Ashley Bloomfield looks on during a press conference at Parliament (Hagen Hopkins/Getty)
Ashley Bloomfield looks on during a press conference at Parliament (Hagen Hopkins/Getty)

The BulletinOctober 19, 2021

Ongoing mysteries of the delta outbreak

Ashley Bloomfield looks on during a press conference at Parliament (Hagen Hopkins/Getty)
Ashley Bloomfield looks on during a press conference at Parliament (Hagen Hopkins/Getty)

Once a footnote in daily health updates, the number of cases that can’t be linked to the greater outbreak has grown worryingly in recent weeks, Justin Giovannetti writes in The Bulletin.

The mystery count is rising rapidly. One of the numbers that underlines the scale of the delta outbreak and the challenge faced by contact tracers is the number of “unlinked cases” reported daily. Cases that scrutiny and contact tracing simply can’t link back to the greater outbreak. When the chains of transmission are well known and the tentacles of cases can be linked, the number of unlinked cases should head back to zero over time. Instead, it’s soaring.

Just over a month ago, the ministry of health began reporting how many cases remained a mystery across the previous fortnight. When Auckland was in level four lockdown, the number was hovering around a dozen per day. The number increased and dipped as new unlinked cases were detected and older ones were solved. However since the move to level three, the number of mystery cases has shot up. As of yesterday, the tally sits at 140 over the past fortnight.

What does this mean for the delta outbreak? Dr. Dianne Sika-Paotonu, an immunologist from the University of Otago, spoke with The Bulletin about what the mystery cases mean:

“Each of these cases represent potential transmission chains that are unknown and are potentially indicative of delta continuing to spread in our community. These mystery cases could have been passing on the virus and infecting people in the community without their knowledge. We continue to see these appear despite the higher restrictions,” she said.

That likely means that there’s a gap between the number of people with Covid-19 in New Zealand and the number of cases reported every day. Each mystery case could speak to a larger unknown chain of transmission.

“We may have reached the point where people are too afraid to come forward to get tested and that might mean the number being reported at 1pm doesn’t truly reflect the precise number of cases out in our communities,” Sika-Paotonu added.

It’s already impacted contact tracing. Stuff reports that the ministry of health has stopped tracking sub-clusters in the outbreak because the growing number of mystery cases has made it unwieldy. There were more than 30 sub-clusters when the count stopped, and nearly half were no longer linked. According to the ministry, the focus has shifted from trying to create epidemiological links and simply identifying, testing and isolating close contacts.

Mystery cases are also at the centre of Waikato’s lockdown. While Northland is headed back to level two, parts of Waikato will remain in level three and face more restrictions than Auckland until at least the end of the week. While newly reported cases are “broadly linked” in the area, the prime minister said yesterday, an ongoing drip of mystery cases in Waikato has raised concerns. That’s the issue with unlinked cases: if they remain a mystery after a few days, they might indicate that you don’t know as much about an outbreak as you’d like.


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