Cases that were infectious while in the community will drive any ongoing transmission under Level 4 and could cause us to rethink our restrictions, Marc Daalder reports
Analysis: The daily rollercoaster of new case numbers may be captivating, but it doesn't tell us much about how well the lockdown is working.
What truly matters is the source of each new case – and where they might have been before they tested positive.
A majority of the new cases we're reporting each day share a bubble with a known Covid-19 case. Given how quickly Delta can spread in a household, these are people we expected to test positive. Almost all of them will have been isolating - or even in quarantine - for the period of time in which they may have been infectious.
Of the 82 cases reported on Sunday the Ministry of Health had data on, 43 fell into this household contact category.
There's another group of people who aren't too worrying either. Those are people who have been at a location of interest and are therefore contacts of a Covid-19 case, even if that case isn't in their household. Because they should be isolating as well, these people don't pose a major risk of onward transmission, provided we find them in time.
This group made up another 13 of Sunday's cases.
The final group are the type of person we really should be worried about. The remaining 26 or so cases are still being investigated, and it's not unusual for that to take a few days to link them to a location of interest or another Covid-19 case. But if we can't find that link, that's a warning sign that Covid-19 is still spreading through the community undetected.
"We do still have cases that are requiring investigation," Prime Minister Jacinda Ardern said on Sunday.
"Often we’re able, over time, once we have different new locations of interest, to build a map of what’s happened, but we do still have mystery cases coming through, and that says to everyone you’ve got to be vigilant. It might be you. So just please be careful. We’re really still at a critical point right now."
Overall, the number of mystery cases is very small. From the 564 cases in the Delta outbreak so far, just 42 had yet to be epidemiologically linked to the main cluster as of Monday. Many of those would have included cases still under investigation from the past couple of days.
Therefore, we shouldn't necessarily be concerned that there are 26 cases from Sunday alone who were the result of undetected community spread at Level 4.
That's one of the problems with relying on the number of unlinked cases alone - it fluctuates, dropping each day as investigations find links and then jumping once new cases are found and the investigations are started once again.
Infectious in the community
There's a more helpful and certain number which we can also use as a proxy for how well the lockdown is holding up, experts say.
"The one number I was very interested in was the cases who generated the risk of onward exposure. That's the number we care about, they're the cases that are at risk of infecting other people," University of Canterbury mathematics professor and Te Pūnaha Matatini disease modeller Michael Plank told Newsroom.
From Saturday's cases, there were 25 people who had been infectious while they were in the community. That didn't necessarily mean they had infected someone, but it's a more certain figure than the prone-to-change unlinked cases number.
Of Sunday's cases, the number who had been infectious in the community dropped to 23. Of those 23, seven were considered to have been infectious prior to lockdown while the balance had been out and about during lockdown.
This doesn't mean people have necessarily been violating the rules - it could also encompass transmission among essential workers or at essential locations, or indicate that infection prevention controls and distancing rules aren't sufficient to stop the highly infective Delta variant.
"I was quite encouraged that 50-odd of the cases were not at risk of spreading it in the community. That's not bad. But I would like to see that other number coming down over time," Plank said.
Ardern said she didn't think it was possible to reduce the number of cases that were infectious in the community all the way to zero.
"At Level 4, there are people who are still keeping the country running and keeping people fed and moving key freight around the country. The idea that you'd get it down to zero, even at Level 4, that's hard. When we look just at our supermarket chains, 55,000 people, so it's not hard to see how you could hit people from different walks of life in an outbreak like this."
However, she said, this concern was what motivated the Government's second look at Level 4 rules. She wanted to make sure they were being followed. If they were being followed and transmission continued at significant rates in essential workplaces, then stronger restrictions might be needed.
'Maximum effort now'
So far, however, this post-lockdown transmission doesn't risk spiralling out of control. Indications thus far are that the lockdown has greatly reduced transmission, bringing the effective reproduction number (how many people each case infects, on average) close to or even below one. That's from a base level of five or six for the Delta variant at Level 1.
Director-General of Health Ashley Bloomfield said the effective reproduction number right now was 0.8, but Plank said he wouldn't go so far as to say it was below one.
"It's a little bit early to say that the reproduction number is 0.8. The way I would phrase it is that what we're seeing is consistent with that scenario but it's too soon to say with any certainty that it's less than one," he said.
Given most new cases each day are household contacts, once that reservoir dries up, transmission could fall further.
"We're seeing now a period where there's a lot of household transmission from cases that were infected before the lockdown. It's now moving through those household contacts. Hopefully, if the lockdown's doing a good job of preventing it getting into new lockdowns, hopefully that will ramp down over time," Plank said.
University of Otago epidemiologist Michael Baker said he'd want to see the effective R number fall well below 0.8.
"Unless we can push the reproduction number down really fast, it means weeks - many, many weeks - of Alert Level 4 to stamp out the virus," he said.
"I think we really need to do maximum effort now, quickly, to get the numbers down and avoid cases and hospitalisations and deaths, as fast as we possibly can."
