Achieving zero cases two days in a row was testament to New Zealanders' efforts that "we could be undeniably proud of", PM Jacinda Ardern says.
With no new Covid-19 cases for the second day in a row, New Zealand is on track to achieve its bold goal of eliminating the virus - but there's a slight hitch.
There's not yet a definition of what elimination actually is - so a group of scientists have proposed one, along with a set of qualifiers for it.
It comes after Prime Minister Jacinda Ardern and director general of health Dr Ashley Bloomfield were last week forced to clarify that New Zealand hadn't yet eliminated Covid-19, having earlier stated so to reporters.
In a blog post published today, Otago University epidemiologists professors Michael Baker, Nick Wilson and Sir David Skegg, along with Te Punaha Matatini director Shaun Hendy, set out the reasons why a concrete definition was needed.
Those included having a better grounding on which to eventually lower the alert level to one; to give businesses more certainty; and to offer a high level of assurance to Australia in the event that transtasman travel was re-opened.
"Many New Zealanders are understandably concerned about the risk of Covid-19," they said.
"Reaching a clear elimination goal will help them to make more informed decisions about things they may have otherwise avoided and which are permitted at a particular alert level."
While there was currently no accepted international definition of Covid-19 elimination, they said the most clear-cut approach would be to require zero transmission within a country or region.
They cited one definition suggested by a group of Australian researchers: "In practice this would mean no new SARS-CoV-2 cases linked to community transmission or unknown sources of infection over two incubation periods since the time of the last known community acquired case, provided a highly sensitive early detection, case and contact tracing and management surveillance system is in place".
For New Zealand, they said elimination should mean there had been no new cases for a specified period, such as 28 days and that a high-performing national surveillance system was testing a certain number of people per day, across the country.
They pointed to Australian modelling that suggested that testing all patients at primary care clinics who have coughs or fevers – potentially capturing 9000 people per week, per million of population – could ensure no new cases were missed.
But they added that level of testing was higher than that currently in New Zealand, which was around 6000 people per week, per million population.
They said the definition could also allow exemptions for new cases among travellers arriving at the border, so long as they were quarantined until they recovered.
If border control failed and any new cases were detected, the country's elimination status would be revoked until it could re-attain it.
As well, they said that special new criteria should be set for border management, contact tracing and surveillance.
Defining elimination would also require agreement across all Government sectors – and other countries and bodies.
"Since the benefits of an elimination definition extend to the international level, it would be important to consider a process that included other countries with which New Zealand might plan to extend travel links," they said.
"An obvious starting point would be to consult with Australian health authorities about the benefits of a shared elimination definition."
In the longer term, the discussion could potentially extend to other countries that were on similar containment paths.
"Involvement of the World Health Organisation [WHO] could also be considered," they said.
"For example, an elimination definition could be recommended by a joint panel of New Zealand and Australian scientists, and this might lead to a process mandated by WHO for verification of elimination."
The scientists also noted the difference between the terms elimination and eradication.
While elimination often referred to a "reduction to zero of the incidence of infection caused by a specific agent in a defined geographical area…", eradication instead typically meant "permanent reduction to zero of the worldwide incidence of infection caused by a specific agent".
