• NZ is the only western country in the world with an articulated elimination strategy
• PM say Covid-19 is effectively eliminated in NZ right now
• That doesn't mean zero cases, but a small number that can be stamped out through aggressive management
• Public health experts say success won't be clear until we see any new cases at alert level 2
As New Zealanders emerge tomorrow after 33 days in level 4 lockdown, Prime Minister Jacinda Ardern says Covid-19 is effectively eliminated in New Zealand.
But what that means exactly is unclear - though politicians and public health experts all agree that it doesn't mean there won't be any more cases.
Elimination broadly means managing a small number of new cases, breaking known chains of transmission and having systems in place to stamp out community transmission, where the source of infection remains a mystery.
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There have been nine straight days of new cases in the single digits, and there is now only one case of recent community transmission - in Tauranga - which director general of health Ashley Bloomfield said could yet be connected to an existing case.
"That does give us confidence that we've achieved our goal of elimination, which never meant zero (cases) but it does mean we know where our cases are coming from," Bloomfield said today.
Asked if Covid-19 had now been eliminated, Ardern said: "Currently."
She added that the number of new cases may well reach zero but then be followed by a small number of cases popping up again.
"That doesn't mean we have failed. It just means we're in a position ... to have very aggressive management of those cases and keep those numbers low and fading out again."
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But public health experts are reluctant to declare mission accomplished until there are defined criteria and a few more weeks of breathing space.
What does elimination look like?
"According to the World Health Organisation, elimination refers to the reduction to zero or to a very low defined target rate of new cases," epidemiologist Sir David Skegg told the Epidemic Response Committee last week.
Skegg suggested that success might not be apparent until level 2, or even level 1.
"Elimination is when we can all go back to normal life, when we can just basically forget about Covid-19, that the public health service will protect us," he told the committee.
"We can stop isolating at home, people like me over 70 don't have to hide, and people can travel around the country, have holidays, you know, and get back to normality."
That sentiment was echoed by Otago University epidemiologist Professor Michael Baker, who said measures which "disguise transmission" like the lockdown should be considered.
"At the moment the country is effectively in home quarantine. The real test is when you come out of lockdown: has the virus disappeared?" he said.
"Proving something is present is easy. Proving it's gone is really difficult."
Baker said there were set definitions for elimination that vary for different diseases.
There was no such definition for Covid-19 as far as he was aware, and no decision should be made without a wide-ranging discussion.
But one crucial criterion, he said, was "no evidence of community transmission for a sustained period".
Breaking the chains of transmission
There are several signs that suggest the chains of transmission have been broken.
These include the low number of cases (1469) amid a high number of tests (almost 125,000), and an R value (the number of people that one case normally infects) of 0.4, much lower than the global average of about 2.5.
"But there's still a chance we have asymptomatic transmission taking place," said Auckland University Professor Shaun Hendy, who has been modelling the spread of Covid-19 for the Government.
"We want the probability of that to go basically to zero, and then we'll declare in our simulation that we've then eliminated Covid-19.
"We won't be completely sure we've managed it for some time."
But Hendy added that it was a matter of definition.
"Looking at the actual case data, strictly speaking you could say we've eliminated now. I'd probably say it's contained."
Baker didn't want to offer a view on whether we were there yet.
"The whole country wants to see those cases drop to zero. But it's not a question of marking a spot in the calendar, but looking at the direction of travel.
"We are definitely heading in the right direction."
He added that he didn't see Ardern's and Bloomfield's comments today as anything absolute.
"I think they're saying we're making very good progress, but they're not really saying we've achieved elimination at this stage - that was my take on it."
The elimination plan - is it good enough?
In the absence of a vaccine, Baker said there were three lines of defence in striving for elimination.
The first is a watertight border - keep it out. Eighty per cent of cases are linked to overseas travel, and at the end of last week, there were 2037 people in managed or strict isolation after arriving from overseas.
Following concerns raised by Skegg, Air NZ is now testing all air crew - who are exempt from the border quarantine - returning from overseas.
The second line of defence - stamp it out - is ring-fencing chains of transmission.
That means identifying the index case - or the source of infection - and then tracing and isolating relevant contacts. In lockdown, such isolation should be automatic.
The third line of defence is for community transmission where, without a known index case, the branches of transmission are harder to identify and isolate.
There are about 45 cases classified as community transmission - and these have infected approximately 340 more people.
But the lockdown should have snuffed out all but the most recent cases, and since April 1, there has been only one case.
"The cases in the last one to two weeks are the ones to look for," Baker said.
"And cases imported are not posing a risk if they are in secure quarantine. They are very different from cases wandering the countryside."
A comprehensive system to fight community transmission was crucial to containing new outbreaks.
That included testing the smallest of sniffles, rapid tracing and isolation of contacts, and surveillance testing to check for undetected transmission.
Bloomfield said each DHB was making surveillance plans to cover all regions and demographics, as well as workplaces such as hospitals, aged care facilities and supermarkets.
Last week he said the gold standard of contact-tracing was being able to trace and isolate 80 per cent of contacts within three days.
"I believe we are at that point," he said today.
The ministry can now trace contacts for 185 cases, but this is still well short of the 1000 cases that was flagged in the independent audit of contact-tracing capacity.
NZ's unique path from mitigation to suppression to elimination
Baker said New Zealand was the only western country in the world to have an articulated strategy to eliminate Covid-19.
(Elimination may be within reach for Australia, but they are not officially striving for elimination and only Brett Sutton, the health chief in Victoria, has said it was possible there.)
New Zealand started fighting the virus with mitigation, which was the official strategy of a pandemic plan based on influenza outbreaks.
The principle was that a pandemic could not be stopped and the best way to manage it was through interventions - such as closing schools - to slow the spread.
"Flatten the curve" was the term Ardern used to describe the strategy on March 14, when she ramped up border restrictions so that almost everyone arriving from overseas had to self-isolate.
But mitigation in Europe had failed to keep health services from being inundated, and four days later, the language changed.
"What we have done to date has been talking about flattening the peak, but even if you do that, you are still likely to exceed your health system capacity," Bloomfield said on March 18.
"So therefore our approach is – and this is what successful countries have done – you want to have a series of small peaks over a long period of time."
Mitigation had been replaced by suppression , which meant yoyo-ing in and out of various stages of lockdown.
Elimination emerged as an option in the early weeks of March and Baker, a member of the technical group advising Bloomfield, was a key figure in nudging the Government towards it.
Baker said a "turning point" was a World Health Organisation report at the end of February showing that Covid-19 was more like Sars and could be stamped out, even after community transmission had been established.
Another factor was the success of parts of Asia. Taiwan, for example, has had 429 cases and six deaths in a population of 24 million.
"We were watching countries like China and Taiwan and seeing the rollout of our pandemic plan as per usual," Baker said.
"Elimination is not a concept that comes up very much, but China did it with 1.4 billion people - stopped it in full flight.
"There has been more transmission at the border there, but they did what many thought was impossible."
Instead of rolling out interventions to fight outbreaks, elimination was the reverse: shutting down schools and staying at home before the virus had a chance to take off.
"You throw everything you have at the start to snuff out chains of transmission, which buys you time to get everything else working," Baker said.
Ardern made the decision to chase elimination in the days between March 19, when she said the borders will close to non-Kiwis, and March 23, when she announced the lockdown was coming.
Baker was elated with the move.
"People were staggered that New Zealand went into lockdown with only 102 cases and no deaths.
"But it's a tribute to our leaders in politics and business and civil society that we could change our direction very swiftly."
New Zealand has so far avoided the thousands of deaths that have hit most other western countries, which are likely to come in and out of lockdown for months.
Success in New Zealand means not returning to lockdown, and a "new normal" with domestic tourism and local businesses thriving once again.
But even if Covid-19 was now eliminated, Ardern said the battle was far from over and called for extra vigilance as alert levels were lowered.
"We must make sure that we do not let the virus run away on us again and cause a new wave of cases and deaths."
Skegg told the Epidemic Response committee that success would be a "historic victory".
"If we are successful, this will be one of New Zealand's greatest achievements."