A new community case in Auckland has been confirmed as the Delta variant and four others are Covid positive. Read Marc Daalder's analysis of the Delta variant, and why the luck we've had with transmission of the original coronavirus won't hold.
ANALYSIS: The Government has issued a number of stark warnings in the past week about the danger that the Delta variant poses to New Zealand.
Covid-19 Response Minister Chris Hipkins took a moment during his weekly vaccine update to say that a Level 4 lockdown would be on the table if Delta was detected. And Jacinda Ardern cited the variant's ability to overwhelm contact tracers as one of the reasons to extend the time between vaccine doses so as many people as possible get partial protection in the coming weeks.
In part, this is because the raging outbreak in New South Wales, despite the layering of heavier and heavier restrictions over the past two months, has reminded ministers that New Zealand is not immune. In part, it's a recognition of speed at which a virus that the US Centers for Disease Control (CDC) says is two to four times more infectious than the original coronavirus would spread in a partially vaccinated population.
And, increasingly, the Government's grim predictions come as it finally grapples with the reality that the coronavirus' odd transmission dynamics are no longer in our favour. Where luck might have once been on our side, the Delta variant has changed the game.
Luck and over-dispersion
A common criticism of New Zealand's Covid-19 response is that it was only possible through good luck.
Certainly during the first wave of the pandemic, New Zealand was lucky in that our outbreak started well after many others, allowing us to determine which of the approaches tried overseas was most likely to succeed.
Since then, however, we have benefitted more from the hard work of health workers and a frank appraisal of the odds. That's different from luck - it's an acknowledgement that, for the past year, the odds have been in our favour.
Take the Covid-19 case who ate at a crowded Wellington restaurant and infected no one. Or the woman with Covid-19 who took a road trip around Northland but didn't transmit the virus to anyone, not even her husband in the passenger seat. Or the man who broke the rules after the first of Auckland's February lockdowns, going to the gym and to university while infected but again, not passing it on to anyone else.
Or the Sydney man who managed to generate 2500 close contacts over a weekend in Wellington, but only managed to infect his partner after they returned to Australia.
We're still playing with loaded dice, but this time the odds are no longer in our favour.
None of this was luck in the sense of an unlikely occurrence coming to pass. It was simply a result of one of the most important traits of the coronavirus, one of the top reasons it is such a "tricky virus".
That trait is called overdispersion. In plain English: Most Covid-19 cases infect no one else. Some pass the virus on to just one or two other people. And a minority (about 20 percent) are responsible for the vast majority (about 80 percent) of onward transmission.
That's why a single case making it through the border is more likely to fizzle out than start an outbreak. It's also why an outbreak has often needed to kick off with some sort of super-spreader event to get traction, or else even a handful of cases might diminish to zero within a couple of weeks. Remember that the initial outbreak in March and April was spurred by super-spreading events: A St. Patrick's Day party in Matamata, a wedding in Bluff, or a cattle conference in Queenstown.
Delta dynamics
One of the most concerning things about the Delta variant is that it appears to change this dynamic. In other words, the odds are no longer in our favour.
While there still appear to be cases that don't pass on the virus - remember the Sydney man infected only his partner, and he was a Delta case - that now makes up a smaller proportion of cases. And those who do pass it on are now likely to do so for more people. For the 20 percent of Covid-19 cases that make up 80 percent of transmission (a rule of thumb that may still be in effect), they are each now likely to infect between twice and four times as many people as they would have before.
"You've still got that variability, you've still got a few people that spread it a lot more than others, that's shifting in the wrong direction," said Michael Plank, a professor of mathematics at the University of Canterbury and a disease modeller at Te Pūnaha Matatini.
"Delta just stacks those odds against you more than they would have been with previous variants. With the original variant of Covid-19, you were sort of reliant on a super-spreading event early on to kick an outbreak off. If you managed to avoid a super-spreading event, chances are you would get away with it.
"Whereas with Delta, that's not really the case. It can get going without necessarily needing a super-spreading event. You can still get a super-spreading event and that will just make it go that much more quickly. But there's a higher likelihood that the outbreak can establish itself without needing one."
Moderate measures which would have once suppressed an outbreak of Covid-19 are no longer up to the task. Level 2 limits the damage a super-spreading event could do by capping gatherings at 100, while Level 3 reduces super-spreading likelihood to near-zero, with no more than a handful of people gathering together in any one workplace or home.
But New South Wales shows the limits of that approach: The virus now appears to be spreading at pharmacies and gyms and supermarkets. Once it was able to establish itself, it could no longer be extinguished through measures that reduce gathering sizes alone. So long as bubbles are intermingling, the Delta variant will find a way to jump between them.
"When Delta gets into a household, it's turning out to infect the majority of people within that household. That just means it's got a lot more chances to then spread onto the next house, and so on, and keep going," Plank said.
What does this mean for New Zealand?
For starters, we need to redouble our community testing and contact tracing efforts.
According to modelling by Te Pūnaha Matatini, an outbreak of a variant as infectious as Delta is likely to be caught at less than 100 cases if 15 percent of symptomatic Covid-19 cases are tested. Even at 50 or 75 cases, Delta would be difficult to corral, but we're only hitting the 15 percent testing target about half of the time, according to data from Flutracking.net.
On scanning, too, rates are far lower than their September 2020 peak, showing that many people are not scanning as much as they once were.
The worse we are at scanning and testing, the greater the restrictions we will need if we do find a Delta outbreak.
Where before a single case of Covid-19 with a known origin might not be cause for worry, Delta is a game changer. Hipkins now says that, if faced once again with the scenario of the Sydney man who had visited so many venues in Wellington, he might have taken a harsher approach than escalating the capital to Level 2.
Every case of the virus must be treated like a potential super-spreader – and a potential super-spreader could now lead to four times as many cases as the original virus. By setting expectations around the possible need for a return to Level 4, the Government has shown it is treating Delta with the right level of caution.
We're still playing with loaded dice, but this time the odds are no longer in our favour.
