Covid 19 Delta outbreak: Modeller answers five key questions about case trends

Why are Auckland's Covid-19 case numbers appearing to come down? And what trends can we expect to see once the city, and the rest of the country, moves into the traffic light system? Covid-19 modeller Professor Michael Plank, of Te Pūnaha Matatini, answers five quick questions.

We're seeing case numbers in Auckland decline. How significant is this drop, in the context of the wider outbreak?

I wouldn't call it a sharp drop. I think we've seen case numbers levelling off, and now, maybe it's the start of a gradual decline.

The numbers have bounced up and down a bit, but the trend is starting to look like it is downwards.

That's down to a combination of two things.

One is vaccinations, and the other is that restrictions are still in place in Auckland - meaning a lot of places are still closed, and many people are working from home.

The vaccine, in a sense, means the virus hits a dead end more, or finds it harder for it to find new susceptible hosts.

At the same time, if we simply did away with restrictions and masks, and went back to life as usual, I think the virus would still be able to spread reasonably rapidly.

At the moment, the vaccines are winning and it's great that cases have levelled off and have started to come down.

But we're about to significantly ease restrictions with the traffic light system.

In a hypothetical scenario where Auckland's restrictions stayed in place, what might happen with case numbers?

We're still vaccinating people, so that will gradually bring the R number [the average number of cases caused by a single infection] down.

It's just under one at the moment, and that value would come down over time, and the rate at which cases are declining would get steeper.

If it stayed like that, for long enough, we would be back at elimination.

But that's probably not realistic, given people are sick of being locked down and you can't keep lockdowns in place forever.

So, what can we expect to see when the traffic light system kicks in?

What the models suggest at the moment is that the restrictions that are in place in Auckland are reducing the R number by about 40 per cent.

So that suggests, if we got rid of all restrictions, that R number would rise to about 1.5.

But we're not going to do that, because we know we'll still need masks, and we have measures like capacity limits under the new traffic light system.

And, of course, we've also got vaccine passes. I'd expect the R number will end up somewhere between one and 1.5 - but it's really guesswork as to exactly where in that range we land.

That said, there's a big difference between the two: at 1.5, case numbers will rise fairly steeply, whereas if it's closer to one, then case numbers will be pretty flat.

This is going to be crucial in determining what happens next in the outbreak.

And what about cases on a national scale?

A lot of that is going to depend on how people respond.

For instance, in the UK, they got rid of all restrictions – but a lot of people have continued wearing masks, and working from home.

There's a lot of precautionary behaviour that's stuck, even when it hasn't been required anymore.

Covid-19 modeller Professor Michael Plank, of Te Pūnaha Matatini. Photo / Supplied

We know that mask wearing can have quite a large effect and that's going to be a factor here too.

If everyone goes out partying the minute that restrictions are eased, that could cause a spike in cases.

But if people do remain a bit more cautious, then that will help keep case numbers down.

We also need to consider that things are a bit different over summer.

Obviously, we have a lot of things changing at once – and the Auckland border lifting will lead to more cases cropping up around the country.

Yet, we're also going into a period where schools are off, a lot of work places are closed, and people are outdoors a lot. That will make it harder for the virus to spread.

So, it's possible that case numbers stutter along and don't really do anything dramatic over summer, although we can't conclude that the virus simply won't spread.

When schools go back, however, things could change and cases could climb quite rapidly again.

If we started vaccinating five to 11-year-olds, that would make a difference.

What other trends are you watching out for?

Whether the virus manages to establish significant outbreaks in other parts of the country.

Once the Auckland border lifts, I think we'll see cases scattered around the country - that's inevitable - but whether they're able to establish will be interesting to see.

We know that areas with low vaccination rates are at higher risk, and it may be that the virus actually finds it quite hard to establish in those high-vaccination areas.