Coronavirus Covid-19: The Three Scenarios Auckland Faces

What does the Mt Roskill “mini pool” mean for Auckland and New Zealand Covid-19 risk?

It is unclear whether we see the case numbers trending strongly upward, downward, or continuing in a trickle that gradually falls over weeks.

Health Minister Chris Hipkins was optimistic when speaking to the media today.

“We are likely to see more positive cases from this new subgroup, but we seem to have done it before, which means that at this point, we are not seeing other cases associated with the subgroup appear. They are not identified contacts yet.”

But when Te Punaha Matatini modelers reviewed the situation this morning, there was still not enough data to make firm estimates.

“We have looked at the numbers in the cases and came to the conclusion that it is too early to tell,” said University of Canterbury mathematician and modeler Professor Michael Plank.

“It all comes down to a combination of a short time at level two and a small number of cases.”

This is what we know

The Auckland group now has 173 cases, 43 of which are connected to a “mini-group” surrounding Mt Roskill Evangelical Fellowship Church.

A subgroup within that is linked to a funeral on September 2 and visits to the home of the bereaved.

Two of the four new cases today were part of the subgroup, which now has 24 cases.

“From what we can tell, they did not know that they had been infected and were incubating and spreading the virus at the time,” the Health Ministry said in a statement.

There are also 101 close contacts associated with this subgroup.

The subgroup was the only additional tentacle connected to the church group, but it already includes a student from St. Dominic Catholic School, a Northern Express bus driver, and several other workplaces.

Discounting cases related to the Evangelical Fellowship Church, the number of community cases in the last eight days is: 0, 0, 1, 0, 1, 0, 1, 0, 2.

As of 8 a.m. today, the labs had logged new tests for 64 percent of the congregation, or 213 of the 332 people involved.

But, because Auckland remains in a generally open level 2.5 environment, it is difficult for modelers to identify the most important basic breeding number of the virus, or the number of other people that a positive case can infect.

New Zealand’s first block pushed that number below one, putting the country on the road to elimination.

However, modelers can talk about the general scenarios that Auckland might face.

A strong uptrend?

If the case numbers start to climb rapidly, it still goes back to whatever that reproduction number is, Plank said.

“If people pass it on to more than one person, the trend will be upward. At first, it could go up slowly, but more than likely it will get faster and faster over time,” he said.

“So that comes down to how quickly we’re reaching them with contact tracing, and if we do it fast enough, keeping people isolated could be enough to prevent them from passing it on to more people.

“But if it doesn’t happen fast enough, chances are, being at alert level 2, people are moving around and not knowing they have the virus, because they may not have symptoms yet.”

In addition to the fact that there may be other “tentacles” that we are not aware of, Plank said there was also “delay” to consider: The average incubation period for Covid-19 generally lasted more than a week.

“We’ve only been at level 2 for a week and a half, so it takes a week and a half to two weeks to start seeing cases show up in the reports.”

Add to that the fact that it also took time for people to get tested, and for those tests to be processed and the results passed on to officials.

“And because some new cases have turned up recently that weren’t initially traced contacts, for example we had the bus driver and obviously the school students, it looks like the tentacles have branched out a bit from this. Mini -cluster.

“It suggests that we have not reached the limit, and we are still a step or two behind, which means there is still a danger that it can take off again.”

A strong downtrend?

This was the least likely scenario, but chance was still a factor.

“At this stage of an outbreak, with a relatively small number of cases, many of them come down to sheer luck.

“If we could imagine a world where the Mt Roskill mini-cluster did not start, we would now be reduced to zero cases, so it is unfortunate that this has happened.

“But what happens next could be due to good or bad luck. Whether an infected person took a bus, went to class with other students, or just went and did what people do on a daily basis, all of that matters.

“Still, I wouldn’t expect it to go away completely in the next few days, given the few cases we already have.

A gradual relaxation?

“We could be more expecting this to potentially rumble for a few more weeks,” Plank said.

“But if we can’t contain it, obviously there is also a danger that it could start to escalate.”

There was a particular risk that the virus could reach a crowded school or workplace, potentially leading to a large outbreak.

“But all of these things start gradually, so even if it starts to go up, don’t expect it to go that fast.

“It may just decrease and then increase very gradually; the nature of exponential growth is that it gets faster and faster over time.

“This is the situation we are trying to avoid.”