Since 1,176 is likely to be two low (we already have 1106 cases to date, we scale again (the first row) by 15% to get the second row above, assuming that New Zealand case numbers have reached or is close to the midpoint.

This estimate is likely to be low too.

We now consider Norway.

Many of Norway’s daily figures are more than double New Zealand’s (peak over 4 x; number of cases over 5 x ) in its Bar Graph.

So we scale (halve) Norway’s results to get the first line in the table below, then scale again to get 1350 (as we did for Australia) as before giving these estimates for New Zealand:

Based	Total	Total	Tot Cases/	Deaths/
on	Cases	Deaths	1M pop	1M pop
Norway	3,000	37	553	7
Norway	1,350	17	249	3

We think that 3,000 cases may be too high and after seeing that 2700 = 2 x 1350, we scale again (take off 10 %) to get the following estimates for New Zealand:

Based	Total	Total	Tot Cases/	Deaths/
on	Cases	Deaths	1M pop	1M pop
Norway	3,000	37	553	7
Norway	2,700	34	498	6
Australia	2,353	11	471	2
Australia	2,706	12	541	2

The first line for Australia is simply double (x 2) the first line for Australia two tables above.

The bottom line adds 15% to the previous line.

We now have a case estimate range from 1350 and 2700 cases.

Some sources think New Zealand’s number of cases have peaked
(passed the midpoint?).
“New Zealand has 82 new Covid-19 cases – and Director-General of Health Ashley Bloomfield has raised the possibility cases may have peaked.”

“It may well be it [Covid-19] has peaked now.”

“The number of cases does seem to be levelling off,” Bloomfield said.” See:
https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=12322478

Assuming the midpoint (peak) for New Zealand may be around Day 33 (31 Mar) and Day 34 (1 April), we see that the sum of the cumulative number of cases on these dates is 1355, very close to 1350.
You will need to look at the posts for China to see why we do this.

33	31/03/20	647
34	1/04/20	708

If the mid-point has not yet been reached, we see that the total for yesterday and today comes to 2145 cases.

38	5/04/20	1039
39	6/04/20	1106

On day 43, the cumulative estimate from my formulae is just under 1400, and the sum of Day 42 and 43 comes to around 2650 to 2800 depending on the formula.

If the midpoint is around Day 42.5, this is in the correct ballpark.

From the above tables we get an estimate for the number of deaths from 12 to 34 for 2700 cases and 6 to 17 for 1350 cases.

Since New Zealand only has one death presently, it is possible that the number of deaths may be half of the above.

We hope that there are a limited number of future deaths from COVID-19.

We will need to wait to see.

The estimates for the number of cases may turn out to be very light.

A range of 1500 to 3000 cases for New Zealand may prove to be a better estimate.

We adopt the estimated range of 1500 to 3000 cases (after adding 10% extra to allow for the tail).

China has continued to have new cases for over a month since the number of new cases was less than 50 for most days.

South Korea has also continued to have new cases for a similar length of time.

It is therefore quite possible that the number of cases may exceed 3000.

In a previous post I mentioned that 50% of cases may be asymptomatic (have COVID-19) but not show any symptoms. Consequently the real number of cases may be close to twice the official figures. It is not known how many people asymptomatic people may infect. It may be the same as symptomatic people.

We note that reports commissioned by the NZ Ministry of Health have far greater estimates. The reports can be found here:
https://www.health.govt.nz/publication/covid-19-modelling-reports

In a worst case scenario up to 27,600 would be expected to die.

The range for deaths is also given from 12,600 to 33,600 deaths with a peak week 21 weeks after virus introduction.

Excerpts from the reports:

See my post:
https://aaamazingphoenix.wordpress.com/2020/04/01/covid-19-in-nz-did-you-think-my-predictions-were-bad-what-do-you-think-of-these-scenarios-modelling/

Below is the graph of the cumulative number of New Zealand Cases. Fortunately the actual number of cases appear to be heading back towards the red straight line.

My other COVID-19 posts can be found here:
https://aaamazingphoenix.wordpress.com/tag/coronavirus/

Data for my posts can be found at:
https://www.worldometers.info/coronavirus/
https://en.wikipedia.org/wiki/2020_coronavirus_pandemic_in_New_Zealand

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https://guestdailyposts.wordpress.com/guest-pingbacks/