Experts say Omicron can spread more easily within indoor spaces - but measures like good ventilation, vaccination and quality masks can reduce the risk. Photo / 123RF
It was dubbed the "bin lid mystery": a strange case of one guest infecting another, somehow, within the seemingly virus-proofed lock-up of MIQ.
Back in September 2020, officials were left scratching their heads at a chain of transmission discovered when a man tested positive with Covid-19 in Auckland.
Working their way backward, the investigation came to focus on a woman and a child, who may have been infected by a person who was staying in the adjacent room while staying in Christchurch's Crowne Plaza MIQ.
The mystery: there hadn't been any direct contact between the occupants.
Neither room had balconies and CCTV footage showed there wasn't a single instance where the guests had been out at the same time.
That led to the curious explanation of the virus somehow spreading by a supposedly contaminated rubbish bin lid nearby.
It wasn't until a year later that a team of scientists, including Otago University virologist Dr Jemma Geoghegan, went back over the evidence to reveal the likelier culprit: aerosols.
Within a window of just 50 seconds, during routine testing in the plaza corridor, each room's doors had been open just long enough for virus-laden particles to cross over.
The case drew the world's attention to a troubling fact about Covid-19.
"We can now conclusively say this virus is airborne - and it can transmit over long spatial scales," Geoghegan said.
"There's no reason to think this won't be any different with Omicron."
Before the pandemic, and even at the time of the Crowne Plaza chain, viruses like Sars-CoV-2 were widely thought to spread mainly through large droplets that we sneezed or coughed, or through the surfaces we touched, like the bin lid.
Since then, we've come to better understand the super-spreading role of aerosols: tiny particles that can be breathed or coughed out by an infected person, and inhaled by others around them.
Not only are these smaller than droplets – a large fraction measured just a single micrometre, while droplets were generally considered larger than five micrometres – they can also linger in the air for long periods, and infect people at much greater distances.
Once breathed in by another person, they deposited in different parts of the respiratory tract, causing infection.
In one recent study, Pennsylvania State University researchers ran experiments which found that spacing people two metres apart – that's more than the one-metre requirements now in place in public venues – wasn't enough to stop aerosol transmission.
They examined three factors: the amount and rate of air ventilated through a space, the indoor airflow pattern associated with different ventilation strategies, and the aerosol emission mode of breathing versus talking.
They also compared transport of tracer gas, typically employed to test leaks in air-tight systems, and human respiratory aerosols ranging in size from one to 10 micrometres.
"Our study results reveal that virus-laden particles from an infected person's talking - without a mask can quickly travel to another person's breathing zone within one minute, even with a distance of two metres," study author Associate Professor Donghyun Rim said.
In spaces without inadequate ventilation, the risk was much higher.
"The results suggest that physical distance alone is not enough to prevent human exposure to exhaled aerosols and should be implemented with other control strategies - such as masking and adequate ventilation."
As New Zealand prepared to move to the red traffic light setting on Sunday, infectious diseases expert Associate Professor Siouxsie Wiles aired concerns that people in places like pubs and restaurants could still sit unmasked.
While the hospitality industry responded by noting measures like separated seating, capacity limits and vaccination requirements, Wiles pointed to how there had already been plenty of examples of spread in these places overseas.
"It doesn't matter if groups of people are seated one metre away from each other, if they are indoors, especially if the place is badly ventilated, then the risk of transmission is high."
Other scientists have echoed Wiles' worries.
"There aren't really any ventilation or air exchange recommendations regarding Covid-19 aerosol transmission, so you would have to assume that most indoor places are going to be high-risk environments," University of Auckland aerosol chemist Dr Joel Rindelaub said.
Indoor spaces where masks weren't worn, such as to allow for eating and drinking, were "definitely places you might want to avoid if you're worried about contracting Covid".
Canterbury University epidemiologist Associate Professor Arindam Basu agreed.
"If we have people sitting down in a restaurant or bar, speaking in a high voice, singing, or shouting, and not wearing masks – all of these are big risk factors in the spread of Omicron."
Importantly, crowded settings were often catalysts for super-spreading events – and studies around the world have shown how just 10 to 20 per cent of index cases have been responsible for around 80 per cent of secondary ones.
1/ SUPERSPREADING IS EXPLAINED BY SHARED-ROOM AIRBORNE TRANSMISSION
— Prof. Jose-Luis Jimenez (@jljcolorado) January 7, 2022
We can use that to estimate risk of transmission for omicron & other diseases
Our peer-reviewed paper was just published (open access) in @EnvSciTech
I'll explain it in this threadhttps://t.co/0NR5mFANJd pic.twitter.com/gFxLNIoEcF
But we know that Omicron can spread much faster: UK data, for instance, has put Omicron's risk of household transmission at three times higher than the Delta variant.
That's partly been put down to Omicron's enhanced potential to evade immunity – highlighting the need for booster shots – and also its ability to replicate within the human bronchus, at a rate around 70 times than Delta.
Did Omicron also pose a bigger airborne threat?
"That's a really good question – and it's still kind of uncertain right now," Rindelaub said.
Because the variant had been observed to affect the upper respiratory tract more – a factor that might partly explain why so many Omicron infections were milder than Delta – the initial hypothesis was a virus that settled closer to our mouths and noses meant there was more of it to be breathed or sneezed into the air.
"But actually, there's been some recent pre-print studies suggesting that might not be the case," he said.
"So, we don't know for sure if there's more expelled virus particles. We do know that immunity evasion is certainly a factor, as is its shorter incubation time."
Whatever the case, experts told the Herald that, quite simply, the more opportunity the virus had to spread – the more it would.
Dr Julie Bennett, an infectious disease and indoor air quality expert at Otago University, said good ventilation remained a vital tool to counter Omicron, alongside vaccination and masking.
She and others have argued what rules New Zealand did have in place - such as through the Building Code's G4 ventilation clause - weren't adequate against the risk of Covid-19.
"Where standards exist, they're fragmented: in other words, they're set and administered by different agencies so there isn't a cohesive approach," Otago epidemiologist Dr Amanda Kvalsvig told the Herald.
"The quality of ventilation that's needed to slow the spread of Omicron is going to be high, and current standards may not be strict enough to be really helpful."
Schools - now subject to red-light masking rules - had extra ventilation protocols in place for new and refurbished buildings, in standards that were about to be updated and which would consider Covid-19.
Yesterday, the Government also announced it would be equipping schools with about 5000 portable air cleaners, along with 2500 carbon dioxide monitors and 8000 internal environment monitors.
Epidemiologists have previously pointed to schools as a trouble spot for Covid-19 spread, with air quality worries front and centre.
A 2019 review by BRANZ found ventilation of schools typically relied on windows being opened, but this hadn't been happening enough to provide effective ventilation.
Further, carbon dioxide (CO2) rates – often used as a proxy for ventilation quality - were often above recommended levels in many classrooms.
The ministry required that average concentration should not exceed 1,500 parts per million (ppm) of CO2 at seated head height, and expected average levels over summer, when windows could be opened, would sit between 400 and 1000ppm.
To minimise spread of Covid-19, the US Centers for Disease Control AND Prevention (CDC) has stated 800ppm as a target benchmark for good ventilation.
Across homes, businesses and schools, experts have already advised building managers bring in as much outdoor air as possible, with a room air exchange rate of four to six times per hour.
That was more than double the rate in a typical office or school building.
"So, I think we do need to consider air quality and building standards going forward – and we need to consider infection control as part of that," Bennett said.
Kvalsvig wanted to see a "whole-of-society" understanding of airborne transmission.
"There will need to be evidence-informed standards, monitoring, and accountability that are appropriate for each type of setting," she said.
"There also needs to be clear public health messaging about air quality so that people can make sensible decisions when they need to."
As we now faced a potentially large Omicron outbreak. she said, there wouldn't be time for long-term protections like legislation and structural alterations to buildings.
"Instead, we'll need a pragmatic approach, aiming to do whatever we can.
"That might include exercising, socialising, working, and learning outdoors whenever possible, and keeping windows open.
"But the long-term infrastructure is important because it's much less effortful and much more useful in winter, which is the time when you most need good indoor ventilation because that's when respiratory infections are spreading.
"So we mustn't get stuck in a short-term loop. Ideally, we should be looking to a future where people can be confident that they're breathing clean air in the same way that they should be confident that they're drinking clean water."
And for those concerned about encountering Omicron indoors right now, experts encouraged people to mask up – and mask up properly.
If you are having trouble finding a P2 or N95 right now, here's a quick reminder on one way to make a surgical mask work harder for you: pic.twitter.com/4SFBEAOs0B
— Dr. Joel Rindelaub (@joel_rindelaub) January 25, 2022
"If you do wear a surgical mask – but have it modified so it does have a tight fit around your face, you may be able to increase your protection up to 90 per cent," Rindelaub said.
"With a tight-fitting N95 mask, you're actually going to get protection that's better than 95 per cent, for most particle sizes."
As for the risk of un-masking indoors?
Geoghegan pointed back to the bin lid case.
"If we could see spread happening in those sorts of situations, then, being in a bar or a restaurant ... it would be exacerbated."
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