New Zealand may have again just escaped another mass outbreak – but infectious diseases expert Associate Professor Siouxsie Wiles says she’d hate for people to get complacent because of it.
Despite the latest incursion generating few cases of onward transmission, Wiles and other experts stress there’s nothing that makes our population any less vulnerable to exposures to the SARS-CoV-2 virus.
Rather, they’ve pointed to luck and individual circumstance, a solid testing and contact tracing system – and something called the Pareto principle.
Otherwise known as the “80-20 rule”, this suggests 80 per cent of disease transmission events in an epidemic are caused by 20 per cent of people.
Having been observed in many other infectious diseases, it’s now a well-established phenomenon in virology.
“There are now several studies that point to the fact the virus that causes Covid-19 seems to follow this 80-20 rule, where 80 per cent of cases result in very little transmission – so no, or one or two extra cases,” Wiles said.
That certainly fitted with New Zealand’s experience last year.
When scientists analysed about 650 cases involved in the main outbreak, they estimated about a quarter led to one other case – and only 19 per cent resulted in a chain of transmission which could be observed as an actual lineage.
The biggest of those – which comprised a lineage called B.1.26 and was linked to a “super-spreading” event at a Southland wedding – most likely originated in the US.
It was just one of some 277 separate introductions of the virus into New Zealand, representing nearly all of its genomic diversity around the world at that point.
But it was true that the newly-emerged variants needed to be treated with particular caution.
The two most recent notable incursions – the Northland cases and this week’s – involved the B.1.351 (South Africa) B.1.1.7 (UK) strains, respectively.
When it came to the UK strain, the average number of new cases per infected person – or the so-called “R number” – was 40 to 70 per cent higher than the original strain.
Where the original typically might infect about 160 cases after five links in an infection chain, the new variant could infect about 1000 cases in the same time.
A just-published Harvard University study has also indicated it came with a longer period of severe infection.
When this strain somehow leaked out of the border and into the New Zealand this month, its heightened transmissibility, coupled with our open level 1 environment, would have put the country’s “R” value perhaps as high as three.
Without knowing the source – there were no genetic matches with any MIQ cases – authorities couldn’t rule out a hidden outbreak, so were forced to order a snap lockdown.
But as with other positive cases that have made it into the community since the Auckland August cluster, for the moment, the latest episode didn’t appear to have sparked a mass outbreak.
In terms of the UK variant being involved, epidemiologists have been at pains to point out that the presence of the strain didn’t mean it would infect more people with every case.
After all, it took thousands of transmissions for scientists to observe its overall pattern.
Again, Wiles explained, much of it came back to individual factors and circumstances, such as how much of the virus a person was carrying – their viral load – or how many people they were interacting with, and where.
“There are definitely some environmental aspects at play.
“For instance, big clusters are more likely to start in poorly-ventilated areas with lots of people, especially in close contact, and singing, shouting and talking.
“And there are a couple of studies coming out now that are pointing to individual effects.”
One recent US study used computer-generated models to simulate sneezes in different types of people, and found that some features – notably a stopped-up nose – could increase their potential to spread viruses, by affecting how far droplets travel when they sneezed.
“Some people might simply expel more particles when they speak or laugh,” Wiles said.
“It could well be that if you have someone with a high viral load, and who does expel more particles when they speak, then you’ve got the perfect conditions for a super-spreader event.”
Otago University clinical microbiologist Professor David Murdoch agreed the opportunity for transmission was much greater where someone was coughing or sneezing, compared with someone who was asymptomatic.
“And yes, if an infected person has lots of other people in their home, or works in a crowded setting, then the opportunity is even greater.”
Otago University and ESR virologist Dr Jemma Geoghegan said those influencing individual variables could also be granular.
“Are there cells that are being infected in someone’s respiratory tract – and if these cells are closer to the surface of a particular person’s nose, or deeper, does that matter?
“There are so many things we don’t know about transmission dynamics with this virus that we need to understand.”
But she said a person’s general susceptibility to the virus, or what environment it was introduced into, remained crucial factors.
In last month’s Northland case, it was a stroke of luck that the infected 56-year-old returnee had only interacted with 11 genuine close contacts, despite visiting 30 locations, and that all returned negative tests.
“This could have been due to the types of symptoms the person had – and perhaps they had a very low viral load,” Wiles said.
“In the latest case, we’ve clearly had transmission from one family to another family, but within the school environment [Papatoetoe High School] we haven’t yet seen any big super-spreading event.
“This could be down to the context of the school setting – what was everybody’s viral load? – or it could fall into the category of expelling less droplets.”
Or, Wiles added, it could have simply been a combination of all factors.
While some people were suggesting New Zealand’s currently warmer weather might have played a part in limiting the recent flare-ups, she noted that outbreaks had now been observed raging in all climates.
Any factor to do with seasonality was likely more to do with how that affected human behaviour.
“You’re more likely to spend more time indoors with the windows closed in winter, while in summer, you have the windows open, or are more often to be outdoors.”
Covid-19 modeller Professor Michael Plank, of Te Punaha Matatini and Canterbury University, agreed luck had been a component of New Zealand’s escapes to date.
“But it is partly that the measures we have carefully put in place have helped us make our own luck.
“We now have a really good testing system that encourages people to get tested, and gives them freely-available access to it.
“If we hadn’t have had these in place, it might well have been that a lot of these border re-incursions could have resulted in big outbreaks.”
And once those first cases had been detected, a sophisticated contact tracing and public health apparatus swung into gear, quickly isolating people before they could catch or spread the virus.
Even during the August outbreak, contact tracing teams were exceeding the gold standard in the time taken to find an infected person’s contacts – 80 per cent of contacts in 48 hours.
Wiles said it was obviously great that New Zealand’s last few incursions had come with little transmission.
“It means there is less of a chance that we have people getting long-Covid, or needing hospitalisation and dying. So this is a good result for us.
“But what I would hate is for people to take from this, oh, actually, these events aren’t so bad, we’ll be fine, and we don’t need to do all the things that are being asked to stop the transmission.
“I’d just hate for us to be lulled into a false sense of security, just because we had two times where things went well for us.”
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