New Zealand's long streak of no Covid-19 cases almost certainly would've been broken earlier had officials been testing all arrivals at the border sooner, an epidemiologist says.
While the country saw its no-case streak between May 22 and June 15, the Government only ordered testing of all arrivals - and not just those with symptoms - from the day New Zealand moved to Level 1, on June 8.
Since the streak was broken with two new cases and revelations of the Government's border blunder on June 16, five more have been reported.
Quizzed about that strangely quiet 24-day period, Director-General of Health Dr Ashley Bloomfield noted that, before last week, of many hundreds of travellers who had gone into managed isolation, just 35 had proven to be infected.
"Because of the increase in the number of people who are coming … and the acceleration of the pandemic offshore … that is why we are now seeing the cases," Bloomfield said.
Prime Minister Jacinda Ardern also hit upon those points today, telling reporters that New Zealand had seen a doubling of returnees over the past month.
Around 900 people were expected within the next few days alone.
"And that in part is because the world is starting to open up, so more flights are available, and more Kiwis are able to take up those opportunities to return home," she said.
She also added that the pandemic was stepping up, reaching eight million cases globally – and 150,000 new cases being recorded in a single day, last Thursday.
"Because this pandemic is growing, we are seeing it in those Kiwis who are being quarantined at our borders."
Another explanation was flights coming into New Zealand were arriving from parts of the world where there were now higher rates of Covid-19.
"We have in the past of course had a high proportion from Australia, where there were fewer cases."
Otago University epidemiologist Professor Michael Baker agreed the pandemic's growth and the higher volumes of people flowing in were two big drivers.
But a third factor was that, before the test-all policy came in, some positive cases may never have been picked up, he said.
"We would've had people who were asymptomatic, or who had flu symptoms and didn't get tested, sitting in isolation or quarantine – and they just didn't become cases because they weren't tested."
While estimates varied, he said around 40 per cent of Covid-19 cases turned out to be asymptomatic.
"You could almost double the number of cases you'd detect in arrivals, if you'd been testing them all," he said.
"So any one of these three changes could explain what we are seeing."
Bloomfield pointed out that, unlike back in February and March, all cases detected in New Zealand were being detected at the border or in isolation facilities.
"That's the difference here … and I think having testing times two, plus the managed isolation, is really a strong border defence," he said, referring to required testing on arrivals' third and 12th days in the country, along with the two-week mandatory quarantine period.
Baker suggested the Government should start reporting data on where all arrivals were returning from, which could help match levels of risk to different regions of the world.
"For months, there has been a strange optimism that [Covid-19] was going to peak and go away again – I don't why people think that, because it's just working its way slowly around the planet," he said.
"Simple back-of-the-envelope calculations suggest that it can kill 30 million people – and it's killed about half a million that we know of – and it may be that it's only five per cent of the way around the world.
"So it's only just starting in many places – and that means the pressure is going to be on our borders for the foreseeable future."