Covid 19 coronavirus: Why aren't we seeing more cases in Northland?

A Covid-19 modeller says it's a stroke of luck that the rogue case's close contacts have all tested negative - which could be due to woman carrying low amounts of the virus.

Director general of health Dr Ashley Bloomfield confirmed today that the infected woman was now considered recovered, and all 16 close contacts she encountered while travelling in Northland have tested negative.

There have also been no new cases in the community connected to the woman, who tested positive for Covid-19 on Sunday after leaving managed isolation at Auckland's Pullman Hotel on January 13.

Professor Michael Plank, of Te Punaha Matatini, suggested several reasons why New Zealand wasn't seeing more positive cases popping up in the Northland community.

"One is that the number of contacts wasn't very high over a week-long period, compared to what it could have been."

The 56-year-old returnee visited 30 locations in Auckland and Northland between January 14 and 22, scanning QR codes as she went.

While five of those places were cafes and two were bars, tracers identified just 16 close contacts, of which 11 were considered "genuine" ones and now isolating.

Close contacts are typically considered a person who was within two metres of an infected person in a closed space, for 15 minutes or longer.

Plank said it was important that she'd had relatively few close contacts, and hadn't attended any potential "super-spreading" events like large gatherings.

With few restrictions in place, New Zealand would have faced a scenario where the basic reproductive number - or the average number of people one infected person passed the virus on to - was at least one.

"So you could have easily gotten one or two cases from where she went, so this is good news," Plank said.

"But again, that all fits with the fact she wasn't mixing with a large amount of people."

It could also be the case the woman wasn't highly infectious, given her husband tested negative even after spending a period of time close to her.

"We know that people have different viral loads, and some people can be more infectious than others, just through random variation."

Otago University and ESR virologist Dr Jemma Geoghegan also said that was possible, noting that in New Zealand's outbreak, only 19 per cent of cases led to ongoing transmission.

She said it would be important to monitor whether the close contacts again tested negative upon further testing.

Plank added it was also possible the virus could have actually been passed on during the woman's travels.

"There's certainly always a risk that a contact has been missed, and that could test positive," he said.

"That's a low risk, on the balance of probability, but it is a risk."

Bloomfield said the woman was isolating at home and was "well again".

There were "no further firm leads" regarding how the woman caught the virus in managed isolation, but Bloomfield said he believed it was unlikely to have been transmitted through the hotel's air conditioning.

Covid-19 Response Minister Chris Hipkins said the negative test results were "encouraging".

Bloomfield also said there were "encouraging signs" in Northland but the situation was still unfolding.

"We're not breathing out just yet."

The close contacts of the Northland woman will remain in isolation for the full 14 days despite testing negative.

The 353 guests at the Pullman hotel at the same time as the woman are being contacted and tested - so far all staff and guests have tested negative.

There are also four new cases in managed isolation facilities today. The total number of active cases is 68 - including the one in the community.