Expert disease modeller Shaun Hendy says his best guess is that the Delta outbreak is currently larger than last year’s August outbreak was at detection, Marc Daalder reports

Delta may have been spreading through the community for two weeks and there might already be 100 cases out there, one of the Government’s top advisors says.

Shaun Hendy, a University of Auckland physics professor who leads the Covid-19 modelling effort at Te Pūnaha Matatini, told Newsroom his team is just beginning to get insights from models run on the latest Delta case.

“At this point, our best estimate would be, potentially 100 cases and around about two weeks of spread,” he said.

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That’s a more up-to-date figure than the one provided by Director-General of Health Ashley Bloomfield at Wednesday’s 1pm press conference. Bloomfield had said any outbreak was likely to involve between 50 and 120 cases at the point at which it was detected. This was based on a weekly situation report provided by Te Pūnaha Matatini, examining the potential size of an outbreak if a single community case was found.

Hendy’s estimate takes into account recent wastewater test results and vaccination in the Auckland area.

“If you compare to, say, the Auckland cluster last year, I think by the time we detected it, there were 50 to 60 cases active at that point. Here, we’re probably looking more than that,” he said.

There are a few things that could affect this estimate. On the one hand, Devonport is less dense than South Auckland, where last year’s outbreak took place. On the other hand, the locations of interest we’ve seen so far – churches, crowded pubs, casinos – are ripe for transmission and even super-spreading events.

The big game changer would be a link to the border. That would give experts an idea of how long the virus has been in the community and therefore how many undetected chains of transmission are likely to be out there. Without that link, however, the estimates err on a few weeks of transmission and up to 100 cases currently in the community.

Hendy said the new cases reported today are not necessarily relevant to the modelling, unless one of them is an upstream case (meaning the Devonport man’s infection is the source of other infections rather than the other way around). 

“The big thing that makes a difference is if one of those people is an upstream case and we get closer to the border. Otherwise those specific details we wouldn’t necessarily try and capture,” he said.

“It’s great that we’re finding them, but there are things we’re probably missing as well. And if we just fed those things into the model, it might bias things. The big thing for us is establishing that length of the chain of transmission.”

One piece of evidence capping the likely size of the outbreak is the negative wastewater test results from August 11. While a negative result doesn’t rule out transmission, a large outbreak involving hundreds of cases would probably have shown up in those tests.

“It gives us a little bit of confidence that we’re not looking at the really extreme upper range. That’s something in our favour.”

Modellers will try to estimate the spread of Covid-19 outside of Auckland next. Photo: Marc Daalder

What sets last year’s Auckland outbreak apart in terms of our response? Hendy said vaccination and the use of Level 4 instead of Level 3 are both helpful, while the transmissibility of Delta is a disadvantage.

If his best guess is right and we do have 100 or so cases on our hands, it will take at least a few weeks at Level 4 to return to elimination.

“If we’re looking at the scenario where it’s similar to the Auckland scenario last year where we’ve just discovered the tip of the iceberg, we’re not talking coming out of Level 4 next week. It’s multiple weeks,” he said.

The modelling team’s next focus will be simulating the likeliness of spread outside of Auckland and the appropriate response in the rest of the country.

Hendy said the quick move to Level 4 lowered the likelihood that large clusters will have been seeded across the country by tourists who came in contact with the Devonport man while he was on holiday in the Coromandel.

“There has been enough time – just – so we can’t rule that out at the moment. But that swift move certainly reduces the risk,” he said.

Te Pūnaha Matatini will also look at the possibility of lowering alert levels in the South Island if cases aren’t found there.

“I haven’t got answers for you yet, but this is where we’ll be going.”

Marc Daalder is a senior political reporter based in Wellington who covers climate change, health, energy and violent extremism. Twitter/Bluesky: @marcdaalder

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