Covid 19 Omicron outbreak: Experts predict three trends to watch for over coming weeks

With New Zealand now having crossed the phase 2 threshold, what can we expect over coming weeks? Covid-19 experts give science reporter Jamie Morton some indicators.

Runaway cases mean fewer brakes

A run of days of case counts in the high hundreds might be a shock for Kiwis who’ve acclimatised to a largely Covid-free country.

But for modellers like Te Pūnaha Matatini’s Dr Dion O’Neale, it’s an Omicron outbreak merely following a trendline they’ve tracked for weeks.

With case numbers doubling roughly every three days, that daily tally could look like 2000 within just half a week – and 4000 perhaps in the space of a week.

“That’s really rapid growth, and much faster than what we’ve seen with previous outbreaks in the country.”

He pointed out a lag effect that was baked into the outbreak: many of those cases being reported now came from people first infected a week ago.

“Given we see around two doubling times between when a person is a new infection, and when they show up as a case, it’s safe to assume the cases being reported now are those from a week ago, and had already been baked in,” he said.

“Because we’re looking at a factor of four, or two doubling times, it’s possible that when we see a day with 1000 new cases reported, that same day there is likely to also really be 4000 new infections.

“These will show up next week as long as people get tested, traced and confirmed one way or another.”

Modelling released by O’Neale’s colleagues, which used February 1 as the starting point of a three to four-month wave, and assumed low transmission and a 90 per cent booster rate, estimated a total of 1.5 million infections, of which 386,400 were reported as cases.

If that booster rate was 70 per cent but transmission stayed generally low, those figures rose to 1.7m infections and 447,000 cases.

And, if transmission was high, the modelling indicated 3.3 million infections and 857,900 cases – even with 90 per cent of the eligible population boosted.

Right now, there was a roughly even split between eligible Kiwis boosted and those still double-dosed.

Several modelling exercises have pointed to a peak hitting some time next month, which O’Neale said wasn’t an unreasonable assumption at this point.

“But a lot will depend on how things go. If we let things accelerate and we get a really steep peak, then it happens sooner and it falls off sharper,” he said.

“If we try and suppress it as much as possible, maybe it happens later, and the peak will be lower.

“The consequence of that is that the wave drags on for longer, but we have fewer total infections and less stress on the healthcare system and critical infrastructure.”

But slowing Omicron would be difficult, given case numbers were soon likely to outstrip intensive contact tracing capacity, lockdowns were no longer an option, and isolation requirements had just been shortened by days.

At the same time, however, soaring case counts might prompt people to take more of their own actions – like masking up, reducing riskier interactions and notifying contacts themselves if they test positive – which in turn could influence the scale of the outbreak.

More infections among young

Around half of cases recorded here since the start of the year have involved people younger than 30 – and O’Neale didn’t expect that trend to change too much over coming weeks.

People in their teens, 20s and 30s, particularly, tended to be the most mobile, and also typically experienced less severe symptoms – or sometimes none at all.

“These are people who are more likely to be socialising and going to work, so we expect to see lots of cases happening in that group.”

He also anticipated a disproportionate number of Māori and Pasifika people among the cases.

While the outbreak had begun with high numbers of Asian cases – something tied to Omicron-linked weddings in the Indian community – demographic trends were now beginning to show familiar skews among Polynesian people.

Māori have accounted for 35 per cent of cases since the start of the Delta outbreak in August, while Pacific people have made up 31 per cent.

Māori vaccination rates – 86 per cent of those eligible and aged over 12 have received at least two doses – also continued to lag behind the national rate of 95 per cent.

The disparity was echoed in pediatric vaccination rates, with 26 per cent of Māori aged 5-11 having received their first dose, versus 45 per cent nationally.

“Everyone is going to encounter Covid, although we won’t all get it,” he said.

“So, we can expect more infections among groups that have lower vaccination rates, and who spend more time in high-risk environments for transmission, such as large households and essential, in-person, workforces.”

The Māori population also happened to have a younger age structure – and many Māori were among the last to become eligible for vaccination.

“Māori have been over-represented in cases to date, and I think this phase of the outbreak will just perpetuate the inequalities we’ve seen throughout,” Otago University epidemiologist Professor Michael Baker said.

Unvaccinated, elderly, at higher hospital risk

O’Neale said age, too, would be a big factor in hospitalisations.

“Older people are more likely to be hospitalised than younger people, with the exception being very young people who aren’t vaccinated.”

Māori, again, could be over-represented among hospital cases.

One modelling study that drew on case data from 2020, and controlled for age and pre-existing conditions, showed Māori were two and a half times more likely to need hospital care than non-Māori – while the risk for Pacific people was even greater, at three times higher.

Another at-risk group, of course, were the unvaccinated.

Of 664 cases who needed hospital care since August, 418 hadn’t been vaccinated, while 121 were partly vaccinated.

In New South Wales hospitals, the unvaccinated have accounted for more than half of patients in intensive care units.

Among the 40 people currently in hospitals around the country, two were unvaccinated, while the status of another 14 were unknown. The average age of patients was 59.

“The only reason we see lots of vaccinated people in hospital wards, or even getting infected, is because the vast majority of people are vaccinated.”

This time around, he said waned vaccine immunity, and a population still shifting between two doses and boosters, would make it trickier than earlier outbreaks to pick out trends around vaccination and hospitalisations.

This month’s modelling report suggested peak daily hospital admissions to range from 200 to 800, and peak daily hospital bed demand range from 800 to 3300, when there was high booster uptake.

Assuming baseline/medium transmission and 90 per cent eligible booster coverage, the modelling pointed to 19,300 hospitalisations and 840 deaths over the course of the outbreak.

While hospitalisation numbers could be expected to creep up in step with overall case counts, O’Neale said there were other confounding factors.

“We can estimate that this is how many people we think will be sick enough that we’d want to put them in hospital, but that doesn’t necessarily mean there’s a bed for them – so, not all of the cases that we might want to be hospitalised, will be hospitalised.”

Outbreaks within hospitals themselves could also affect hospitalisation numbers, he said.

Despite those scarily high numbers in modelling reports, Baker anticipated the country would fare comparatively well.

“I think New Zealand, by world standards, will get through this in good shape – but there’s still that critical need for more people to get boosted.”

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