Experts suspect that a suggested source of New Zealand's latest community cases - food and laundry connected with air travel - could just be a coincidental red herring.
Covid-19 Response Minister Chris Hipkins said there were "gaps in our knowledge" around the three new cases - but confirmed that one worked at LSG Sky Chefs in Māngere, in the laundry and also preparation of food.
Amid resulting speculation that one of these items could have been infected, resulting in surface - or fomite - transmission, epidemiologists say such cases are extremely rare.
They instead point to a much more likely explanation: the SARS-CoV-2 virus being passed through its main route of transmission - respiratory droplets and aerosols in the air - from an infected person.
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Otago University epidemiologist Professor Michael Baker said that, when the pandemic first emerged, scientists were worried about three modes of transmission: droplets, aerosols and contaminated surfaces.
One early study suggested the virus could linger on plastic and stainless steel for days, seemingly backing World Health Organisation (WHO) guidance at the time that it could spread through fomites.
That prompted a world-wide surge in deep cleaning.
"But as time has gone on, there has been less focus on contaminated surfaces - and in fact no cases have ever been documented - while concern about aerosols has risen," Baker said.
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Advertise with NZME."While there might be potential for observation bias here - and these contamination events are also indeed very hard things to document - one would think that, after millions of cases, we'd be seeing more examples of it. That isn't happening."
The US Centers for Disease Control now states that fomites are "not thought to be a common way that Covid-19 spreads" - and some studies have indicated the risk is exaggerated.
Following his review of available evidence for The Lancet Infectious Diseases, for instance, Rutgers New Jersey Medical School microbiologist Professor Emanuel Goldman concluded that the chance of transmission through inanimate surfaces was "very small".
And this only applied in instances where an infected person coughed or sneezed on the surface and someone else touched it very soon after, or within one to two hours.
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Baker said there was a general view that, to infect someone, the virus on a surface would somehow need to be "re-aerosolised and inhaled".
"That said, we have to keep in mind, just as we had to with the frozen packaging investigated during the August Auckland cluster," he said.
"It's true that you can culture viable virus for some days on some surfaces - but we're just not seeing documented examples of transmission in this way.
"It doesn't mean it can't happen - it doesn't seem to be common with this virus," he said, however adding the route had certainly been demonstrated with other pathogens like norovirus and influenza.
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Advertise with NZME.Otago University clinical microbiologist and infectious diseases expert Professor David Murdoch agreed said surfaces had sometimes been suggested as potential sources when causes of infections couldn't be proven in any other way.
"We can speculate, and that's often all it ends up being - speculation," he said.
"So I suspect this is going to be difficult to prove one way or the other - and I think we still need to focus on airborne and droplet spread, and look at where all of those opportunities might have been."
Murdoch said further testing and contact tracing would shed more light here.
"Hopefully, what we'll see is that this is contained, and there are no extra cases, regardless of the source."
The Ministry of Health said today that an "intensive" source investigation around the latest cases was ongoing.
