Coronavirus: Contact tracing 'glitch' has experts worried COVID-19 cases might have been missed

Infectious disease experts are warning a "glitch" in the latest COVID-19 investigation means Aucklanders should remain vigilant as the city moves a step back towards normality.

The super city left level 3 overnight, moving to level 2 even as the source of the latest cases of the virus remains unknown. Genomic testing hasn't yet linked any of the cases, picked up in Papatoetoe, to a known source in managed isolation and quarantine.

Of the six cases found to date, there's only a single genetic mutation between them.

"The cases are highly likely to be linked, so we're confident we're dealing with a single chain of transmission," Associate Minister of Health Ayesha Verrall told The AM Show on Thursday.

"It rules out the possibility we're dealing with two separate chains of transmission that are unconnected." 

The lockdown this week was sparked by the discovery of three cases at the weekend; the other three are considered 'downstream' cases, having caught the virus via the 'upstream' cases found at the weekend. 

"We probably will see probably another couple of downstream cases - those are cases that picked it up from the original cases, perhaps at the school," University of Auckland disease modeller Shaun Hendy told The AM Show.

"What would be alarming is if we discovered some upstream cases - that would suggest that maybe there is a larger cluster out there."

Shaun Hendy.
Shaun Hendy. Photo credit: The AM Show

With the initial cases having no direct link to the border, that worry remains real. 

"It is a concern that although onward transmission is being effectively managed, the source is still unknown," said Amanda Kvalsvig, an epidemiologist at the University of Otago. "Despite some reassuring results, stepping down alert levels does introduce risk because there's less protection against unknown transmission from potential earlier missed cases."

"This shift in alert levels is not cautious enough from my public health perspective, and also from an economic perspective given that regaining successful elimination is also best for the economy," said Nick Wilson, also an  epidemiologist at the University of Otago. "We still don't have any clear idea how the pandemic virus got through the border and many test results are outstanding." 

The new cases not only had no matches in our MIQ database, they couldn't be matched to any cases overseas either, Dr Verrall said.

"The one glitch is we still don't know the origin of this cluster - that's a little bit unnerving," said Dr Hendy. "It's possible we won't know the origins of this cluster. If they had been able to figure that out, that would have given us a lot more certainty. That's really the one glitch this time around." 

When new cases popped up in August, Auckland was plunged into a three-week level 3 lockdown - this most recent one lasted just three days.

Dr Verrall says the difference now is we have wastewater testing, which would likely reveal any wider spread of infection that hadn't been picked up by the usual testing. 

"One of the key things here is we've had 20,000-plus tests done in the Auckland region since the weekend, and haven't found any unconnected cases to this chain of transmission. We also have the sewage testing which would detect a large cluster, and that remains negative in Auckland. That's really reassuring." 

"I think, with the wastewater testing for example, that's probably unlikely," added Dr Hendy. "We might still see a couple of downstream cases." 

Dr Hendy said the Government was becoming increasingly confident in its contact tracing, despite few Kiwis regularly using the COVID Tracer app - none of the index cases in the latest cluster were using it

With the source still unknown, Dr Hendy says there is still a chance someone out there has the virus and could be infecting others without even realising it. While any undetected spread isn't likely to be widespread yet - wastewater testing would reveal that - it would only take a superspreader event for dozens or hundreds to be infected. 

"With this virus about 80 percent of infections are caused by 20 percent of people," microbiologist Siouxsie Wiles told The AM Show on Wednesday

"That's the worry - if there are some people out there that aren't detected, once we move back to alert level 1 and we're all meeting in large gatherings and things, there's then that potential for a super-spreading event."

Ayesha Verrall.
Ayesha Verrall. Photo credit: The AM Show

Gatherings are limited to 100 people at alert level 2. Last year when Auckland came out of level 3, more caution was applied - the city moving to level '2.5', with a cap of 10 on gatherings. 

Dr Wilson says the Government should have done that again, mandating masks in indoor public spaces and workplaces, saying they involve "minimal inconvenience and could speed up the end of all lockdown measures". 

Dr Hendy acknowledged the Government was perhaps moving quicker that most epidemiologists would like.

"They've followed their own rulebook this week. They've done a lot of thinking about how to handle cases like this. While there are still some things that are unknown, they've not been able to demonstrate there is a cluster out there. So moving to alert level 2, that's basically what that playbook tells them to do."

Dr Kvalsvig said the new mandate to wear masks on public transport was good, and she would like to see "much more emphasis on safe indoor ventilation, and exercising and socialising outdoors", but had confidence the Government knows what it's doing.

"New Zealand  now has a superb testing and contact tracing system, with people working around the clock and great uptake from the school community. We also have a Prime Minister whose understanding of the principles of outbreak control is extremely impressive and on [Wednesday's] evidence, well up to postgraduate level. 

"This ability to ground decisions in evidence and transparency in communicating how decisions are made is a key element of New Zealand's elimination strategy."

No new cases were picked up overnight.