Covid-19 infections came from around the world, say experts who study the virus' genome.
In a Science Media Centre online briefing last week, ESR lead bioinformatics Dr Joep de Ligt said that genomic sequencing of New Zealand's COVID-19 cases suggests at least 35 unique introductions from overseas.
"They've come from all over the world, so we've seen them from Europe, Iran, North America," Dr de Ligt said.
"After that border closure, we've not seen any new introductions or at least direct links to travel. This supports border closure as a very effective measure to prevent that further importation."
University of Otago senior lecturer Dr Jemma Geoghegan said our knowledge of the new coronavirus was "actually quite remarkable".
"If you think it was only five months ago that this virus was completely unknown to us and today it's a subject of research on an unprecedented scale really."
She said the "secret" to the virus' success was that it was often a mild infection.
"This means contagious people are unknowingly infecting others by walking around, transmitting the virus."
In contrast, SARS makes people far more sick and with a higher mortality rate.
"When people are hospitalised, like they were often when they were infected with SARS, they weren't going around infecting people, which is the major difference here."
Although an early study suggested there were two major types of the SARS-CoV-2 virus, Dr Geoghegan said those findings were based on a small number of mutations and there was no evidence that mutations would lead to any functional changes in the virus.
Dr de Ligt said genomic sequencing of positive cases helped inform the public health response, especially in cases where there was initially no link to previously-known cases.
As an example, a Queenstown case was shown to be linked to the World Hereford Conference.
ESR has so far sequenced 125 samples and hoped to eventually sequence all New Zealand's positive cases.
"The virus almost certainly originated in bats. There is a very closely related, similar virus called the RaTG13 virus which was sampled from a bat species in China. It's possible that the virus first jumped on to an intermediate host before it jumped to humans. And that could possibly be a type of animal that had been in close contact with humans, perhaps at a live animal market for example," Jemma Geoghegan said.
"So we know that bats have likely transmitted the virus to other species of wildlife to because there is a closely related virus species that has been in pangolins, for example, that is a type of anteater."
There have been rumours about the virus escaping from a lab. What does genomic data tell us about its origins?
"All evidence points to the fact that Covid-19 has naturally derived from a non man-made source. And we know this by comparing the available data and genomic data from known coronavirus strains. It has been firmly determined the virus originated through natural processes.
"So the closest known relative of the virus is this bat virus. It's being kept at the Wuhan Institute of Virology. So there has been a lot of unfounded speculation that the virus was the origin of the pandemic through a lab escape (or something similar), however this virus was sampled from a different province in China where Covid-19 first appeared and the level of genome sequence divergence between the virus that causes Covid-19 and this other bat virus – termed RaTG13 – is equivalent to about 50 years of evolutionary change so these viruses are very different.
"It's very clear that it wasn't an escape of this bat virus. Moreover, there have been no reports of people getting sick that worked in that virology institute lab and also finally the first outbreak of this virus was associated with a live animal market in Wuhan, which is exactly where we would expect this virus to jump hosts so all evidence points to this as a natural process."
"The virus is continually mutating, like any RNA virus does, so we need to monitor whether or not these mutations lead to any changes in the way that the virus affects us, or the type of disease it actually causes. So we need to know how much the virus changes because that will be really important in the future therapies like a potential vaccine or antiviral treatments and how effective they are in combating the disease."