The combination of the new COVID-19 Protection Framework and the removal of internal borders later this month could cause a massive spike in infections, says microbiologist Siouxsie Wiles.
On Friday, there were only 92 new community cases of COVID-19 in the country - the first time in weeks the number had been below 100 - with 98 new cases reported on Saturday.
Dr Wiles said the smaller tallies are a sign that vaccinations and the now defunct alert level system have been suppressing the Delta outbreak.
"It was showing that our vaccinations had been playing an incredible role in keeping case numbers low and that the changes that had been made to the alert level system were not contributing massively to an increase in cases," she said.
But that picture could change quickly, she said, as the new 'traffic light' system allows many more interactions between people.
"There's far more mixing of people now, far more interactions, and it wouldn't at all surprise me if our case numbers start to rise again. The question is whether we can keep them at a level that protects our health system and our hospitals."
It could take a week or two before the case numbers show the impact of the new system, Dr Wiles said.
The combination of the new system and the removal of Auckland's regional boundary on December 15 has the potential to cause a massive spike in infections, she said.
"We obviously have the unvaccinated and the under 12s and they are not protected from severe disease, so depending on where the virus spreads, we could see a massive rise in cases and hospitalisations if it starts to circulate in those people and communities that are not already protected by vaccinations."
Dr Wiles said she thought Aucklanders should put off their holiday plans.
"I'm imploring Aucklanders to 'staycation' rather than to head to other areas, especially those areas where the communities have asked us to stay away, because they are busy getting the vaccination numbers up."
Summer and vaccination rates could help
Otago University epidemiologist Michael Baker said he suspects case numbers will trend upwards more markedly.
But that trend may not be so steep for Auckland, he said, as vaccination coverage is now high and the warmer weather means people will spend more time outside and open windows more often, increasing ventilation.
"We probably expect the numbers to start tracking up in Auckland because we are putting viewer controls on the virus, but it may be quite a small effect because we are entering summer and people are doing far more things outside and vaccination coverage is getting into quite a high range now."
Baker is more concerned about areas outside of Auckland.
"That is a bigger problem because we have the virus present in decent numbers outside of Auckland, and some places with largely low vaccine coverage - in particular in Northland and pockets of the Waikato - they are places to watch."
Dr Baker said he is watching carefully to see if the 'traffic light' system will be equally successful at suppressing the virus, but noted that he doesn't expect signs either way for some weeks.
He agreed the lower case figures for the past two days are an encouraging sign that New Zealand's strategy for combating the virus has been working.
He said three important barriers erected against the virus have been doing their job: "Vaccination coverage - that's really helping, the other barrier is contact tracing, and of course the third is the alert level 3 system that we did have."
Spread of Omicron internationally could change the picture
The spectre of the Omicron variant is also weighing on Dr Wiles' mind.
"Depending on what happens in the next weeks or months with that variant, is it more transmissible? Are people who are vaccinated susceptible to infections? Does it cause a more severe infection?
"All of those things should impact on how we behave in the future and may well have implications for our [international] borders.
"We'd be fools not to pay attention to the data. The data is what has served us well so far. We would be idiots not to see what it says before moving into the next phase of our response."
RNZ
