Covid-19: What will drive NZ’s next wave?

Covid-19 modeller Professor Michael Plank said the variant that steered our next wave would need to have the right machinery to evade the immunity that past infection and vaccination had left us.

“I guess there’s two ways it could go: one is that we get another sort of Omicron: so a complete game-changer variant that has very high immune escape and can cause very big waves,” Plank said.

“The other possibility is we continue to see these sorts of sub-variants that evade immunity to some extent, but which still aren’t in the same league as Omicron was when it burst onto the scene.

“These would typically cause smaller waves, which is what happened with BA.5.”

In the latter scenario, particularly, Plank said prior infection would deliver some degree of cross-protection, although immune waning might also prove a factor.

“Although it was clear that Omicron evaded immunity against getting infected and caused waves that grew rapidly, in a lot of countries there was still a high degree of immunity from previous variants – and also from the vaccine – against severe illness,” he said.

“Everything we’ve seen so far in the pandemic points to that pattern continuing.

“But there certainly doesn’t seem to be anything too dramatic on the horizon at this stage.”

Otago University virologist Dr Jemma Geoghegan agreed the next wave-maker would bear “some sort of antigenic novelty”.

“It could have an ancestral backbone with additional mutations, like Omicron did, or it could have Omicron mutations, but more of them.”

Bodies like the UK’s Scientific Advisory Group for Emergencies (SAGE) have highlighted the many different ways future variants might arise, ranging from immunity-tricking antigenic shift, to reverse zoonosis and spillback.

That involved the virus being passed from humans to animals, then sent back in an entirely new, mutated form.

But Geoghegan pointed out the next big variant didn’t necessarily need to be dramatically distinct.

“We should remember that BA.5 caused a wave and it was only slightly different from BA.2 – just a handful of mutations,” she said.

“So, the immunity profile we’ve gained from our waves and vaccinations should help us with any further Omicron waves – but we still can’t say the next variant won’t look very different from Omicron.”

How would we know when it had arrived?

University of Auckland computational biologist Dr David Welch pointed to the way in which Omicron, and Delta before it, entered the pandemic.

“We’d be seeing something that’s genomically very different – it has a lot of mutations – and begins making up a significant proportion of new cases in some country,” he said.

“Those are the two warning signs we’d be looking for.”

As Geoghegan and other experts have noted, with our borders now re-opened, New Zealand might not have the head-start it once did to prepare.

This made Covid-19 surveillance – something soon to be improved by infection prevalence surveys – all the more important, Plank said.

“Because of genomic sequencing, we could see the BA.5 wave here coming a long time before it really started to impact case numbers.”