The Government is dropping the last of its Covid restrictions and says the last masking mandate and isolation requirements will be lifted at midnight tonight.
Video / Mark Mitchell
Experts are proposing a smarter way to gauge Covid-19′s spread in New Zealand, as a heavily mutated new subvariant puts global health authorities on high alert.
So far detected in just a handful of countries, Pirola has been designated a “variant under monitoring” by the World Health Organisation, amid growing concern it represents a large enough evolutionary leap to drive new waves.
“There are two really important questions with this variant,” Covid-19 modeller Professor Michael Plank told the Herald.
“One is the growth rate: if it doesn’t or can’t compete with variants that are currently in circulation, then it’s not going to have much of an impact.”
New BA.2.86 risk assessment from UKHSA. Key points: - high uncertainty (only 6 samples so far!) - detection in countries with good genomic surveillance suggests established intl transmission & likely rapid growth - no severity estimates yet (and likely won't be for some time) https://t.co/cANMpdMHfu
“The second one is, if it does have the legs to take off, then what does the severity look like?” Plank said.
“That’s a harder question to answer – or will at least take longer to do – because estimating severity is difficult without a reliable estimate of the number of infections.
“Because, while you might see a rise in hospital numbers, you don’t really know whether that’s an increase in the number of infections, or there’s actually a genuine increase in severity.”
The agency did, however, suggest there was now “established international transmission”, along with potential for rapid growth, at a time another Omicron off-shoot, EG.5 or “Eris”, has been fuelling rises in case and hospitalisation rates in the US.
On Pirola’s striking number of mutations, again, the UK agency said it was too soon to predict what this meant for impact – but added there was “sufficient information to expect significant antigenic change”.
Like his international counterparts, Plank expected we were only now seeing the tip of Pirola’s iceberg.
“It’s certainly a reminder that we need surveillance – including genomic surveillance - to keep an eye out for new variants that could really have an impact.”
It comes after New Zealand’s reported cases dropped off by about 40 per cent over the week since the Government announced an end to the seven-day isolation requirement for infected people.
Even before the rollback, Plank estimated the ratio of cases picked up or reported versus total overall infections – something called the case ascertainment rate, or CAR – sat somewhere between one in three and one in four.
While the fraction of reported infections had been clearly falling over time, Plank said the numbers could still be useful for surveillance.
“If we see those reported case numbers growing rapidly, that’s potentially an early warning signal that could translate into significant demand on healthcare systems.”
In a new Government-funded study, released online ahead of peer review, Plank and others propose a new statistical model that combines those different indicators to give an overall picture of infection rates in the population.
That included giving the CAR at any point in time, along with the effective reproduction number, or Reff, which represented the average number of people a given infected person was likely to pass the virus onto.
Fellow study author and University of Canterbury researcher Dr Leighton Watson said the model could be applied nationally or regionally, and used by any country where most people were connected to the wastewater system.
“While the results are not the only piece to the puzzle, the model could be used as an additional source of information to inform public health policy decisions and hospital capacity planning.”
Watson said this approach took into account the fact that restrictions and testing guidelines had eased dramatically since Omicron first washed over the country 18 months ago.
“At first, people would test every time they got a sore throat,” he said.
“Anecdotally, now it seems like many people are assuming they are just under the weather because, for example, their kids bring every single bug possible home from school.”
Wastewater surveillance has been crucial in gauging Covid-19's prevalence in New Zealand.
Fewer cases could mean fewer infections or fewer people reporting - and the difference was crucial to understand.
In their study, the researchers used their new model to demonstrate that, while reported case numbers made 2022′s first wave appear twice as large as the winter wave a few months later, the true numbers of infections in both surges were actually similar.
Plank said the study highlighted the value of keeping wastewater surveillance running, in an environment where fewer people were testing.
“Everyone who lives somewhere that’s linked up to the town wastewater system is going to shed the virus into the wastewater if they have Covid,” Watson added.
“If they are plumbed into the wastewater system and we are sampling it, we can pick that up independent of whether people are testing or not.”
Over the longer term, there remained many troubling unknowns about Covid-19 and its potential to drive fresh waves, kill or hospitalise more people, or widen the number of people left suffering ongoing symptoms.
“Unlike something like the flu, we don’t really have a clear sense of how Covid is going to behave,” Plank said.
Meanwhile, two modelling reports – just released but handed to the Government in early May – explore alternatives to the just-scrapped isolation mandate, while estimating the impact of removing it.
Based on case data around the time the Government decided to stick with the isolation policy to ease strain on hospitals over winter, the reports were prepared by Covid-19 Modelling Aotearoa after a government steering group requested analysis on six scenarios.
Those were mandated isolation times of seven and five days; unmandated isolation periods of seven days, with high and low compliance scenarios; and two “test-to-release” regimes with minimum five day isolation periods, but with different maximums, of seven and 10 days.
One report found that moving from the mandated seven days isolation to a test-to-release policy could have shortened isolation time without much effect on transmission rates.
Slashing the case isolation time from seven to five days, but without a test-to-release requirement, would have led to increased transmission, although the rise was modelled to be considerably less than scrapping the mandate altogether.
We have published the modelling we produced on the effect of alternative isolation policy settings.
— Covid-19 Modelling Aotearoa (@covidaotearoa) August 21, 2023
“Estimated reduction in transmission, compared to no isolation requirements, ranges from 10 per cent to 25 per cent, taking into account various assumptions and uncertainties,” the researchers said.
The other report, meanwhile, used a special methodology to look at potential effects of the six options on infections, hospitalisations and deaths over the six months afterward.
The modelling accounted for potential seasonality effects at the time it was prepared, including scenarios where transmission rates increased gradually from April to July due to seasonal factors.
The findings indicated that removing the isolation mandate, even under high compliance with non-mandatory guidelines, could lead to a six to 21 per cent increase in hospitalisations, and a six to 24 per cent increase in deaths over the subsequent six months.
In low-compliance scenarios, the estimated increase in hospitalisations ranged from 11 to 30 per cent, with a corresponding increase in deaths ranging from 12 to 35 per cent.
“However, these estimates acknowledge uncertainty due to variations in testing and compliance rates,” the modellers said.
“While these reports provide valuable insights, they also highlight the complex interplay of variables and uncertainties in predicting the outcomes of isolation policy changes.
“As New Zealand continues to navigate its pandemic response, these findings offer critical information for decision-makers seeking to balance public health and societal needs.”
