When scientists discovered a remarkable new variant in South Africa just 67 days ago, the world reacted with renewed dread.
As experts discussed B.1.1.529's freakish number of mutations - and its potential to beat vaccines and drive another deadly global wave – New Zealand swiftly joined other nations in blocking travel from the outbreak's epicentre.
Then, as South African doctors reported this hyper-spreading variant appeared to be causing much milder infections overall, the media narrative flipped to giddy optimism.
Would Omicron sweep the planet, write the dominant Delta's obituary, and finally bring the Covid-19 nightmare to an end?
Unfortunately, as the World Health Organisation reminded us this month, Omicron certainly won't be the last variant in this far-from-over pandemic.
In other ways, however, those early Omicron forecasts have been borne out.
Omicron indeed did go on to beat out Delta as the number one variant in many countries.
It's now washing over Europe at such a rate that half its population may be infected within weeks.
Two months ago in England, Delta accounted for 98 per cent of recorded cases. By 2022, it represented just four per cent of them.
Any cases found at our own border are now simply being assumed to be Omicron.
Still, experts told the Weekend Herald that we shouldn't bank on Delta's demise just yet.
"It might disappear from our radar a little if Omicron really swamps it," University of Auckland senior lecturer Dr David Welch said.
"But I don't think we can say it'll necessarily eliminate it."
The Omicron effect
Scientists cite two big reasons for Omicron's rise to supremacy.
One is that it simply spreads faster and further.
Its infectiousness has been put at between two and four times higher than Delta, partly because of its shorter incubation time - estimated at around just three days, compared to Delta's four.
That meant someone could be exposed to the virus, catch it and pass it on before they got a positive test back.
This factor influenced its generation time – or the interval in which infections could double – which has also been placed at only three days.
The faster it took an infected person to become contagious, the faster an outbreak spread, and the higher case numbers climbed: hence the staggering peak projections modellers have suggested for a fast-building Omicron wave here.
The other reason was the built-in machinery that helped Omicron to more easily escape immunity, be it through vaccination or pre-exposure to the virus itself.
While two doses of the Pfizer vaccine have proven impressively effective at blocking symptomatic disease with Delta – something that's helped New Zealand climb on top of its Auckland outbreak – that wasn't enough to stop Omicron.
Given UK data has indicated the vaccine's Omicron effectiveness fell to just 10 per cent four or months after the second dose – but could be restored to up to around 70 per cent with a third – the Government has been pushing to get as many Kiwis boosted as possible now.
"At the moment, we've still got a lot of people with two doses of the vaccine and less people boosted – so from an immunity perspective, we can expect to see Omicron spreading more," University of Auckland immunologist Associate Professor Nikki Moreland said.
"But that doesn't mean Delta is going to go away. We know it's still here and it will still be able to find people to infect, even though we've almost vaccinated our way in front of it."
Of course, those most at risk of encountering lingering Delta, and certainly Omicron, were the unvaccinated.
As at this week, more than 210,000 Kiwi teens and adults hadn't received a single dose.
Dampening down Delta
While researchers are still learning about what long-term problems Omicron infections might cause, successive studies have backed early predictions that a given case was typically less severe.
One pre-print study out of the Imperial College London suggested that, compared with Delta, Omicron cases carried an average 15 to 20 per cent lower risk of any hospitalisation – and a 40 to 45 per cent lower risk of actually being admitted.
Another US study that compared Omicron's surge with Delta's found the risk of Omicron patients needing to go to the emergency department fell from 15 per cent with Delta to five per cent with Omicron, while the risk of being hospitalised dropped from four to two per cent.
Closer to home, Delta cases still made up around a third of those in New South Wales intensive care units this month, yet just a small portion of those found in the community.
Because of that higher case risk, experts have said contact tracers should give high priority to any transmission chains involving Delta.
In reality, that might prove a challenge, Welch said.
"If we're only sequencing one per cent of all cases, and Delta ends up being one per cent of all cases, then the chances of actually sequencing a Delta case is very low."
At the same time, he said, Delta cases were still fairly widespread around New Zealand.
"Even if we have the largest Omicron outbreak anyone's predicting, that still leaves a lot of people in which Delta can carry on spreading."
Otago University virologist Dr Jemma Geoghegan agreed.
"We keep seeing multiple Delta chains sampled every so often, which does suggest there's undetected transmission still happening," she said.
"Unless we went into a lockdown – and we know only something like level four or stricter can help stop its transmission – I'd be surprised if Delta burnt itself out here completely."
Massey University disease ecologist Professor David Hayman said that, while collective immunity was the biggest factor, the way we tried to curb Omicron would also impact Delta.
"So, it'll be interesting to see what happens when we change strategies and enter the next phases the Government's just announced."
There's a further way Delta might be further dampened down – although it's one scientists definitely don't want to advocate.
That's what's called hybrid immunity.
"There's now some data emerging that, if you are vaccinated and have an infection with Omicron, you can end up with broader immunity, that can then help protect against Delta," Moreland said.
But she added that this pattern had only been observed in studies with relatively small numbers, and not in large populations.
And despite some suggestions that New Zealand's lack of direct contact with Covid-19 had left Kiwis with more exposed to Omicron, studies have shown the new variant could also evade immunity from prior Delta infections.
That offered yet more validation to our collective efforts to keep Delta out, and also put us in a better position to confront Omicron, Covid-19 modeller Professor Michael Plank said.
"It's actually one of the biggest advantages that we have now: we're going into a potential Omicron wave, which is still a serious threat, but we're not going into it with full hospitals and an ongoing Delta epidemic."
When it came to modelling the Omicron wave, he and his Te Pūnaha Matatini colleagues were effectively assuming Delta had been pushed to levels so low the variant had only a negligible effect.
"Now, that may or may not prove to be true, but what we've seen in other countries is Omicron has just taken over incredibly quickly."
Notwithstanding Omicron's own threat – the variant is still capable of making people seriously ill, after all, and experts are at pains to point out higher case numbers mean more stress on an already-stretched health system – Plank said a Delta outbreak of this size would be a much bigger headache.
"So, in the sense of hospital capacity, you can afford to have more Omicron cases than you can Delta cases."