Covid-19 NZ: Explaining what is going on with the daily case numbers –

Posted: September 1, 2021 at 12:21 am

There had been a steady rise in cases since lockdown. This caused particular angst on Saturday as the numbers were expected to peak by then. But Sunday came along and 83 cases were announced, pretty much the same as the day before. Keith Lynch looks at what is going on.

We are in a strict lockdown and the reason is simple: it is all about pushing down the R number of Covid-19. This is the average number of people that one infected person will pass the virus on to.

If this number is above 1, the virus is spreading, if it is below 1 the virus is dying out. On Sunday, Director-General of Health Dr Ashley Bloomfield said models suggested the R number might now be about 0.8.

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Michael Plank, a professor at the school of mathematics and statistics at the University of Canterbury, and principal investigator at Te Pnaha Matatini, told Stuff last week he believed the R number for Delta was about 6 or more. For some context, the first Covid-19 variant to emerge had an R value of about 3.

There is something else about the R number that is worth going back to. It defines whether the virus is growing exponentially or not. On Wednesday, Bloomfield struck an optimistic note, stating Covid-19 case numbers were not increasing exponentially.

It is important to note that there are degrees of exponential growth. As my colleague Charlie Mitchell points out, if the R number is above 1, the spread is exponential.

Say there are 10 cases of Covid-19 and the R number is 1.2. Now get out your calculator and multiply 10 by 1.2. You will get 12. Now multiply 12 by 1.2. Keep multiplying the result by 1.2 and after enough cycles you will see more dramatic increases. This is technically still exponential growth.

Nowhere near as dramatic however as when there are 10 cases of Covid-19 and the R number is 6. Multiply 10 by six. Multiply 60 by six and continue. After a few cycles, you will see staggering numbers.

There are two groups of people that will define the outcome of this outbreak.

The R number in these two groupings is, and was, very clearly different.

The good news (and that term is very relative here) is that 70 of Saturdays 82 cases were linked to the largest cluster. And it appears most cases are now within the households of infected people.

If cases remain almost all within Group 1, that would be great. It would mean the lockdown is working keeping the rest of us walled off. It would lead to case numbers plateauing off over time, simply because there would not be anyone else within Group 1 to pass the virus on to.

On Saturday night, Plank posted on social media pointing out: there is a downward trend in the per cent change in average daily cases. This could suggest the R number within Group 1 could well be falling.

This trend continued on Sunday. But it is important to add a caveat, as Plank did: you should not read too much into one or two days.

Mark Mitchell

Prime Minister Jacinda Ardern said New Zealand is not running out of vaccines.

Some believe the case numbers in Group 1 may have already peaked, essentially knocking its R number under 1. This may be because of a lag in reporting of positive test results, particularly in those who caught the disease just before lockdown.

It would be very bad if the R number was above 1 in Group 2.

On Sunday, Prime Minister Jacinda Ardern said there had been transmission of Covid-19 among staff in four essential workplaces. This is clearly worrying and something to keep a close eye on.

If the virus is spreading, or starts spreading, among essential workers or those not adhering to the lockdown, then we have a real problem. It means the lockdown moat protecting the rest of the population has been breached.

(Obviously the more people in Group 1 that catch the virus, the more chance there is of a leak into Group 2.)

This is why University of Auckland Professor Shaun Hendy, another Covid-19 modeller, suggested that if this is the case the Government may need to take another look at what businesses are allowed to open at level 4 if the case numbers continue to rise. It is why Ardern suggested on Sunday that her Government may need to tighten level 4 up even further.

If the daily case numbers continue to shoot up this week, it may be that basically all of Group 1 has been infected and the virus has leaked into Group 2. If they plateau, that is a positive.

A few important things to finish: we should not read too much into one days numbers Saturdays or Sundays. We will learn a lot more in the next seven days.

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Covid-19 NZ: Explaining what is going on with the daily case numbers -

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