New Zealand's milestone of having no active Covid-19 cases has put the country well on track to achieving its bold goal of elimination – but there's still confusion and debate about what exactly that is.
Kiwi researchers have now renewed calls for countries to agree to a specific and universal definition, which they say would give Governments clearer targets to work toward, and help reopen borders with other virus-free states.
New Zealand's position was that elimination was not a point in time, so there could never be a date on which it could say it had eliminated Covid-19.
Rather, the Government has opted to take elimination as being "confident" that the chains of transmission in the community have been extinguished for at least 28 days, as well as being able to effectively prevent or contain any cases coming in from overseas.
It set its 28-day count as starting at the point at which the last case of community transmission finished their self-isolation.
"The last case of Covid-19 in New Zealand that we haven't been able to confidently identify the source of infection was a person who tested positive for Covid-19 on April 29, and they were in self-isolation until May 18," a Ministry of Health spokesperson said.
New modelling released over the weekend by Te Pūnaha Matatini researchers showed a 95 per cent probability that Covid-19 had been eliminated in New Zealand – and that rate was expected to climb to 100 per cent this week.
The researchers also defined elimination as New Zealand having no active chains of transmission left.
"The models show that the risk of new cases being transmitted within New Zealand is now about the same as the risk of new cases making it across the border," explained Professor Michael Plank, of the University of Canterbury.
"That could be a new arrival into New Zealand infecting somebody in the same hotel who is about to leave quarantine. Or it could be via people who work at the border, such as cabin crew or airport personnel."
Either way, the risk of domestic transmission dropped with each day with no new reported cases, while the risk at the border stayed about the same.
But Plank said defining elimination was tricky, because it was about "not" having something – and it was impossible to test all 5 million New Zealanders to check that none of us has Covid-19.
"So that means we need to use models to estimate the probability of elimination using the data available."
He agreed that, as countries start to open up their borders, having a set of agreed criteria for when people are allowed to travel between different countries would be important.
"This might vary from country to country. For example, some European countries are allowing international travel despite having lots of cases," he said.
"For transtasman travel, we would want to be confident that both New Zealand and Australia had eliminated Covid-19."
Last month, epidemiologists Sir David Skegg, Professor Michael Baker and Professor Nick Wilson, along with Te Pūnaha Matatini, set out several reasons why a concrete definition of elimination was needed.
Those included giving businesses more certainty and offering a high level of assurance to Australia in the event that transtasman travel was reopened.
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 could ensure no new cases were missed.
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 reattain 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.
In the longer term, the discussion could potentially extend to other countries that were on similar containment paths.
They also suggested 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 World Health Organisation 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".
Speaking to the Herald today, Baker said a clearer definition would give New Zealand a tangible point to measure progress.
"And particularly, if there are setbacks – which would be quite possible if we get a reintroduction of the virus – it gives us a reference point, in terms of control measures," he said.
"It's also important, if we are going to be looking at increased travel to other countries that have also claimed to have eliminated the virus, to have a definition that we can apply to them, and which they hopefully also might accept as reasonable."
While the SARS-CoV-2 was a new virus, Baker said decades of work on other infectious diseases had created a framework with which elimination of Covid-19 could be defined.
"The terminology around managing serious infectious diseases has been around for 80 years – so it's very established."
As for today's milestone of zero active cases, and the Government's call to move to level 1, Baker saw it as only the end of the first battle in what will be a long-term war against the virus.
"The threat from Covid-19 obviously remains while this pandemic continues across the globe," he said.
"This risk will rise again in New Zealand as we gradually increase the numbers of incoming travellers.
"It will also rise during the coming winter when coronaviruses are more transmissible."
Fellow University of Otago epidemiologist Dr Amanda Kvalsvig also sounded caution, pointing out that having no active cases wasn't really a meaningful landmark for pandemic control.
"The numbers of 'active' and 'recovered' cases don't tell us how many people are still infectious, and don't answer the really important question which is whether there is still virus circulating in the population," she said,
"If we continue to find no new cases despite ongoing testing, that's much more informative and so far the results are encouraging."
She added that the official definition of recovery from Covid-19 and the reality of that recovery process could be quite different.
"People recovering from Covid-19 infection are reporting a variety of concerns ranging from feeling unusually tired to more serious post-viral complications," she said.
"True recovery in the sense of feeling completely back to normal may take much longer than expected and this issue will need attention in the weeks and months to come."