Covid 19 Delta outbreak: Why the case for level 2 was strong, and for 2.5 even stronger

Prime Minister Jacinda Ardern will announce Cabinet's alert level decision for regions outside Auckland at 4pm today. Photo / Mark Mitchell

ANALYSIS

It's been more than 300 days since any Covid-19 was in the community in the South Island.

The risk that the current Delta outbreak had leaked from Auckland over the Cook Strait was real in the first few days, especially in light of the thousands who fled the City of Sails for their Queenstown or Wanaka pads.

News: NZ outside of Auckland to level 2 at midnight tomorrow - new mask and gathering rules; Covid checks at border

But there has been no sign of any cases there despite constant community and wastewater testing.

There's also been no sign of any cases in the North Island except in Auckland and Wellington, and it's been more than two weeks - a full incubation cycle - since the last time a case in Wellington was infectious in the community.

Those are the cases we know about.

There can't be certainty about a complete absence of cases outside Auckland without testing everyone outside Auckland multiple times, so public health officials can only give their best guess based on the most current information.

We don't know the status of 5300-odd contacts who are yet to be formally contacted - some of them might be carrying Delta in the community as they head to the supermarket.

Contact tracers have been working through the 38,000 contacts - including 500-odd in the South Island - according to priority, so while 5300 is a lot of people, most of them are likely at the lower end of the risk scale.

Less than 0.5 per cent of contacts have so far tested positive for Delta.

The rate for very close contacts - including those who live or work with known cases - is much higher; last week it was over 15 per cent.

But there are only about 10 or so of these contacts who are yet to be contacted, and they may not be outside Auckland.

There are also the contacts we don't know about yet, but the total number of contacts has been plateauing, along with the daily case numbers.

Director-general of health Ashley Bloomfield will dial in to today's Cabinet meeting with the latest data about the outbreak. Photo / Mark Mitchell

Two key clues about the spread of the outbreak are the number of mystery cases, and the number of cases who were infectious in the community.

The former dropped to 33 today, and there's a declining trend in the latter: 27 on Wednesday, 17 on Thursday, eight on Friday, six on Saturday and five on Sunday.

All of these cases have been in Auckland, and given the level 4 restrictions, it's highly unlikely that any missing links around the mystery cases are outside the city.

So as long as testing rates have remained high enough, it would be hard to argue that there's a good chance of Delta hiding somewhere outside Auckland.

The harder judgement call is how much restrictions should be eased.

It's a huge leap from level 3 to 2 - everyone back to work and school, inter-regional travel, and gatherings up to 100 people - and Otago University public health experts are arguing for level 2-plus.

The alert levels were based on a time when we thought the virus was most likely caught during a 15-minute interaction within two metres of a case.

But we've seen how Delta can be caught from strangers passing in a mall.

That means tighter restrictions make sense where transmission is most likely, such as crowded indoor spaces with poor ventilation.

It also makes sense for more indoor mask-wearing - level 2 requires it on public transport, taxis and flights - which reduces the chances of transmission.

Such measures would keep Delta from getting a massive head-start, should there be any cases hiding anywhere outside Auckland.

Any change requires tight borders around Auckland, and an efficient surveillance testing regime for essential workers travelling in and out of the city.

Ongoing compliance for whatever the future holds remains vital.