The threat of new coronavirus variant Omicron is becoming clearer by the day: but what will it mean for New Zealand? Science reporter Jamie Morton looks at the latest evidence – and the three fundamental questions.
With Omicron now busy establishing itself in dozens of countries, Omicron's arrival in New Zealand is just a matter of time.
When it does get here, there's now plenty of reason to suggest it will out-compete the tricky variant we've been battling for the better part of a year, Delta.
Last week, the UK Health Security Agency predicted just that: B.1.1.529, as its coded, wasn't just likely to better Delta at household transmission, but also in secondary attack rates and growth rates.
That key growth advantage could be down to higher transmissibility, increased immune evasion, or just some combination of the two.
While not yet confirmed, early data indicated a greater risk of re-infection – or the ability to get around immunity from prior infection with earlier strains.
There's been a lot of new data for Omicron in the last day, much still preliminary, but here's my summary of the good and bad pic.twitter.com/pL8uiD7hfV
— Eric Topol (@EricTopol) December 10, 2021
Otago University epidemiologist Professor Michael Baker said Omicron's duel with Delta ultimately hinged on its effective reproduction number (Reff) - or the average number of cases caused by one infection in a population with mix of susceptible and non-susceptible people.
That Reff value would be partly influenced by its inherent level of transmissibility in human populations - something measured by the more familiar basic reproduction number, or "R0".
"But its Reff will also be increased by it having ability to partially evade immunity provided by vaccine or natural infection," Baker said.
That meant that Omicron could dominate Delta, even if it wasn't fundamentally any more transmissible - though obviously having the same or greater transmissibility than Delta would help.
"In other words, the increase in Omicron may be largely because of its ability to infect people who are already immune to Delta through vaccination or previous infection."
This happened to be a familiar evolutionary path for respiratory viruses.
"Their success comes more from immune evasion rather than increases in infectivity, and this is similar to what we see with the existing seasonal coronaviruses and seasonal influenza," Baker said.
"We obviously need more data on Omicron to help us understand what evolutionary path it is on."
Hanging above all of this were two all-important questions: whether Omicron would come with different symptoms, and how severe those symptoms would be.
"I think the main confounder with severity, is that most people who are catching Omicron have some sort of immunity already," University of Auckland senior lecturer Dr David Welch said.
"This is exactly the sort of scenario we expected to see: it's not so much the virulence of the virus that changes, but our immunity to it.
"So, basically, everyone who's getting infected at the moment – particularly in Europe, and probably in South Africa – has had some form of Covid before, and they're mounting a decent immune response, and not getting very sick."
Despite promising early signs from abroad - nearly all of the cases reported in the US have also been mild - the answers to those questions would only come after analysis of several weeks' more case data.
In this case, the variant's remarkable transmissibility mattered.
Covid-19 modeller Professor Michael Plank offered some numbers around two hypothetical variants to illustrate that.
One was "variant A" - which spread relatively slowly, with an R value of just 1.1, but carried a hospitalisation rate of 4 per cent - and the other was "variant B", which spread twice as fast, with an R of 2.2, yet was half as virulent, with a hospitalisation rate of 2 per cent.
If we started off with 1000 cases of "variant A", 40 of them would end up in hospital.
After eight generations of transmission, or about 40 days, there would be about 2144 cases, of which 86 would require hospital care.
But, if just 10 cases of "variant B" came in across the border, after those same eight generations, there would be 5488 cases – including 110 hospitalisations.
Yet, just two more generations later, while variant A would have produced about 2594 cases and 104 hospitalisations, variant B would have created 26,560 cases – and 531 hospitalisations.
Another 10 days later, that difference widened even more dramatically, with 126 hospitalisations from variant A, but a whopping 2571 from variant B.
"So, it's clear that while variant A may look worse to begin with, variant B eventually catches up and then quickly overtakes A," Plank explained.
"These numbers are just an example, so how does this relate to Omicron? Omicron cases are doubling every three days in England, 2.3 days in Scotland."
First modeling analysis of potential omicron wave in England.
— Michael Plank (@MichaelPlankNZ) December 11, 2021
TLDR serious possibility of hospitalizations reaching Jan 2021 levels or higher and UK now in a race against time to get booster doses out https://t.co/VWWcjEkhkA
While it wasn't yet known what Omicron's hospitalisation rate really would be, Plank said there was reason to think it could be lower than Delta, given more Omicron cases would be in vaccinated or previously infected people - and therefore were likely to be milder.
"If the doubling time is 2.5 days, halving the hospitalisation rate buys your health system an extra 2.5 days, quartering the hospitalisation rate buys you five days," he said.
"So, although milder is obviously better than more severe, it's unlikely to be a get out of jail free card for Omicron."
Of course, Plank's point rests on the assumption that our vaccines hold up against this fresh threat.
Will they?
Early lab results from one small South African study suggest the Omicron variant may be able to bypass some of the antibody immunity brought about by the Pfizer vaccine.
Researchers studied samples from 12 double-jabbed people, finding a 41-fold drop in antibodies' virus-blocking ability in some of the samples, compared to those generated against the earlier Beta strain.
Omicron mutations also appeared likely to reduce the effectiveness of monoclonal antibodies, which are given as therapy and mimic the immune system's ability to fight off harmful pathogens such as viruses.
Another preliminary German study showed Omicron resistance to neutralisation by casirivimab and imdevimab - the two monoclonal antibodies in the Regeneron drug - alone or in combination.
The Immunisation Advisory Centre's medical director, Professor Nikki Turner, explained that high levels of neutralising antibodies, delivered via vaccines, were important to prevent entry of the virus into the body.
That meant a reduction in levels was likely to particularly affect mild and asymptomatic disease.
Yet, immune response to disease also included cellular protection – or "T-cell immunity".
One reassuring finding in a further, Pfizer-led study was that this T-cell immunity appeared to remain highly protective after a two-dose course.
"This would suggest we can continue to expect reasonable protection to severe disease, which is a vital feature of an effective vaccine," she said.
"This should continue to reassure us of the importance of continuing the heroic efforts of our vaccination programme to ensure as many of our population can access the vaccine as possible."
Moreover, the study suggested a third booster dose of the Pfizer shot may be able to stop Omicron in its tracks.
"A booster dose, or having previously had Covid, appears to be important in restoring much of the reduction in protection to the vaccine," Turner said.
"There is a question as to when the best timing of a booster dose will be, many countries are considering what the best timing for a booster dose would be, and that is a decision that New Zealand is also currently reviewing."
She felt it crucial that New Zealand continued to support people to receive a primary course of vaccination, followed by a booster.
"Further data will accumulate rapidly, allowing New Zealand to make a decision about the best timing of the booster dose that is expected to improve the protection for when we do see the Omicron strain in our community."
Welch expected that, once Omicron did find a foothold here, it would precede a large, fast-moving wave of infections.
Along with a strong border system, testing requirements and top-class surveillance, New Zealand faced this threat with the advantage of a high vaccination rate.
If current momentum continued, the Herald's vaccine tracker projects nearly 92 per cent of the eligible population will be fully vaccinated by this time next month.
Vaccinations of children aged 5 to 11 are due to start next year, and more than 150,000 booster shots have been administered to combat risk of waning vaccine immunity.
Our weakness, however, could be found in the fact, besides children, who are at lower risk, large pockets of vulnerable Kiwis remain at risk.
Only about three quarters of eligible Māori have received both doses of the vaccine – and researchers have noted many of these people live in areas with poor access to healthcare.
As well, hospitals remain poorly prepared to meet an avalanche of cases – and dealing with Omicron might have to happen as New Zealand also tries to manage a nationwide unleashing of Delta.
While increased community transmission is expected to result from Auckland's re-opening next week, in the face of the Omicron question, experts have suggested New Zealand delays relaxing its international border settings.
The Government is currently planning to allow quarantine-free entry – first to fully-vaccinated citizens arriving from Australia in mid-January, and then to Kiwis coming in from all other countries a month later.
"The big thing about the MIQ system is that, people not only get isolated when they come here, but there's only a few thousand arrivals a week," he said.
"When the MIQ requirement gets dropped, we might move to a few thousand a day – which massively increases our rate of infected arrivals."
Welch said that, if New Zealand did want to shift that timeline back, it needed to decide soon.
"A lot of people will be booking their tickets for January right now."
Baker said New Zealand had taken a cautious approach at nearly every step of its response to Covid-19, and had been rewarded for it.
"I would say the same should apply now," he said, recommending a one-month delay in the planned relaxation of border entry requirements in January 2022.
Much of the South Island is set to plunge below 0C tonight and tomorrow.