Experts say we'll never know just how Wellington escaped a major Covid-19 outbreak. Photo / Mark Mitchell
A string of near-misses with Covid-19 – most recently Wellington's week-long scare - has thrown up some crucial questions about our exposure to the virus and its worrying new variants. Science reporter Jamie Morton looks at three of them.
The scare happened to be New Zealand's first real big-risk brush with Delta – a Covid-19 variant already being overtaken in the risk stakes by the fast-spreading lambda.
Had the virus got loose in a city with few restrictions in place at the time to curb spread, the consequences could have been disastrous.
Here's what we know happened.
An Australian man visited the capital over the weekend of June 19-20, after being infected with the Delta virus variant at the centre of Sydney's outbreak.
He visited nearly 20 locations, including bars, cafes, restaurants and a crowded Te Papa, and several of them for hours at a time.
Any of these busy indoor settings could have hosted outbreak-sparking "super-spreading" events.
The man only learned he was infected when he returned a positive test result, after landing back in Sydney.
While the man's partner, who was travelling with him, later also tested positive, somehow, none of the 2710 other people identified as contacts did.
We also know the man had received one dose of the AstraZeneca vaccine before flying to New Zealand.
With limited vaccine data, we can't say the extent to which that single shot may have lowered his viral load – or how much of the virus he was carrying - while he was here.
Given that the timing suggested the man's partner became infected while they were still in Wellington or on their way home, it was most likely the man had entered his infectious period towards the end of his stay in New Zealand.
Still, experts say that, without being able to step back in time and collect test data, we'll never truly know precisely why Wellington dodged a bullet.
"He was partially vaccinated, which could have played a role, or it could have been he was early on in his infection," said Dr Jemma Geoghegan, an ESR and Otago University virologist.
"Or it could have been that the circumstances just weren't right – although we know he did spend time indoors with other people.
"And from his partner's case, we know that he can transmit. It's a mystery, quite honestly, and we're very lucky."
University of Auckland infectious diseases expert Associate Professor Siouxsie Wiles said the key questions to consider in such cases were how infectious a person was; when were they infectious; and what were they doing at the time.
Because we didn't have much of that information, officials naturally had to err on the side of the caution, quickly raise the alert level, and assume they could be dealing with the makings of a calamity.
"The reason this case was a real worry, was because, if the person had been highly infectious, then, with the kinds of things they were doing, we could have expected lots of further transmission," she said.
"The fact that they didn't transmit, but did to their partner who they were spending a lot of time with, would suggest that their dose - or how much of the virus they were shedding – was low, and it required prolonged contact."
Dr Amanda Kvalsvig, an Otago University epidemiologist, said the experience of the last 18 months had shown that if officials waited passively for an outbreak to declare itself, it was then extremely difficult to control.
"Many people could become ill, or even die, before the outbreak is stopped.
"That's similar to the logic of wearing a seatbelt: in any given car journey, you know that your chance of a major accident is small, but if it happens the consequences could be serious."
Notwithstanding the Auckland August and Valentine's Day clusters, New Zealand has now lucked out with a handful of near-misses – most notably an infected woman who visited 30 places in Auckland and Northland in January.
As with the Wellington case, it also remained difficult to make conclusions around why the other run-ins didn't trigger flare-ups, or draw any links between each of those episodes.
"It could be any one or a combination of factors to do with the individual's own immune response and behaviour, their environment and the public health response," said Dr Rachelle Binny, a modeller at Te Pūnaha Matatini.
Because most people had the highest chance of testing positive about four days after they'd been infected, just before they were most likely to show symptoms, officials needed to detect cases as early as possible.
"Te Pūnaha Matatini is currently analysing New Zealand test data to measure how an infected person's probability of testing positive changes over time, and how this might differ depending on age and other factors."
Yet there were broad patterns we could point to.
In particular, epidemiologists refer to something called the Pareto principle - otherwise known as the "80-20 rule" - that's now a well-established phenomenon in virology.
This suggested that 80 per cent of disease transmission events in an epidemic were caused by 20 per cent of people.
That trend was seen in New Zealand's main outbreak, when one in five adults were responsible for up to 85 per cent of the virus' spread.
In virology, that proportion of people responsible for large amounts of disease transmission is also called the "K number".
Yet the reproduction, or "R0" number – that's the average number of people an infected person passed the virus on to – of the Delta variant has been estimated at five, which was twice as high as the original strain.
So, did a higher "R0" number in turn mean a higher "K" value?
"Delta means that people have an increased viral load, so they're more likely to shed more virus, so I'd assume that the ratio would increase if you had people transmitting to more people," Geoghegan explained.
"But it's still hard to know how exactly those ratios change – maybe you've got more of a 50 per cent chance of transmitting. I just don't think there's an answer yet, and I'm not sure if there will be."
Wiles added: "We know now that with the Alpha and Delta variants, if you've got them, then your whole family, or your close contacts, are much more likely to get it, too.
"The question, really, is whether you correspondingly transmit more in a super-spreader scenario."
Sydney's experience had suggested that might indeed be true: of 30 people exposed to the variant at a birthday party in West Hoxton last month, the only guests spared were the six who'd been vaccinated.
Wiles described the circumstances of any given case – how infectious it was, when it was infectious, and how many places it visited and people it interacted with – as sliders on a scale.
"We have to remember that the sliders are now extending out into the really bad department."
In Wellington's case, she said, the sliders had luckily been set on the low side of the scale.
"So we got away with it. If the settings had been on the high side, then it could have been Sydney, basically."
Kvalsvig said New Zealand could appear to be lucky for a very long time, even when our systems could have been stronger.
A new study she co-authored with other Otago experts, and published online before peer review, found Australia and New Zealand had 32 failures arising from quarantine systems, up to June 15 this year.
Nine of those failures led to lockdowns and Melbourne's major outbreak involved an estimated 800 deaths.
That data, which didn't include Sydney's outbreak, put the failure risk for those transiting quarantine at five failures per 100,000 travellers - or 6.1 failures per 1000 positive cases – and these rates likely would have been lower had all frontline border workers been vaccinated.
In separate modelling, Nic Steyn and colleagues at Te Pūnaha Matatini assessed the border risk from overseas arrivals with New Zealand's 14-day stay in MIQ.
Their model suggested, on average, one case out of every 100 infected arrivals would still be highly infectious when they left MIQ, assuming there was moderate transmission within facilities.
But with no transmission in MIQ, that risk dropped to zero.
"Another way the virus could spread into the community is via an infected frontline border worker, which is why our border workforce were prioritised for vaccination and are regularly tested," Binny said.
"If a vaccinated border worker gets infected, our model predicted around a 12 per cent chance of this resulting in a major community outbreak, but this assumed a less transmissible variant than Delta and that only border workers were vaccinated.
"We're now updating the model to account for more transmissible variants and vaccination of the New Zealand population."
Ultimately, if a case slipped through the net and sparked a community outbreak, the size of that outbreak depended on how quickly we detect it, and how many people were vaccinated.
Yet so far, only about 10 per cent of Kiwis have received both vaccine doses.
For a highly transmissible variant like Delta, and a moderate level of testing, Te Pūnaha Matatini's modelling suggested health officials could be dealing with between 50 and 280 infections by the time a community case was first detected.
"This is nearly three times more cases than if we were dealing with the original Sars-CoV-2 strain," Binny said.
"The chance of eliminating the outbreak before it reached 1000 cases would be less than 50 per cent and it could take a long time.
"By vaccinating more people and doing more community testing, we improve our chances of detecting an outbreak while it is still small and more easily managed by contact tracing or lower alert levels."
Amid the Wellington scare, Kvalsvig and other experts called for a raft of other improvements, including an alert level overhaul to give more clarity around mask use, mandating scanning in places like cafes, bars and gyms, and measures to tighten the transtasman bubble, such as pre and post-flight testing.
The Government was considering several of those options.
"The appearance of new variants and the strong motivations for keeping the bubble open are nudging the outbreak risk upwards," Kvalsvig said.
"Collective immunity from vaccination will help enormously, but we still need to tighten up our baseline protections - the ones that are always working in the background to dampen transmission risk."
That meant wearing masks while using public transport or visiting, health clinics, improving indoor air quality in public spaces, including workplaces and schools.
It also meant staying home when sick, and getting tested if we had Covid-19 symptoms.
"New Zealand has demonstrated that this virus is controllable," Kvalsvig said.
"We may have been lucky, but we can make our own luck too."
Wiles said a crucial way Kiwis needed to play their part was by scanning everywhere they went.
Although the Wellington scare saw daily scan rates more than double from over 415,000 to nearly a million within a few days, by the weekend, numbers had dropped back to around 657,000.
"The Sydney case shows that when we have these travel bubbles, they're low risk, but they're not no risk," Wiles said.
"So we just need to have using the app as a standard thing."
The diver was rushed to Christchurch where the nearest hyperbaric chamber is.