Covid 19 coronavirus Delta outbreak: Airborne virus spread 'vastly underappreciated'

The Covid-19 pandemic has revealed how respiratory viruses can travel through spaces far more easily than first thought – a long-overlooked risk that international scientists say warrants a rethink around current social distancing measures.

Before the global coronavirus crisis, the main ways these viruses spread were thought to be via large droplets that we sneezed or coughed, or through the surfaces we touched and contaminated.

Now, a fresh review published in the journal Science has pointed out how these two vectors alone couldn't account for how effectively the coronavirus had spread – especially in enclosed spaces and "super-spreading" settings.

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Its international authors instead turned to a mode that global health authorities were slow to acknowledge: aerosols.

Like droplets, aerosolised particles were breathed or coughed out by an infected person.

But not only were they smaller – a large fraction measured just 1 micrometres, while droplets were generally considered larger than 5 micrometres – but they could linger in the air and infect people at much greater distances.

Exactly how they travelled depended on a mix of factors - such as temperature, ventilation, airflow and relative humidity – but they could accumulate in poorly ventilated spaces, and be inhaled "at both short and long ranges".

The Covid-19 pandemic has revealed how respiratory viruses can travel through the air far more easily than first thought. Image / US CDC

And once the virus-laden particles were breathed in by another person, they could deposit in different parts of the respiratory tract and cause infection.

"Given all that we have learned about SARS-CoV-2 infection, the aerosol transmission pathway needs to be re-evaluated for all respiratory infectious disease," the authors said.

They recommended extra protective measures such as ventilation and filtration of indoor environments, along with mask use and greater physical distancing.

University of Auckland aerosol chemist Dr Joel Rindelaub noted how it wasn't until May this year that the US Centers for Disease Control and Prevention (CDC) acknowledged that aerosols – once largely ignored - were the main mode of transmission.

That was nearly a year after a group of more than 230 scientists came to the same conclusion.

"Thanks to the urgency driven by Covid-19, scientists have learned valuable information about the transmissibility of respiratory viruses - with no small credit going to the meticulous investigations of virus transmission here in New Zealand that have included events in airplanes and MIQ facilities."

Mask-use and ventilation are critical virus defences in indoor spaces, scientists say. Photo / Michael Craig

Last September, the virus appeared to have been passed through aerosol particles suspended in the air - within just 50 seconds - during routine swabbing in Christchurch's Crowne Plaza Hotel last September.

Similar cases of the virus travelling from one room to another in a matter of seconds were also observed at the Jet Park quarantine facility this month, and likely also at Auckland's Crowne Plaza MIQ: the suspected ground-zero of this outbreak.

"We know that aerosol transmission is really frequent so it would be surprising if it hasn't occurred in the current outbreak," Otago University virologist Dr Jemma Geoghegan said.

Since scientists now knew that the smallest aerosol particles - the ones that travelled the farthest - were enriched with the virus, Rindelaub said current recommendations regarding one or two metre distancing "are not – and never were – sufficient to guarantee protection against Covid-19 transmission".

"This is especially relevant for indoor areas where ventilation and airflow are reduced compared to outdoor settings."

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Because the varying distances aerosols could travel, he said it might not be straightforward to recommend a universal distancing protocol, as many countries had.

"That being said, physical distancing of at least two metres, along with focus on ventilation, airflows, air filtration, UV disinfection, and mask fit can be used to help prevent transmission of Covid-19."

Leah Grout, a research fellow at Otago University's Department of Public Health, said the issue of ventilation still hadn't been properly addressed in New Zealand.

"This will be an important consideration for controlling the Covid-19 pandemic and preventing future respiratory disease outbreaks.

"Some of the methods for improving ventilation are very easy, such as opening windows, and may have benefits beyond the prevention of disease transmission, with studies suggesting that improving indoor air quality may increase worker productivity and improve learning in schools."