THE
CLUSTER
PARADOX

‘Cluster’ has surged frighteningly in the national lexicon during the Covid-19 crisis. But, as it turns out, the larger they’ve loomed, the more reassured we can be.

Watching the number of New Zealanders infected in our coronavirus clusters blossom, then spread through the country like blood through our veins can feel deeply worrying. 

One friend has turned those figures into something of a comfort blanket: each day he updates a spreadsheet he maintains to track each cluster. 

He’s got the right idea. Sort of. Because all those statistics we hear about our clusters should be deeply reassuring, says Professor Shaun Hendy, director of Auckland University’s Te Pūnaha Matatini centre. It means we are still in control. For the most notable thing about our clusters is how much we know about them.

Countries like the US have gone far beyond the point of tracking and encircling their clusters. We could have reached that point if we had developed a sudden, huge cluster, such as the one in South Korea where 500 cases exploded from a single church meeting. Our contact tracing system would have been overwhelmed, our capacity to ring-fence the cluster gone. 

“It can be alarming to look at the clusters and see so much detail about them,” says Hendy, “but other countries are so well beyond the point of looking at clusters any more.

“If the disease did get out of control here, we would very quickly lose the ability to trace those clusters. The fact we’ve seen them and are able to have a really good description of them is showing that … we are on top of it.”

As Prime Minister Jacinda Ardern said at one of her media briefings, the clusters “tell us something helpful” - where the disease came from, how it spread and to whom. Linking every case in a cluster to an ‘index patient’ - the source - gives health officials confidence they have contained the spread and not missed branches.

It’s the cases that aren’t attached to clusters that cause concern - where did they come from? Hendy says what clusters don’t tell us is the “low-probability stuff”: do we have chains of asymptomatic people who haven’t been tested, and who could be essential workers? 

Dr Harriette Carr, the deputy director of public health, says our big clusters (barring those in rest homes) have mostly followed a shared pattern: a ‘seed event’, where one person has infected another 15 or 20, usually in situations involving alcohol, and lots of repeat close contact over a long period of time. Then those 20 people go home to unwittingly infect workmates and colleagues. 

From studying our clusters, says Carr, officials have learned that in New Zealand, there’s two degrees of separation, and Kiwis are very mobile. Because symptoms begin gradually, people hadn’t realised they were sick until after they’d started spreading infection. And, they’ve confirmed, sticking to our bubbles works: one cluster had an unexpected post-lockdown branch caused by some illicit bubble-bursting.

Officials also discovered something about, well, our willingness to lie to public health officials. 

“People aren’t always honest about who their close contacts are - and you can have the best contact tracing in the world but if they aren’t honest, or forget, we can only go so far,” says Carr.

It’s too early, Hendy says, for science to give a detailed description of the environment that Covid-19 would best thrive in. But New Zealand’s five biggest clusters have come at a school, a wedding, a pub, a conference and a rest home. If, say, a month ago you had pressed Hendy to predict where our clusters might be, he probably would have picked those sorts of locations.

Actually, he would also have expected some at major events, like rock concerts or sports fixtures. We’ve been lucky in that regard, he says. Lucky too, not to have one overwhelmingly large cluster like the South Koreans (though our spread-out population will have helped).

Infectious diseases usually create clusters of infection. A cluster is defined as a group of 10 or more cases with a common cause.

Mapping those clusters could be done for all such illnesses - for example, with seasonal flu, but because it is less deadly, and we have a vaccine, no one bothers. It is done when we get outbreaks of measles, which is much more infectious than Covid-19 and occasionally fatal. It was done with SARS in 2002 but was much easier because people with SARS all had very strong symptoms.

What we know is that on average, each person with coronavirus will infect between two and three others. But some won’t infect anyone, and some will infect many (so-called “super spreaders”) - leading to a huge range of outcomes. All cases in New Zealand are imported - that is, they should link back to someone who caught the virus in Europe, China or the US then returned here with it. 

We have 16 major clusters in New Zealand, with the five biggest being Marist (the school, in Auckland), Bluff (the wedding), Matamata (the pub), Hereford (the conference, in Queenstown) and Rosewood (the rest home, in Christchurch). Here’s what we know about them.

There are two key statistics about the clusters that can offer some comfort. 

The first is the way their growth - once rapid - has slowed and even stopped.

The Marist cluster, for example, took 12 days to jump from 19 cases to 84. But only one new case was recorded in the week to Wednesday April 22. Similarly, Matamata saw only three new cases in that time, and Bluff, which grew by 81 cases over 15 days, saw just nine in that week.

On Saturday, April 25, the ministry declared one of the 16 significant clusters - the Boomrock wedding in Wellington - closed, as there has now been two incubation periods (28 days) since a case.

"We expect more clusters to be closed in coming days," the ministry said in a statement.

The second is the rise in the number of cases attributable to clusters. That means there are fewer unexplained infections, which means a growing confidence that the clusters have been effectively identified and ringfenced.

Three weeks ago, major clusters accounted for 20 per cent of all cases; two weeks ago it was 33 per cent; now it is 40 per cent of all cases.

And if you want extra reassurance: if we were in another country, then the different ways of measuring infection could mean our clusters would have been reported as much smaller.

Carr says we have developed one of the broadest ‘case definitions’ in the world. It means cases which might have been disregarded have been picked up, analysed and added to our statistics. For example, people reporting a runny nose (who might be dismissed elsewhere) are tested, even though they have a low probability of being infected. 

With Covid-19’s reproduction number now down to 0.48, Basu says we won’t see numbers now “explode” in the way the Marist and Bluff clusters did. But he notes, “if there were no restrictions on people’s movements, then clusters would form very rapidly across the country.”

It’s not impossible, Basu says, once we get more freedom of movement, that we will see new clusters form - it depends on how far elimination has progressed. “My guess is we have reduced it, but not all the way.” 

Carr says it all depends on whether every case has now been identified, or whether there are a few that have slipped the net. “If we can track and manage those, there’s a good chance it won’t happen. It’s whether we’ve been able to do that - that’s the million dollar question.”

There’s always a chance we will be surprised, agrees Hendy, and there remains a risk another cluster will form. But what we should all do, he says, as we move towards level 3, is take some lessons from the big five clusters on how to behave.

“[They] illustrate some of the risk factors if we do go to level three or level two, to do as much as we can to avoid the situation that has occurred in those clusters,” he says. 

“It is a really good lesson for us as we step down the lockdowns and move away from our more controlled situation. Everyone should take a look at those clusters, and think about the risks you are taking when you go back to engaging with other people.”