Covid 19 coronavirus: Bubble's outbreak risk to NZ still 'very low'

The transtasman bubble poses a "very low" risk of a fresh Covid-19 outbreak here, despite a flare-up plunging Melbourne into lockdown again. Photo / AP

The transtasman bubble generally poses a "very low" risk of a fresh Covid-19 outbreak here, despite a flare-up plunging Melbourne into lockdown again.

That's according to a new study, whose authors say New Zealand could look at extra measures - like requiring bubble visitors to wear masks over their first two weeks - if Australia kept seeing spates.

In their study, published in the journal Scientific Reports, Otago University researchers calculated Covid-19 data for Australia throughout last year and in early 2021.

Assuming there was one flight to New Zealand each day, and that no interventions were in place, they found one in every 110 flights could result in a local outbreak, or that there'd be a 96 per cent chance over the space of a year.

But that scenario didn't reflect Australia's current, effectively virus-free state.

Assuming there were only two new community infections reported each week in all of Australia, the annual estimated risk of an outbreak here was just 1.2 per cent.

That was equivalent to a median time to an outbreak of 56 years - or after an average of 29,400 flights.

In contrast, the risk that came with travellers from "amber zone" countries like Japan - and "red zone" countries like the US became last year - was much higher.

Even with a 14-day quarantine period in place, the modelling suggested that, given the high prevalence of infection in US, the risk of an outbreak coming from there within a year was 100 per cent - and the median time to an outbreak was only 20 days, or an average 29 flights.

The equivalent values for Japan were better, but still troubling, and suggested that an outbreak could come every 1.7 years, or after 870 flights.

In a blog post discussing their findings, Otago epidemiologists professors Michael Baker and Nick Wilson, along with Professor Martin Eichner of Germany's Tübingen University, said these risks would "certainly" change over time.

"They should be reduced if more people in the source countries for travellers and in New Zealand get vaccinated, although this protection depends on the effectiveness of vaccines against new variants," they said.

"But before this happens to a substantive degree, the risks might even temporarily increase with the spread of more infectious variants ... and if international travel volumes are increased."

Nevertheless, they said their analysis indicated a quarantine-free travel bubble with Australia was typically very safe, even with occasional small outbreaks.

"But where there is large uncertainty about outbreak size in Australia – it seems very prudent for air travel to New Zealand to be temporarily suspended from the relevant Australian state or territory as per the recent actions of the New Zealand Government."

Last week, Covid-19 Response Minister Chris Hipkins extended a "pause" on quarantine-free travel with Victoria for a week, with the order to be reviewed on Friday.

Anyone who'd been to a location of interest in Melbourne could not travel to New Zealand for at least 14 days since visiting the location.

Earlier today, Victoria, now in its third day of a seven-day lockdown, recorded five new cases of Covid-19, bringing the state's outbreak to 49 cases.

The Otago researchers said that, if there proved to be frequent outbreaks in Australia, then New Zealand could consider adding in other interventions, such as pre-flight and post-flight testing and requiring mask use for the first two weeks here.

"But currently for amber zone countries such as Japan and probably the US at present given its progress with vaccination, we see no viable alternative to 14-day quarantine," they said.

"Added to this could be tighter constraints on traveller numbers, especially for red zone countries."

With the spread of more infectious variants, they said it'd be "critical" that ongoing and rigorous scientific risk assessment was used for decision-making here.

And also, they added, "that all aspects of border control are appropriately optimised for the differing risk posed by green, amber, and red zone countries".

The modelling comes as the Ministry of Health today reported two cases of Covid-19 in Managed Isolation and none in the community.

One confirmed case travelled from Lebanon and tested positive on day 10, and the other case travelled from Saudi Arabia and tested positive on day zero, bringing the total number of active cases in New Zealand to 16.

The Ministry of Health confirmed 4626 people flew from Melbourne airport to New Zealand between May 20-25, compared to the initial estimation of 5000 travellers.

However, 89 people from Melbourne are yet to be found after an order to get a Covid test and self-isolate.

Less than half of the 4626 people identified have been tested and returned a negative result.