The main Omicron subvariant in this country is becoming less dominant as new subvariants rise up, recent data suggests.

But warming temperatures in the lead-up to summer could help limit an expected rise in new Covid cases.

Case numbers remained relatively flat last week, but that could change in the months ahead and it may be, in part, driven by new variants.

The latest Covid report from the Ministry of Health says that BA.5, which drove the recent winter wave now accounts for about 75 per cent of cases in the two weeks to September 30. That's the variant's lowest share of cases in months.

The BA.4.6 variant's share increased sharply in the latest data to 15 per cent, while the BA.2.75 variant is also gaining and is up to 10 per cent.

The ministry's report also says the Omicron BQ.1.1 lineage has not been detected in New Zealand in the fortnight to September 30.

"However, as BQ.1.1 is rising rapidly in Europe at present, we might have a situation where we expected multiple new variants to be circulating, all with different immune evasion and severity profiles."

It is worth noting, however, that just how Covid cases are sequenced has changed. For example, pre-departure testing is no longer required meaning we have an incomplete picture of what is coming in.

In a statement to Stuff, ESR says the latest variants' counts should be interpreted with some caution, if for instance, you are comparing to older reports.

"The latest report reflects changes in New Zealand's reporting and surveillance system alongside the current Covid-19 situation.

"Major factors contributing to this are the lower volume of PCR results available for sequencing (RAT tests can't be sequenced). In addition, there is a discontinuity as a result of no longer separating community and border samples. And both the local and global picture is shifting which changing variant frequencies."

What about the severity of the new variants then? Well, a Ministry of Health report, dated September 27, suggests: "There is no strong evidence of large increases in transmissibility or disease severity associated with the most recent variants.

"Vaccination, including boosters, remains effective at decreasing the risk of hospitalisation and death from Omicron. It remains difficult to predict the impact of variants here in New Zealand based on overseas trends as our infection history and hybrid-immunity differ from other countries."

Professor Michael Plank, from the University of Canterbury, says it appears there's an increase in the share of some variants, but the picture is complicated.

As mentioned above, that's partly because the end of testing at the border has made it harder to distinguish between cases in the community and those at the border.

"That said, it does look like there is an increase in some of these other variants. That is consistent with what is being seen overseas. It's a bit difficult to quantify the speed of it," says Plank.

Internationally, a "whole host" of different lineages were showing up. They are all somewhat related, and are picking up similar sets of mutations.

It's hard to categorise them as just BA.4.6, as there were several versions of Omicron.

Covid cases and hospitalisations are rising in many countries, says Plank.

"It looks like some of these new variants are probably going to cause a wave in the Northern Hemisphere.

"We would expect a similar thing to happen here at some point, a few weeks later."

Countering that, warming temperatures as summer approaches could be an advantage for this country.

"It maybe that our wave isn't as big because the Northern Hemisphere is going into a colder time of year and people are indoors more.

"Whereas, here we're moving towards summer, which makes it a bit harder for the virus to spread. It might reduce the size of the wave."

This article was originally published on Stuff.