The emergence of historical COVID-19 cases in New Zealand isn't surprising and there could be other cases we weren't aware of, an infectious diseases expert says.
The Ministry of Health reported six "historical" cases on Wednesday, made up of one confirmed case and five probable. The old infections from February - believed to be New Zealand's first - have only just come to authorities' attention and pose no risk to the public.
It comes as Auckland moved to alert level 2 on Wednesday night, allowing gatherings to have up to 100 people.
The historical confirmed case recently developed a sore throat and got tested. A weak positive result, combined with serology test results and case history was "consistent with an old infection", the ministry reported.
Associate Professor Mark Thomas, an infectious disease expert at the University of Auckland, told Newshub historical cases were inevitable.
"I think it's inevitable given the excellent work that is going on in New Zealand to really understand, as carefully as possible, all cases and any degree of spread from those cases," he said.
"It won't surprise me if a few others cases and maybe clusters turn up in the future as people get diagnosed with a positive test, but presence of antibodies and an illness in the past that was probably COVID.
"These sorts of stories will be put together to explain why somebody who turns up with a positive test and had an illness and an exposure way in the past, how it happened."
He said there was no reason for Kiwis to be concerned about the cases, something microbiologist Dr Siouxsie Wiles agrees with.
"Such cases have been found in many other countries and are because we now know that the COVID-19 virus was already beginning to infect people in parts of Europe while the world was focused on the growing outbreak in China," she said.
"We may yet see more of these cases though it will still only amount to a small fraction of New Zealanders. The rest of us remain very much susceptible to this virus."
The infections occured in late February following a Waikato family's exposure to an infected person - a relative - who had arrived in New Zealand from Italy and became unwell with COVID-19 symptoms.
At the time, New Zealand's case definition for testing people for COVID-19 was limited to people with defined symptoms who had travelled from or through China, where the virus' global spread is believed to have begun.
"Italy had not at that point been identified as a country of concern. This meant the New Zealand household was not tested at the time," the Ministry of Health said.
Thomas said the cases show how much has changed since February.
"While we were watching COVID in China at that point, people weren't thinking about COVID in Italy. If those people had become unwell in the present scenario, it would have been identified as being COVID. But back then, it was not recognised, so not tested."
With the historical confirmed case having symptoms on February 21, they are now understood to be New Zealand's first case. Previously, our first case was reported on February 28.
"Consequently, the other household cases would represent the first locally acquired cases of COVID-19 in New Zealand, given the onset of symptoms from 29 February for the confirmed case. Further investigation will continue," the ministry said.
"As part of the investigation, close contacts of this man during his recent mild illness have been tested as a precaution and have returned negative tests."
'A very, very unusual event'
The Ministry of Health also told Kiwis of three new community cases on Wednesday that are not linked to the August cluster. Instead, they're connected to the Christchurch-Auckland charter flight that a returnee who last week tested positive after leaving managed isolation also flew on.
That individual arrived back in New Zealand on August 27 from India, completed his 14 days of isolation, had two negative tests, but was found to have COVID-19 days later.
Questions remain about how the man was infected. Genome sequencing has found his case is consistent with several others on a flight to New Zealand.
"It might well be that this man, who was in isolation in Christchurch, was infected at the time as his Delhi to Fiji flight. He maybe was then incubating the infection while in managed isolation in Christchurch," said Thomas.
"He presumably either had an inadequate second swab that didn't detect the infection or he had a perfectly acceptable second swab that was negative but he had a very long incubation period. A very, very unusual event."
But Thomas said it showed our system for detecting the virus was working.
"This is a sort of bizarre episode that we are likely to turn up with all the careful testing and contact tracing that we are doing. I don't think there is any reason to be particularly concerned about it."
Eighty-six people were on the charter flight, all of whom have been contacted and are being tested. Sixty-three have returned a negative test.
While most of New Zealand - excluding Auckland - is now at alert level 1, Thomas said there is still value in wearing masks on planes and it's little inconvenience for passengers. Air New Zealand will still require customers to wear masks while Auckland is at alert level 2.