"It's not scary just yet, but they clearly are creating a longer tail to our winter wave."
Still, he expected BA.5 could soon be overtaken by one or several subvariants that have been causing fresh Covid-19 waves overseas.
"Probably, by the end of the month, BA.5 won't be our main variant anymore."
The latest hospital surveillance results – covering the last weeks of September – showed 90 per cent of cases were BA.5, compared with five per cent BA.2.75, four per cent BA.4.6 and one per cent BA.4.
Experts have already warned that winter surges in the Northern Hemisphere are likely to translate to another wave here before the year is out – with the possibility of several subvariants in the driver's seat.
"What will happen here is going to depend on a lot of different factors, but it's clear that we're going to be importing positive cases, because, with no pre-departure testing in place now, these will be moving directly into the community."
Just like BA.4 and BA.5 swiftly replaced the subvariants responsible for New Zealand's first Omicron wave – BA.1 and BA.2 - de Ligt expected the next changeover to play out with similar speed.
"If our borders were closed and we weren't importing anything, then this would likely happen with BA.4.6 and BA.2.75," he said.
"But the one that we're really closely watching, is BQ.1.1, which is heavily on the rise in the UK and has that potential to cause another large wave here."
While little is yet known about BQ.1.1 - a descendant of BA.5 that was discovered mere weeks ago – scientists are concerned over mutations that help it evade immunity, and also reduce the effectiveness of monoclonal antibody treatments.
According to the latest surveillance report, no cases of BQ.1.1 have yet been reported here – but with our diminished surveillance, it was possible the subvariant has already begun spreading in our communities.
"Generally, what we'd be expecting is co-circulation of several of these variants, where one is more likely to drive a peak, while the others just cause a continuous grind."
Covid-19 modeller Professor Michael Plank agreed our coming wave would be much more complex than any before it.
"We have a sort of variant soup, with different lineages growing at different rates, which makes it difficult to predict how big this wave will be," he said.
"It's unfortunate that now we can't so easily distinguish border cases from community ones in the sequencing data, as it's become harder to tell how quickly some of these variants are spreading in the community."
When the next wave did begin, Plank expected this would be heralded by a shift in wastewater surveillance trends, along with a clear increase in reported case numbers.
While the rolling seven-day average had gradually climbed from around 1340 in mid-September to nearly 1600 on Monday, it was too early to tell if this was already happening.
"We've been seeing an increase of 10 per cent week-on-week – but it's not a rapid one yet."
The Ministry of Health this evening reported there'd been 11,205 cases in the past week - a figure 1800 higher than it had stated earlier in the day.
