As few as 35 cases of COVID-19 coming into New Zealand led to an outbreak, according to genomic data.

In a Science Media Centre online briefing held 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 hopes to eventually sequence all New Zealand's positive cases.