There was likely spread of COVID-19 in New Zealand before the earliest current known case, experts say, backing the unprecedented decision to put the whole country into lockdown.
For most of 2020, it was believed a man who arrived in the country from Iran who was reported being positive for the virus on February 28 was our first case. But in September, a man in his 60s with a cough and sore throat tested positive for the virus - despite no clear link to the cluster that recently emerged in Auckland.
It later emerged he'd been one of six people who'd fallen ill in late February after a family member had returned from Lombardy, Italy.
"At the time, laboratory testing capacity and the case definition as it stood (requiring travel from China) were insufficient to allow broader testing of individuals who had arrived from other countries," Waikato DHB Medical Officer of Health Richard Vipond told Newshub.
"It wasn't until later that we learnt that there were other countries experiencing significant COVID-19 transmission early on in 2020 (in this case it was Italy)."
Blood testing suggested he had a historic infection, pushing back the date of New Zealand's first known and probable cases to February 23. This was widely reported at the time.
Doctors at the Waikato DHB have now taken a closer look at the six cases, saying there's a chance "social events our cases attended prior to lockdown may have resulted in some transmission - which lockdown then helped to stamp out".
No one knew it at the time, but COVID-19 can be spread asymptomatically - when a person never develops any symptoms - or presymptomatically, before they start showing symptoms.
"It would be speculation without having any confirmatory laboratory testing, however, it seems likely that other cases went undetected in early 2020," said Dr Vipond.
Italy's first known case wasn't until February 21 - just days before it travelled from there to here. Later research found evidence of antibodies for the virus in Italians as early as September 2019, when no one even knew it existed, let alone was spreading outside of its origin in China.
There were no other cases linked to these six at the time, despite "frequent contact with the public" as part of the man's work. But that doesn't mean there weren't any, with "very limited testing capacity" in February.
"There were COVID-19 cases in New Zealand from the period (March-April 2020 while we were in lockdown) for which we were unable to identify a source," said Dr Vipond.
"By definition these are cases of 'community transmission' and supports that there were other missed cases and possibly clusters at the time."
In the report, published in the New Zealand Medical Journal on Friday, Dr Vipond and his colleagues said this backed the decision to go into a strict lockdown a month later.
"This approach saw New Zealand avoid any significant health system shock, and the virus was eliminated from within the community for 102 days," the article, Keeping up with COVID: identification of New Zealand's earliest known cluster of COVID-19 cases, says.
"The identification of this household cluster, which was epidemiologically linked to a region of Italy that subsequently became a region of international concern, provides evidence that undetected local transmission of SARS-CoV-2 was occurring from at least late February 2020.
"This finding offers additional evidence that supporting the New Zealand lockdown action and following public health advice for any influenza like illness were effective strategies in reducing transmission, from both identified cases and unrecognised clusters, to the point of elimination."
Another finding from the study is that genetic material from the SARS-CoV-2 virus, which causes the disease, can still be picked up six months after even a relatively mild illness. None of the six cases required hospitalisation, all recovering at home in self-isolation.
The low level of genetic material picked up in September, when the man fell ill again, and the presence of antibodies suggested he wasn't infectious, Dr Vipond and colleagues said.
"This case investigation... supports international evidence that SARS-CoV-2 genetic material can be detected on a nasopharyngeal PCR test for at least six months after initial COVID-19 infection."
Only 26 people in New Zealand have died from COVID-19 to date, one of the lowest per capita rates in the world.
"I remember when we made the decision in early February to restrict movement between New Zealand and China - that seemed like a monumental decision, yet only weeks later we were making the decision to lock the country down and effectively close the border to everybody," COVID-10 Response Minister Chris Hipkins told The AM Show on Thursday. "Things moved very, very fast."
"Like the entire Western world, we were following our influenza mitigation strategy - and we changed at this moment," epidemiologist Michael Baker of the University of Otago added.
"When the decision was made to go to level 4, we took a completely different direction to the rest of the Western world, and that was to basically stamp out the virus entirely."