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"I don't think we should see this as a surprise, I've been saying this all along. There will be more lockdowns in 2021 I'm afraid," he told Nine to Noon.
The prime minister confirmed this morning the latest cases had the UK variant, which Sir David says is more infectious and serious. But he said what was even more concerning was that they've been unable to trace any link to managed isolation and quarantine facilities (MIQ).
"That immediately raises the question of how this person became infected and how many other people might've been infected in the chain that led to them.
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"I think the most likely thing, and obviously this is speculation, is that this woman was infected by one of her colleagues at work who has been going airside ... and perhaps was in contact with someone who in transit who was infectious but wouldn't have been tested here in New Zealand.
"But, of course, if it was someone passing through the airport, we may never find a link with the original case."
Jacinda Ardern too said that was a possibility and they would be in touch with their counterparts to see if the genome sequence matched anyone who had recently transited through Auckland.
Sir David agreed with experts in saying that transmission through the laundry was unlikely. However, he said we may never find the primary source, but it was crucial to know how many others were infected or were in the chain of transmission.
He said it was good to hear that two close contacts were so far not showing symptoms.
"This is one thing we need to bear in mind, it's a very infectious disease but actually not everyone is as infectious as others.
"One of the most interesting things that has come out from another scientist at Otago [University] Dr Geoghegan ... she reported that during our outbreaks last year, only 19% of people passed the virus on to someone else. So that's only one in five, and that partly reflects their behaviour [movements] ... but also it will reflect their viral load and where it was in their system."
'We do also need to tighten our protections'
Although people get annoyed to hear we've been lucky as officials take stringent measures, he said we had indeed been lucky in this sense, with the border failures not becoming worse or spreading more rapidly.
"It's not meaning it's all luck, it's certainly not, but a chance factor in how many people get infected.
“That again really points to the importance of tightening up our procedures, both in terms of protections at the border and also within the community, there is such complacency, I hardly see anyone using the QR code as they go into places."
He said the country had done a good job last year compared with countries around the world, but while others were dealing with worse Covid-19 figures, New Zealanders became complacent.
"I think people really do believe we've somehow beaten this virus. We're having a wonderful time compared with the rest of the world, living a normal life, but we do have to accept this will only continue if do things right."'
'Stop dragging our feet' on saliva testing
Sir David said border workers were doing a good job and they shouldn’t be criticised if there was fault or gaps, but rather the system.
"Recently, I have a sense that the Australians, we're lagging behind them in some of the precautions. For example, in NSW now, there's mandatory daily saliva testing of everyone who works at the border or hotels which are MIQ facilities.
"But we're still doing weekly tests, or in the case of this woman who wasn't herself been exposed to travellers, but she was working with people who were - she was having fortnightly testing and because she happened to be away the day they came around, it went [on for] four weeks."
He said slowly piloting voluntary saliva testing once a week was not good enough.
"We need to stop dragging our feet and get on to that quickly."
Ardern told Morning Report there were questions around the accuracy of saliva testing results, but they were looking to use it in conjunction with nasal swab testing at higher risk MIQ.
Sir David agreed with comments from Professor Nick Wilson that even if there were shortcomings with saliva testing, then doing it daily would compensate for its shortcomings.
“If you're doing it every day it'll be better... also it'll give us an earlier warning that we need to act."
He also acknowledged it would put a load on labs. "I have the impression that perhaps some of our labs don't have the equipment that enables saliva specimen to be processed as quickly for PCR testing, but we need to get that equipment."
'Proud of what New Zealand has done'
Speaking on the vaccine, he said prioritising border workers would greatly reduce any risk there.
"It would be shameful if New Zealand was at the front of the queue for the vaccine, because thousands of people are dying everyday around the world, so there isn't the same urgency for the rest of us, and we might be better to wait a few months to find out which vaccines are the most effective."
While he said the public health system was poorly prepared for the pandemic, the efforts of all those involved had paid off.
"You just have to stand back and look at how our experience is compared with nearly every other country on the planet. You have to be proud of what New Zealand has done.
"Obviously, it's the job of people like me to point out where there are deficiencies, but we need to keep a sense of proportion here ... but please let's not be complacent."