Explained: Why the timing of Omicron's arrival matters

New Zealand is preparing for an avalanche of Omicron cases. When we do confront the new variant, how much does timing matter? Jamie Morton explains.

What would an Omicron outbreak look like?

That's precisely what modellers are busy trying to calculate right now. But it's clear the inevitable wave we're facing will be both large - and swift.

Te Pūnaha Matatini's Professor Michael Plank said it was possible we may face an outbreak big enough to peak with tens of thousands of new daily cases.

It may kick off much as Auckland's Delta crisis did, with an unlinked community case pointing to a hidden iceberg of infections, but escalate much faster.

Across the Tasman, it took just a month for new daily cases in New South Wales to swell from a few hundred to nearly 40,000, forcing the state to bring public health restrictions back.

That owed to the sheer speed at which the variant can spread: one recent Danish study found it to be 2.7 to 3.7 times more infectious than the Delta variant among vaccinated and boosted individuals.

Plank said states like Queensland, which just recorded nearly 16,000 new cases, and South Australia could act as good guides of what to expect here, as they had high vaccination rates but hadn't seen the Delta surges their neighbours had.

But there were steps that could soften the blow.

One was vaccination coverage: while around 93 per cent of eligible New Zealanders have received both doses, wider uptake of the booster shot was also critical.

While it wasn't yet clear how well the booster shot prevented transmission of Omicron, there was now plenty of data to show it greatly reduced the risk of severe illness.

UK monitoring showed that, at about 20 weeks after the second dose, effectiveness against symptomatic disease was only around 10 per cent with Omicron. But at two to four weeks after the booster dose, that protection rose to 65 to 70 per cent.

Around 830,000 Kiwis have received booster doses since 1.2 million people became eligible this month.

"It's a big unknown at the moment as to what level of coverage we'll have when Omicron does start spreading here," Plank said, "but the more people who are boosted, the more we can bring case numbers down".

Our overall lack of exposure to the virus was also important.

By contrast, in the UK, which has high vaccine coverage, high booster uptake, and almost two years of exposure to circulating virus, about 97 to 98 per cent of adults test positive for Sars-CoV-2 antibodies.

Even so, Britain logged a daily high of more than 180,000 cases early this month.

Otago University virologist Dr Jemma Geoghegan noted that New Zealand's child vaccine had only just gotten underway - and nearly 220,000 unvaccinated teens and adults were also vulnerable.

Around 830,000 Kiwis have received booster doses since 1.2 million people became eligible this month. Photo / Supplied

"While kids have always been the least likely to have severe disease, looking overseas, the vast majority of people who end up in ICU are unvaccinated – and I'd say they're the most vulnerable group here."

As well, the same health inequities seen over Auckland's Delta outbreak – where Māori and Pacific populations were hit disproportionately hard – could again feature in an Omicron wave.

Another factor was what health interventions officials tried to block it with.

As director-general of health Dr Ashley Bloomfield has acknowledged, the traffic light system that's performed so well against Delta may now need a change-up – and the outcome of a Cabinet meeting discussing current settings will be announced tomorrow.

Bloomfield has indicated that more restrictions were likely to be put in place as the country.

Working from home and lowering the number of people allowed at gatherings were all back on the table, but he said the health response wouldn't necessarily be around more lockdowns.

With Omicron's potential to quickly overwhelm our test-and-trace regime, Otago University experts have suggested pivoting to a mitigation approach of "flattening the curve" to protect the health system.

University of Auckland senior lecturer Dr David Welch said Australia hadn't done a good job at using such measures against Omicron.

"They've really gone for a 'let it rip' approach – so I think New Zealand does have greater ability and social licence to control it a bit better than Australia has, and hopefully take the edge off."

Does timing matter?

Omicron's lightning-fast march across the globe – it could well infect half the population of Europe within two months – showed that its transmissibility didn't solely depend on inherent seasonal factors that influenced other respiratory viruses.

"Most respiratory infections have their peak incidence in winter, at least in temperate countries like New Zealand and the UK, hence terms like 'seasonal flu'," Otago University epidemiologist Professor Michael Baker said.

"But pandemic respiratory infections can strike during any season, as we have seen with the rapid spread of Omicron in summer in a number of Southern Hemisphere countries such as Australia and South Africa."

Director-general of health Dr Ashley Bloomfield has acknowledged the traffic light system that's performed so well against Delta may now need a change-up. Photo / Supplied

However, he added, Omicron was still somewhat easier to tackle in summer, as there was slightly greater potential to slow spread with measures like encouraging people to shift activities outside, or open windows.

The biggest headache with battling Omicron in the colder months, of course, was that health services would be already stretched dealing with winter ills like the flu or RSV, which caused a national crisis last year.

"This winter, New Zealand can expect to see more severe respiratory infections as several of the common respiratory viruses - notably influenza - have been largely excluded for the last two years, so immunity to them is likely to have declined."

While around two thirds of nationwide ICU beds were already occupied – and only a few of them with Covid-19 patients – that capacity could be surged to around 550 ICU-capable beds, the Ministry of Health told NewsHub last week.

Baker expected the country would probably achieve "peak immunity" to Covid-19 at some point in the next three to four months, immediately after a high proportion of adults had received a booster, and young children were fully vaccinated.

"Arguably, that would be the timing when New Zealand would be best placed to manage an Omicron outbreak with the least health impacts."

That's why, despite it being further from the very warmest time, Baker and colleagues have been pushing the Government to try to delay Omicron's arrival until at least March.

"We can also use this time to get other measures in place including supplies of rapid antigen tests, higher quality face masks, and better protocols for schools, workplaces and healthcare services."

Otago University epidemiologist Professor Michael Baker. Photo / Supplied

After calls to "turn down the tap" on overseas arrivals and lower the case load at the border, the Government last night put a halt on releasing more MIQ rooms.

Will we be dealing with an outbreak before winter anyway?

Baker said it was highly likely New Zealand would confront Omicron well before winter.

It could happen when New Zealand relaxed its border controls – as had been planned after February – or indeed at any moment, if Omicron slipped into the community via MIQ, airport staff or air crew, or a sea port.

That danger was highlighted yesterday, when an airport worker tested positive despite having received both vaccine doses and a booster, and also by the handful of other Omicron scares recorded since Christmas.

With another 56 new cases detected at the border today – the vast majority, if not all, likely to Omicron – an outbreak-causing failure might occur within weeks, or even days.

An analysis of previous breaches indicated a failure occurring with every 100 to 200 cases in MIQ – although that risk would have changed with higher vaccination levels in travellers, and the more infectious nature of Omicron.

There's been a ten-fold increase in cases at the border compared to December.

Currently, the seven-day rolling average of border cases is 33.

As Baker and colleagues wrote this week: "While not all of these failures will result in a community outbreak, a certain proportion almost certainly will."

And if an outbreak did happen now, Welch said we'd at least have the advantage of the virus having a lower effective reproductive number (Re) - that's the number of secondary infections caused by one case in a population with mixed susceptibility – than in winter.

Whereas Omicron's current Re might sit at around two, it could be three or above over winter.

"That partly might be why we're seeing so few cases right now. Yet, by the same token, New South Wales and South Africa are roughly in the same seasons we are, and they've had very large outbreaks."

In the meantime, Welch said New Zealand had an opportunity to prepare.

"I think there needs to be some urgency in getting those boosters out and getting children vaccinated."

He stressed that, while Omicron infections appeared to be milder at an individual level, it was the variant's systemic risk that made it such a threat.

"Everyone becomes at much greater risk if the hospital system stops working."