There were 20 new cases of Covid-19 in the community on Sunday. Video / Mark Mitchell / Brett Phibbs / Dean Purcell / Michael Craig
New Zealand's Delta outbreak could total fewer than 900 cases, new modelling indicates, with lockdown squeezing the coronavirus down to levels potentially lower than seen in April last year.
But the University of Canterbury mathematician who's run the latest numbers warns these predictions still rely on Kiwis sticking to their bubble, wearing masks and curbing spread as much as possible.
As of today, the Delta outbreak numbered 801 community cases - 784 of which had been detected in Auckland - with just 20 new ones reported yesterday.
Director general of health Dr Ashley Bloomfield said the falling case numbers showed the country was now "heading in the right direction" - but that this week would be "critical".
Associate Professor Alex James said her modelling had shown the R value - or the average number of other people an infected person passed the virus on to - had consistently been sitting at below the crucial threshold of one for about a week.
"In fact, it's around the same level as we saw in April 2020 at this stage, and lower than we ever saw in August 2020 during alert level 3," James said.
"The model currently predicts the total number of cases in this outbreak to be less than 1000 - and likely to be less than 900."
Source / Associate Professor Alex James, Canterbury University
James' model - a variation of a "branching process" model she developed last year with modellers at Te Pūnaha Matatini to help provide advice to Cabinet - analysed dynamics, such as how long it took for symptoms to appear, and when someone was most infectious, to predict when new cases would occur.
"I fit the output of these dynamics to the daily case data to find the R value, that is how many people, on average, each case is likely to infect," she explained.
"This changes with the alert level, with contact tracing effort and different variants of the virus."
She said a key difference between Delta and previous variants was that the R value didn't stay constant throughout the lockdown period.
"As Delta is so much more contagious especially between household members, you still had high transmission at the start of lockdown, as people brought it home to their families before they even knew they were infected," she said.
"This explains why case numbers continued to rise for so long after lockdown started."
Last April, during the country's main outbreak, the R value dropped below one quickly - and the case numbers started dropping after New Zealand introduced its MIQ system.
In August last year, case numbers also started dropping less than a week after the alert level was introduced.
"With this Delta outbreak, there were about three days at the start of lockdown with a lot of household infections - and the R value was around two or three, so case numbers continued to go up for much longer," James explained.
"After that R dropped to a similar value to the last time we were in level 4 and the case numbers started to turn around.
"This is a huge accolade to the contact tracing teams who have managed to achieve the same transmission levels with a much more contagious disease."
Going by the current trajectory, James expected there might only be a handful of cases each day by the end of this lockdown.
"When you get to low numbers of cases, what matters is the detail of every case – were they a household contact that has been in quarantine, or are they a mystery case and infectious in the community?
"At the end stages, these details matter as much as the daily case numbers."
Importantly, she pointed out that her model couldn't make any predictions about these details - or where the cases might occur.
"As the Delta variant is so contagious coming out of lockdown, even one infectious person still in the community could send us back to square one," she said.
"To stay on track, we need everyone to keep doing what they are doing now, stick to your bubble, wear a mask, minimise contact with other people.
"Without that, we could easily see transmission rates shoot back up to where they were during the first few days of lockdown."
The modelling comes as the Government is poised to announce whether the rest of New Zealand, outside of Auckland, may be able to move down from level 3 this week.
