Covid-19: NZ's third Omicron wave a 'puzzling picture'

New Zealand's third Omicron surge is presenting a "puzzling picture", a Covid-19 expert says - and may possibly prove smaller and shorter than our last one. Image / NIAID

New Zealand's third Omicron surge is presenting a "puzzling picture", a Covid-19 expert says - and may possibly prove smaller and shorter than our last one.

Modellers are closely tracking what's shaping up to be our most complex Covid-19 wave yet, with driving factors ranging from loosened restrictions to waned immunity and tricky new subvariants.

As at today, the daily rolling average of new cases had crept up to 2211 – higher than 1745 a week before – and more than twice some daily totals reported last month.

While some commentators have suggested New Zealand could be at the start of a rapid, exponential growth curve, University of Auckland computational biologist Dr David Welch estimated the doubling time was still only about three weeks – and not rising quickly.

If that trend continued, he said, the wave would prove much smaller than our last, when daily cases peaked at just over 10,000 in mid-July.

There also appeared to be little growth in reinfections – the seven-day average today stood at 228, compared with 183 a week ago – which accounted for only about one in 10 new reported cases.

"So, there's no signal we've suddenly got an escape variant that's causing lots of new infections – what we're dealing with is something a bit more complex," he said.

"It's difficult to ascribe these trends to any one thing right now."

Although the driver of our winter wave, BA.5, remained the dominant subtype, prevalence of BA.2.75 and BA.4.6 - which the latest data put at about 10 and 15 per cent of sequenced samples – had risen over recent weeks.

Scientists were keeping an especially close eye on the recently-detected BQ.1.1, which appeared to have a similar growth advantage over its parent BA.5 than BA.5 did over BA.2, which helped fuel our first Omicron wave.

"There are new variants out there, but I'd say that they're working their way around various types of immunity, rather than the immunity you received from your last infection," Welch said.

"It also doesn't appear that it's variants, generally, that are driving an increase in numbers – and I'd still suggest that this bump in cases appears most likely caused by relaxation of restrictions."

The Government announced an end to a swathe of measures in September – including masking rules in most settings, and vaccination and testing requirements at the border.

Modellers anticipated that waves that have been observed across Europe would inevitably translate to case rises here.

Welch noted many of those countries now appeared to be at the peak of their waves, yet these hadn't been powered by any specific new variant.

"So quite what has caused them is a little bit open to interpretation – and it's possible we're seeing the same sort of thing here."

He estimated New Zealand's current effective reproduction number - that's the average number of people that one infected would spread the virus to, in a population with mixed susceptibility – at 1.15.

"Unless we get a real escape variant coming along, I'd expect to stay in that range, where it might slightly increase then come back down again."

Ultimately, he said time would tell what this wave amounted to: but it was possible it could peak sooner than we expect.

This week, the Government announced it was winding down special powers that enabled it to order lockdowns, vaccine mandates and managed isolation and quarantine.

Much to the relief of public health experts, requirements for infected people to isolate – which Australia recently scrapped – along with masking mandates in health and aged-care facilities have been retained.

They continued to urge people to ensure they'd been boosted and vaccinated, but also avoid infection altogether, pointing to the ever-present risk of long Covid.