New Zealand's first Omicron wave may have left the population with more immunity than first thought, experts say, as new modelling indicates cases won't reach worst-case heights.
In his final media conference today, outgoing director-general of health Dr Ashley Bloomfield said updated modelling indicated the current wave would peak at around 12,000 daily cases – although numbers were now tracking below that.
Today, the seven-day rolling average of community cases stood at 8111, compared with 9367 the same time last week.
While hospitalisation figures could reach 1000 occupied beds, the new modelling showed the country moving closer to a peak of about 850 – a point nearly reached last weekend.
"What is apparent is that the worst-case scenario that our modellers had suggested a couple of weeks ago - with up to 1200 beds occupied, and over 20,000 cases a day - is now highly unlikely," Bloomfield said.
"I'm sure we're all relieved to see this."
Bloomfield pointed out two critical factors which had improved the picture.
One was the benefit of second boosters, of which more than 270,000 have been delivered so far, on top of 2.7 million given in the first round.
The other was previous infection with Omicron appearing to provide stronger than expected protection against BA.5 - the subvariant now accounting for 61 per cent of cases nationally.
The rapid ascension of the fast-spreading BA.5 initially caused concern among experts, with preliminary studies indicating it to be four times more resistant to antibodies from vaccines than BA.2, the type that mainly drove our first Omicron wave.
There was also worry over its potential to evade immunity from infection with BA.2, given its slightly different structure.
But studies have been increasingly showing immune memory from prior vaccination or a previous infection – giving what's called "hybrid immunity" - has been lingering on to reduce risk of severe illness.
That memory included T-cells – one specific group of these has just been documented to linger for 20 months - as well as B-cells, which could be activated to fight a new infection.
Dr Dion O'Neale, of Covid-19 Modelling Aotearoa, agreed there now appeared to be more immunity transfer from BA.2 than BA.5 than had been initially estimated.
It was also possible the level of infection across the country during the first wave may have been higher than suspected.
As the true level of infection was estimated at two to three times higher than the 1.5 million cases recorded officially, at least two-thirds of Kiwis had now likely been exposed to the virus - giving them some immune memory in the process.
But without the data that a national seroprevalence survey would bring, the true state of our "immunity landscape" wasn't clear, O'Neale said.
His colleague Professor Michael Plank added that, when BA.5 first emerged and began spreading here and overseas, scientists hadn't had much time to gather data.
"We're constantly learning. Every time a new variant comes along, it takes time to work out what properties it has, and how it's responding to the immunity that exists in the population," he said.
"Of course, that also differs a lot from country to country, because each has its own immunity landscape that depends on vaccination levels and the size and timing of previous waves of variants."
Recent modelling carried out by Plank and colleagues, but based on BA.2, included one scenario in which half of the second wave would be made up of reinfections.
Even with BA.5, that scale of reinfection hadn't come to pass.
Known reinfections now 5.97% of known new cases pic.twitter.com/cBsHGLjZjz— David Hood (@Thoughtfulnz) July 27, 2022
The modelling did, however, show that wave would come over winter – and that infections would shift into older age demographics, which had proven a major driver of our high hospitalisation rates.
"So while [immunity] has been constantly shifting, some of the scenarios that we looked at, in broad terms, were similar to what's happened," Plank said.
University of Auckland computational biologist Dr David Welch also acknowledged immunity had been the biggest unknown in earlier forecasts.
"If you look at the comparator countries we've been using, most of these haven't had such a recent wave as we have – and they also haven't had a wave that's been so dominated by BA.2," he said.
"Of course, we also don't yet have a good idea of exactly how long immunity from infection – or indeed from the vaccination – is actually going to last.
"You only need to make a small change to a model with these parameters, and it can make for a pretty big change to the outcomes."
Shifts in human behaviour – such as the timing of the school holidays, but also more people being cautious amid growing case numbers, getting boosted and isolating when required – also might well have contributed to the lower peak.
Whether New Zealand would see yet another wave before the year's end was unclear, but Plank said it was a possibility – particularly if a significant new variant turned up.
Welch also pointed out that case numbers were still running high, and would remain elevated for a while to come.
Weekly Covid Genomics Insights Dashboard (#16) will be posted soon https://t.co/4bxUhtyCPf. BA.5 most prevalent: 61% of genomes analysed - matches 73% BA.4/5 in wastewater (thanks @wilderlabnz for the wastewater spike barcoding). BA.2.75 in the community (2 cases). Check it out. pic.twitter.com/jEDrdAEaes— Michael Bunce (@BunceDNA) July 21, 2022
"But perhaps if no variants come along, we might actually drop down to a slightly lower plateau than we had before, because there'd be even greater immunity in the population."
Otago University epidemiologist Professor Michael Baker pointed out this wave had come at a colder time when more people were indoors, and amid fewer public health restrictions.
"So, given most of the factors are favouring the virus at the moment, the main thing that's contributed to this wave peaking not as high as expected must be growing host immunity."
Baker said the pandemic still remained an "arms race" between our own immunity and vaccination efforts, and the virus with its constant stream of new variants and subvariants.
"This is the equilibrium that, more than anything, will determine where the curve goes in the future – and it may be that it does flatten out to a relatively low and manageable level some time in the future," he said.
"But, as evolutionary biologists have found, this virus has continued to surprise us with its evolutionary range – and very few people are predicting what it will do in the medium term."
Further, Baker singled out another factor people needed to consider when thinking about waves and cases.
"The burden of long Covid is real – even if we don't yet fully understand how common, prolonged and severe it is overall," he said.
"While it might be good that our immunity is being boosted, it's bad that we're getting it from being infected with this virus."
Bloomfield closed out his briefing today with an appeal to the thousands of eligible Kiwis still to get their second booster.
"My message to those people is please do so. It could save your life."