Covid 19: Omicron wave has likely peaked, Michael Baker says

Health workers hand out rapid antigen test kits at a Covid-19 testing station in Otara. Epidemiologist Professor Michael Baker expects the national Omicron outbreak has peaked. Photo / Alex Burton

New Zealand's Omicron outbreak has probably peaked but a top Covid-19 expert warns smaller waves are likely to follow.

Otago University epidemiologist Michael Baker expects hospitals across the country to remain under pressure for months to come – and urged people not to think of the virus as following a come-and-go pattern like seasonal influenza.

As at Sunday, New Zealand's rolling seven-day average stood at 17,278 cases – the vast majority of them self-reported from rapid antigen tests – with officials increasingly optimistic the outbreak has peaked in Auckland.

While the Ministry of Health announced another 12,020 community cases - down from the 18,514 reported on Saturday - it warned weekend testing numbers were generally lower.

The ministry said there was a "steady decline" in case numbers across Auckland while cases in other parts of the country continued to fluctuate.

Baker, who has been tracking case trends, said Auckland cases appeared to have peaked in early March, before the city's hospitalisation rates peaked last week with more than 630 cases.

Given the pandemic wave in other places seemed to be trailing Auckland's case count by one to two weeks, he said, we could infer New Zealand's overall outbreak similarly peaked some time in the last fortnight.

"It looks like it was about 10 days ago that the moving average of reported cases in New Zealand reached the peak, and it's now slowly tracking downward."

DHBs in the South Island, along with more rural DHBs in the North Island, were still seeing drawn-out peaks, he added.

"This is just reflecting the fact that this virus will obviously be spreading into different populations at different times," he said.

"We can look at this as like a Mexican wave, where we've got a whole succession of waves peaked at different times.

"But the picture we see when you add it all up together and smooth the data out, is a peak across the whole country lasting two to three weeks."

Baker expected the climb-down from the peak would take much longer than what was a relatively short-and-sharp rise to it.

"In Auckland, it took around four to five weeks between the first noticeable rise in cases and reaching a peak – and it does look like it's going to have a slower descent," he said.

"At this point, I think it's reasonable to say that the whole country might reach a new baseline level by early May."

If that happened, he said New Zealand would have made it through the thick of the outbreak before winter: a period where we traditionally saw the highest rates of respiratory infections.

"So, if you're designing the ideal time to have an Omicron wave, I think now would be it."

But he stressed that, unlike seasonal nasties like the flu, the post-peak equilibrium of Omicron would be much different.

"Based on the experience of Australia, that baseline may still be quite high."

While Australian cases peaked at more than 110,000 cases a day in January, and subsequently dropped to around 25,000 a day in February, numbers had since climbed again to more than 40,000 – partly on the back of the more transmissible BA.2 subtype.

"In New Zealand, it looks like hospitalisation numbers are going to peak at around 1000 – and if we follow the Australian experience, we might drop down to there being 200-300 people in hospital with Covid-19 for the next few months, on a typical day," he said.

"All of this is really a warning for the health system that it's going to be under pressure for many months."

Covid-19 modeller Professor Michael Plank said the hospitalisation numbers would be the key metric to watch in tracking the post-peak trajectory.

"Queensland, which has a similar population size to us, peaked at around 1000 hospitalisations and they're now down to 200 to 250," he said.

"So that's a couple of months after the peak – and it's quite plausible that we'd follow that same sort of pattern here."

Plank, of Covid-19 Modelling Aotearoa, pointed out that a "significant" proportion of hospital cases weren't people who'd been admitted to hospital with the virus, but had tested positive while there for unrelated reasons.

Another scenario that could play out could be that of Denmark's outbreak, which was now reporting just over 8000 new infections each day on average – or about 15 per cent of what its peak was around two months ago.

"We might think of a scenario where [cases] come down steadily and maybe plateaus out at some level; or the other scenario is that we get a bit of a bump over winter."

While other countries were seeing second waves fuelled by BA.2, it was possible New Zealand wouldn't experience these, given our outbreak was being driven by a mixture of both that subtype and the original BA.1.

"I'd add that BA.2 is still not making up 100 per cent of all [sequenced cases]. So, there's still room for it to increase a bit, which could prolong this sort of flat top of the mountain, rather than give us a pronounced second peak."

As for whether New Zealand was indeed past the worst of the Omicron wave, Plank said this was difficult to say for sure – but expected we'd either just passed the high-point, or were approaching it.

"If you're designing the ideal time to have an Omicron wave, I think now would be it," Professor Michael Baker says. Photo / Supplied

"My take is that we're pretty much at the peak, nationally, for cases."

Although reported case numbers were no longer the solid indicators they were before mass transmission and the switch to rapid antigen testing, Plank said they were still telling us something.

"We know that there are a whole lot of infections that will go unreported, but my sense is that the fraction that do get reported has probably now stabilised from when we switched to RATs," he said.

"So, although we don't know the absolute number of cases, I think that when we see a clear peak in cases, that's likely to be real – and not just some sort of artifact of the testing."

Looking out further, Baker said we could expect more Covid-19 waves for a many months, as we reached some form of balance with the coronavirus.

Just how intense they'd be, or what would be driving them – perhaps a new immunity-evading variant created by a recombination event – remained unclear.

Because we can't predict the virus' evolution, nor could we change the biology behind waning immunity, Baker saw it all the more critical that we continued to do as much as possible to control the third big variable: our pandemic response.

He hoped that New Zealand would learn from the experience of countries overseas and anticipate future pandemic surges by only removing control measures cautiously to avoid a "yo-yoing" in cases and interventions.

"We need to work out a suitably sustainable set of controls for this virus, which is another way of saying: don't put all of your eggs in the vaccination basket.