New Zealand appears to have lucked out at another roll of the Omicron dice, an epidemiologist says, with just one of an infected MIQ worker's 86 contacts so far testing positive.
The worker, who was based at Auckland's Stamford Plaza and is fully vaccinated, returned a positive result on Friday after being tested two days earlier.
Despite the person having been infectious from January 10, the Ministry of Health today reported that only a household contact who was already isolating – had been confirmed as a secondary infection so far.
"Further case interviews are under way but, at this stage, there are no exposure events associated with this case," the ministry said.
Six other household members had returned negative tests, as had 70 other close contacts.
From the exposure events linked to the initial case, 10 people from two bus journeys were yet to be tested and are being followed up in person.
"No other close contacts linked to this cluster have returned positive results."
Otago University epidemiologist Professor Michael Baker said it appeared the initial case hadn't been very infectious.
"Contact tracing is often like peeling back layers of onion: you work your way through people with less and less intense exposure, until you see a positive," he said.
"But generally, if the person is highly infectious, that inner ring of the onion gives you your first indication."
Baker added that, as the incubation period for Omicron was relatively short – possibly only three days – any positive results would have been returned quickly.
"So, I'd say this is looking very low risk at this point."
If so, it would prove another brush with the high transmissible variant that didn't result in a community outbreak.
The others included a household contact of an Air New Zealand crew member and DJ Dimension – real name Robert Etheridge – who visited a number of Auckland's hot spots on Boxing Day before learning he was infected.
Experts have told the Herald that, although the Delta outbreak showed it only took one stray case to spark a flare-up, single community incursions often came to nothing.
Factors included the person's vaccination status, their behaviour – or whether they'd been mixing with many others – and whether the infectious period happens to coincide with the period when they're out and about in the community.
Although much about "super-spreading" remains unclear, researchers point to several factors that raise the risk.
They include the amount of virus a person is carrying (viral load) – especially if also coughing, sneezing or talking loudly among others – and possibly even individual biological or genetic traits.
The variant involved also mattered – Omicron's effective reproductive number is estimated to be three to five times greater than Delta's – as did pure luck and random variability.
Epidemiologists have often referred to something called the Pareto principle – otherwise known as the "80-20 rule" – a well-established phenomenon in virology.
This suggested that 80 per cent of disease transmission in an epidemic were caused by 20 per cent of people.
That trend was certainly seen in New Zealand's main outbreak, when one in five adults were responsible for up to 85 per cent of the virus' spread, but it was unclear whether this still applied to the faster-spreading Omicron.
"It's always a roll of the dice," said Baker, who has been warning that high numbers of Omicron cases in MIQ are unsustainable if New Zealand wants to delay a mass outbreak.
At the same time, an earlier-documented case of transmission from one MIQ room to another – within just a 50-second window during routine testing when both rooms' doors were open – demonstrated how easily the virus could spread.
The investigation into the transmission route of the latest leak was continuing, as was testing of all staff at the plaza, the ministry said.
"We're reiterating our call for anyone who lives in Auckland with symptoms – no matter how mild – to get a test, even if you're vaccinated and to please stay at home until you return a negative test result."
- by Jamie Morton