University of Otago epidemiologist Professor Michael Baker said case numbers were not spiking as in past waves, but that could be because of under-reporting of testing and positive cases.
Hospitalisations were a more reliable guide, Baker said, and showed a distinct rise in the number of people being admitted with Covid infection.
“Numbers have been rising for seven or eight weeks. We’ve seen a doubling of hospitalisations over that time. That’s quite significant, and means ... more than 10,000 people winding up in hospital over a year and displacing other things.”
Dr David Welch, senior lecturer in computational evolution at the University of Auckland, said the rise in cases was likely to be driven by waning immunity and possibly the beginning of the winter flu season.
He expected cases to keep rising but not reach the peaks of previous waves - largely because of increased levels of immunity among the population and widespread vaccination.
He said newly available bivalent vaccines targeted the current variants of Covid, “so there’s a real opportunity to significantly reduce your chances of getting very sick by getting that vaccine now”.
Most of the new cases are being driven by the Kraken subvariant of Omicron.
But a new Covid subvariant which has gained a foothold overseas is being closely watched in New Zealand.
The Arcturus subvariant, also known as XBB1.16, has now been detected in 29 countries, including New Zealand.
The World Health Organisation (WHO) has designated it as a “variant under monitoring”, which means it could have a “growth advantage” over other sub-variants, though its exact impact on people who catch it is not yet clear.
It is driving a new surge of Covid cases in India at a time when positive cases are flat or falling in many countries.
Reports that it could cause a rare symptom in children - sticky or itchy eyes - were at this point speculative, experts said.
Genome sequencer Dr Jemma Geoghegan said the Kraken subvariant was still the most dominant in New Zealand, and a handful of Arcturus cases had been picked up in surveillance monitoring.
Some overseas studies had shown that Arcturus was more transmissible, she said.
“It’s important to know this is in laboratory settings and it will be important to see what will happen in the real world.
“It does seem to be spreading more rapidly than other variants right now in certain countries. But this often happens, a new subvariant comes along, spreads quite rapidly and then peters out over time, ultimately to be replaced by another one.
“What will be important to know is if it really does change the disease, so if it makes it more severe. So far there is no data to show that it is more severe.”
Dr Jemma Geoghegan said there some some evidence that Arcturus was more contagious than other variants, but it was not yet clear whether it led to more severe symptoms. Photo / Supplied
Dr Welch said Arcturus was believed to be between 5 and 10 per cent more contagious than Kraken.
“When it gets around 10 per cent above ... that is when it starts causing a new wave maybe or a noticeable rise in cases. Below 10 per cent, it doesn’t really push case numbers up significantly.”
Professor Baker said the subvariant was not significantly different from previous ones.
“There will be some speculation that it might have a different mix of symptoms or it might be slightly more harmful but I think you have to be very wary about speculating … unless you have a lot of data.
“At the moment, it is not a game-changer - that would be my impression.”
Baker said the new subvariant re-emphasised that Covid-19 continued to evolve, with new subvariants “out-competing” previous ones.
“And a lot of that is by being slightly novel and by evading our existing immunity.”
This is what made Covid different from other diseases like the flu, he said. Its changing nature meant people could be reinfected multiple times a year, while a person may get reinfected with flu once or twice a decade.
