What does it take to stop a cluster?

One case of Covid-19 can lead to many. Farah Hancock looks at clusters and what it takes to stop them.

One case of Covid-19 can lead to a cluster with devastating results - even when there are measures in place to limit transmission.

A number of recent deaths have been linked to clusters. Amid calls to end Level 4 lockdown early, Prime Minister Jacinda Ardern likened the current situation to a sports game. 

“... we can't squander a strong half-time lead by letting up or getting overconfident.”

She said there would be no early exit from the current alert level.

“We are definitely not in a position to move out of Level 4 early. Nor can we afford to change anything we are doing while we are here.”

A clear example of where one case has become many is the cluster centred around Marist College in Auckland. 

It started March 22 with a single case. It’s thought ‘patient zero’ is a teacher with no travel history. As well as no travel history, to date, this teacher has not been linked with a known case.

By the time lockdown started there were five cases linked to the school. By March 31 this had grown to 48 cases. At the time of writing there are 93 cases of Covid-19 - all sparked from a single case.

The lockdown, contact tracing and self-isolation are hoped to help halt clusters in a ‘ring-fencing’ strategy.

What is ring-fencing?

In the current situation, Covid-19 is a bit like a hungry hitchhiker. It moves about in people: new people represent a new ride - and a free meal. 

With some infectious diseases you can ‘starve’ and stop a virus by making sure plenty of people are vaccinated. The virus is held in by a ring of vaccination.

Massey University’s distinguished professor Nigel French is part of the Ministry of Heath’s technical advisory group for Covid-19. He describes ring vaccinating as a bit like a fire break.

With no vaccine, or immunity, due to previous exposure to act as a fire break, the only hope is keeping people from spreading it. The lockdown is ring fencing and “walling off a community” said French.

Lock down the people, you lock down the virus. Once the virus works its way through a person’s bubble, it has nowhere left to go. 

There are a couple of exceptions to this. Bubbles with essential workers in them may have had a chance to unknowingly spread the virus prior to learning their bubble has an infected person.

The other exception is where bubbles are broken, or if an infection has potentially occurred via surface contamination.

If we’ve been in lockdown for over two weeks, why are there new clusters?

So far there have been 1205 breaches of the Level 4 lockdown, which resulted in 138 prosecutions and 1038 warnings.

As well as the public breaching the rules, Ministers have also been in the spotlight, with Health Minister David Clark saying he felt "like a complete dick” after admitting to travelling beyond a local area to take a walk on a beach. He was stripped of some of his responsibilities as a result.

Ardern holds a dim view of Easter Weekend bubble-breakers.

“I say then to those who broke the rules this weekend, it could take one case amongst you to have an outbreak that could lead to dozens of infections and possibly death.”

Her words came close to the news a person connected to a wedding cluster died.

When asked why there were new clusters emerging after two weeks of a nation in lockdown, the Ministry of Health gave a number of reasons. 

Deputy director of public health Harriet Carr suggested some clusters may have originated before Level 4 came into place. Another possible reason was timing. 

“Because the incubation period of the virus is up to 14 days, some initial cases that have resulted in clusters may not have been detected prior to the alert coming into place.”

Finally, a possible reason for cluster growth after Level 4 is a lag in a case being known, and contact-tracing linking people with a case.

How much immunity is needed for a cluster to fizzle out? 

If the goal is to starve a virus of new human meals to hitchhike on, the number of people who need to either be immune, through catching it, or through a vaccination, all comes down to the ‘R0’ number. 

This number gives a guide to how many unprotected people a virus can infect. Measles, which is highly contagious, has a R0 of around 12 to 18. This means every person with measles would be likely to give it to 12 to 18 people if there were no measures in place to stop spread. For herd immunity to stop the virus spreading you need around 95 percent of the population to be immune.

The R0 for Covid-19 is lower, sitting at around two. 

“The number the average person would infect is two, then if 50 percent of the population are vaccinated or immune as a result of a previous infection, then every contact that person has only one of the two would be infected. That means infections would gradually die out over time.”

The R0 number also impacts whether everybody in a cluster is likely to get infected. 

“Within these clusters, like the Marist cluster, if they’re a walled off community, after a certain period of time, if enough have been previously exposed then on average the number of vulnerable people you’ve come into contact with diminishes. That’s why the infection dies out before everybody is actually infected themselves.”

To stop the clusters spreading beyond bubbles, physical distancing is key.

A deadly second wave

French is personally in favour of Ardern’s half-time analogy. In his view, any shift from current measures should only be made on evidence. 

French warned of a potentially “devastating” outcome should a second wave gather momentum.

“History tells us from the 1918 pandemic that this [a second wave] is really bad news, particularly for vulnerable populations. The November second wave in 1918 pandemic was more devastating than the first.”

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