Scientific experts say two or three key factors specific to Auckland's latest Covid-19 cases forced the Government to impose a seven-day lockdown.
The snap move applies from 6am today and follows confirmation of two new Covid community cases.
A man, 21, known as case M, went to the GP on Friday afternoon for a Covid test - then went to the gym. His positive test result was confirmed this afternoon. It has since been confirmed the man's mother has also tested positive.
Epidemiological and public health experts agreed the length of time the new Case M was exhibiting Covid-19 symptoms while mingling in public would have concerned Cabinet and the Ministry of Health.
Otago University infectious disease expert David Murdoch says the change in alert levels was the right decision.
He told Francesa Rudkin it's unlikely the family member that's tested negative three times produced false-positives.
"It is always a possibility, but to have three in the row for someone who is positive, that would certainly be an unusual event."
University of Auckland Professor Shaun Hendy said Case M had shown symptoms since about Tuesday so may have been infectious all week.
"This may mean that there are downstream cases in the community that are also infectious.
"Secondly, although it is possible that the chain of infection leads directly back to the cluster of cases at the school, the school pupil in the family tested negative three times and has not had symptoms.
Hendy said although it is "not impossible" for someone to test negative three times, "it is unusual".
"Thus it is possible that the infection came via another route and that may mean there is another cluster of cases in the community, linked to the school or otherwise," Hendy said.
"Finally, assuming the whole genome sequencing does establish a link to the cluster at the high school, these new cases have shown us the limitations of our contact tracing system when people are not able or willing to cooperate.
University of Auckland School of Medical Sciences Associate Professor Siouxsie Wiles said Aucklanders will be surprised and shocked by Saturday night's lockdown announcement.
"The details of today's case are too concerning not to. Once again we have an unclear chain of transmission," Wiles said.
"We know that they are a sibling of a student at Papatoetoe High who has already returned three negative tests and has not had any symptoms. Though it would be highly unusual, it is still a possibility this could be the source of case M's infection.
"The serology testing may help shed some light on this as will the genome sequencing. We also have the fact that the case has been infectious in the community for as long as the last week and has visited a number of locations. Moving Auckland to alert level 3 and the rest of the country to alert level 2 puts us in the best position to get on top of this outbreak as quickly as possible."
University of Canterbury Professor Michael Plank said two key factors made Case M's situation so concerning.
"One is that there is no established link between the new case and the existing Papatoetoe cluster," Plank said
"The second is that the new case has likely been infectious since 21 February and has visited a number of popular locations in the community.
"[So] there's a high chance there are other cases we don't know yet about, and they have potentially been out in the community for some time."
Plank said it was "highly likely" these new cases are the more infectious B.1.1.7 variant, which means the outbreak has the potential to spread faster.
"For these reasons, a seven-day period at level 3 makes sense," Plank said.
"This will give our testing and contact tracing system the time they need to track down any extra cases and shut off chains of transmission. As frustrating as it is, this is the right move to keep Auckland and New Zealand safe."
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