A new analysis has highlighted how geographic isolation wasn't the only factor that enabled New Zealand and three other island nations to quash Covid-19.
The collaborative study, which compared our early experience with that of Australia, Iceland and Taiwan, has underscored the effectiveness of chain-breaking lockdowns - and the power of modern data.
It's also revealed how some cases were incorrectly linked to New Zealand clusters - while others should have been.
The paper's lead author, Dr Fabio Kuriki Mendes of the University of Auckland, said all four countries shared some measure of success in controlling the first wave of disease a year ago.
"Our hope was that the geographic isolation these countries enjoy would provide us with a cleaner statistical signal when studying the progression of Covid-19 - particularly under the light of varying human movement."
The research team drew on genetic data, collected from genome sequencing, and then used it to reconstruct how the virus spread, with "phylodynamic" tree diagrams.
"It's an alternative to other mathematical approaches that epidemiologists like to use, which do not look at genetic data."
Drawing on Apple phone data, the team also designed a model that showed a consistent drop in human movement across all four of the countries, and which informed their analysis of the trends in genetic data.
Lastly, they assessed the overall effectiveness of contact tracing by checking whether genetic samples, matched to New Zealand clusters by the Ministry of Health, stacked up with their own model.
The results showed that our contact-tracing efforts were generally accurate, even at the height of the national outbreak.
"Early on in the epidemic, the New Zealand systems weren't in place to have the sort of real-time genome sequence that we are now using," University of Auckland research fellow Dr Jordan Douglas explained.
"So this paper uses genomes to take a look back at how contact tracing performed in that first period and how successful it was at identifying clusters.
"Generally, contact tracing did a good job, but the genomes tell us that some cases were not put in a cluster they belonged to - and others were assigned to clusters when they clearly were infected elsewhere."
After analysing 217 genomes - representing about 19 per cent of cases in New Zealand at the time - they found five cases appear to have been mismatched to clusters, while 18 "unclassified" ones actually could have been linked.
"This tells us that having both systems running in tandem is essential for accurate understanding of how Covid is spreading," Douglas said.
While a lack of available genetic data made it harder to delve into similar detail in the other studied countries, some similarities were clear.
The four countries all fared far better than other nations, where the virus quickly became widespread.
In the studied countries, the average number of people directly infected by a single infectious individual - or the reproductive or "R" number - was one to 1.4 at the onset of the pandemic, compared with two-to-six in others.
That reflected the fact that a large fraction of cases in the four countries were travel-related imports and that community transmission rates were low, even before the "R" number was pushed below one.
But University of Auckland scientist Dr David Welch said Taiwan stood out in movement data, given it was able to control Covid-19 without major restrictions.
"This is in contrast to the other countries we looked at which required strong lockdowns."
Taiwan - population 23 million - also saw fewer cases per capita than New Zealand, despite our lower population density.
The researchers put Taiwan's success down to the lessons it had learned from the first SARS outbreak in 2003, with the country rolling out border controls and mass-masking early.
Still, they noted New Zealand's hard response enabled it to become the first country with more than 1000 confirmed cases to return to zero active cases, and reopen its economy.
"Conversely, while Australia also saw a decline in human mobility and initially enjoyed some measure of success against Covid-19, its more lenient approach, compared with either New Zealand's or Taiwan's, might underlie Australia's longer path to elimination," they added.
Welch felt the study ultimately showed how modern data could help understand outbreaks - and how to prevent them.
"Genomes show a large number of cases coming across the border - about 40 per cent of all confirmed cases in the first few months were introductions," he said.
"Controlling the border and introducing the lockdown, which radically reduced people's movement around the country, showed up strongly in the data as effective controls, greatly reducing the reproduction number and controlling the disease.
"Similar results are seen in the other island nations we looked at - Taiwan, Australia, Iceland - which, like us, had some of the most effective responses in the world.
"While this study is retrospective, much of the work reported was feeding directly into the response at the time and with these models now well established, we are able to respond even more quickly."
New Zealand's success has also been described in a new study, just published in the major journal Nature Immunology.
It found how New Zealand had avoided the worst of the pandemic due to decisive government leadership informed by molecular virology, infectious disease epidemiology and immunology expertise.