Where is New Zealand at with Omicron? Covid-19 modellers tell science reporter Jamie Morton three key trends they're closely watching.
After weeks of soaring case numbers, New Zealand appears to be finally climbing down the other side of Mt Omicron.
A week ago, our reported seven-day average stood at 14,515 cases, and today that figure had fallen to 11,791.
But, as Covid-19 modellers continually remind us, the path down from the peak is likely to be much longer than our short and sharp ascent to it over February and March.
Dr Dion O'Neale, of Covid-19 Modelling Aotearoa, said Auckland was now "almost certainly" already into the tail of infections, while other regions were still closer to their peak.
"In general, based on looking at what has happened in various states in Australia, we expect cases after the peak to drop down to a rate of about 1000 new cases per day per million of population," he said.
"This works out at about 5000 new cases per day for the whole country if all the regions have managed to reach that plateau at the same time."
Roughly, that represented about one per cent of the population, per week, being newly infected.
"Without any additional measures being introduced to drive infections lower, we expect cases to bounce around at this level before starting to rise again after a few months."
In Queensland, which shares a similar population and other factors with New Zealand, it took around a month for its Omicron peak to fall to a plateau – and it was possible Auckland, and then the rest of New Zealand, might follow similar timelines.
Fellow modeller Professor Michael Plank said it was difficult to say precisely how far down the track we were.
"This is a bit of a how long is a piece of string question as there isn't always a clear separation between when one wave ends and the next one starts," he said.
"I think we're likely to see a long tail on this wave - and we could potentially see a second wave during winter and spring, although the timing of that is very uncertain."
With higher case numbers came higher death rates – the current weekly average for mortalities linked in some way to Covid-19 stands at 17 – and Plank was hopeful this trend would fall too.
"Vaccines have already massively weakened the link between cases and deaths and the percentage of cases that die is much, much smaller than in the pre-vaccine era," he said.
"Hopefully over time, as immunity and antiviral treatments improve, this percentage will decrease further. But there will always be some proportion of cases that die."
O'Neale added that a demographic shift in cases might influence reported death rates.
"Over the last month or so, we've seen cases move from being in predominantly younger age groups to being increasingly in older age groups who have higher hospitalisation and mortality rates."
I entirely get if people are over COVID graphs, but for those that aren't here's this weeks updates.
— Alex Kazemi (@KazemiAlex) April 7, 2022
The all NZ comet tail is stretching out but slowly getting darker and the weekly change is flat to decreasing in all age groups now. 1/ pic.twitter.com/e0As2c5G67
Given Omicron has only been circulating in the world for about four months, scientists are still learning about how long we can expect our natural and booster immunity to hold against it.
Recent surveillance data collected in the UK indicated vaccine effectiveness against symptoms from either BA.1 and BA.2 fell to less than 20 per cent about 25 weeks or more after a second dose.
That protection climbed back to around 70 per cent two to four weeks after a booster, but we could expect it to drop away, again, over several months.
It's because of this risk that officials are now mulling whether to offer a fourth dose of the vaccine, which would likely be targeted at vulnerable groups like the elderly and immunocompromised.
If one was rolled out, O'Neale said this could play a role in what happened to case rates around the middle and later parts of the year.
What was less clear, Plank said, was by just how much infection-induced immunity and hybrid immunity – gained from being both vaccinated and infected – in the wider population would wane.
"Most Omicron infections have occurred in the last three to four months, so we don't really know what our immunity will look like more than three to four months after infection."
While some parts of the world were already seeing second Omicron waves, these were largely being driven by the BA.2 variant hitting countries where the original BA.1 type had struck early – whereas New Zealand has been grappling with both at once.
Still, the modellers were closely following reports about new sub-variant XE – a hybrid of BA.1 and BA.2 - which might be 10 per cent more transmissible than even the quick-spreading BA.2.
"If it turns out to be more transmissible than other variants, either because of immune evasion, or because of inherent transmissibility, then it will spread to become common and will arrive in Aotearoa relatively quickly with open borders," O'Neale said.
Regardless, borders reopening to vaccinated Australian tourists this month – and international visitors from May – would inevitably mean more Covid-19 cases being seeded here.
O'Neale however expected the impact of these to be relatively low, given new domestic infections here would still number around several thousand per day as tourism resumed.
"The main risk that comes from reopening the border is that for any new variants, we will be unlikely get a chance to prepare for it, before it is already circulating in the community," he said.
"With Omicron, keeping it outside the country for an extra month or so gave us the opportunity to get booster rates up and start vaccinating tamariki, which reduced spread and reduced both protected people from more severe health consequences."
Rather, it was the return of other respiratory illnesses like influenza that might pose the bigger problem with New Zealand re-joining the world – and right at the start of the colder season.
"Wearing a mask when indoors will help protect both you and others around you, including those who may be especially vulnerable because of other health conditions or because they are too young to be vaccinated," O'Neale said.
How people responded to Covid-19 restrictions – especially if more were rolled back – was another factor to consider, but a difficult one to model.
"It could be that behavioural change doesn't have a major impact in the short term, but combines with waning immunity to produce a second wave later in the year," Plank said.
"But if there is significant behavioural change, particularly if it results in increased mixing with older age groups, this could potentially prolong the current wave of hospitalisations and deaths."
Modeller Dr Emily Harvey said ensuring that workers and students stayed home when sick, particularly, would make a big difference this winter.
"For example, we know from past studies during winter cold and flu seasons that workplaces with policies that require workers to stay home - with paid sick leave - as soon as they develop symptoms have reduced absenteeism compared to those who don't."
She said this should apply regardless of what results rapid antigen tests turned up.
"For adults, this is only going to be possible if employers enable and encourage people to stay home if they are sick, with paid sick leave or working from home if they are well enough," she said.
"For children to stay home from school when sick, this requires caregivers whose employers enable them to take paid time off work or work from home when they need to look after a sick child.
"Despite the increase in sick leave entitlements last year, we know that many workers have been forced to use these up already due to Covid isolation requirements."
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