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The COVID-19 pandemic has revealed critical knowledge gaps and assumptions concerning how respiratory viruses spread between hosts. Traditionally thought to be spread mainly through large respiratory droplets produced by the coughs and sneezes of sick individuals, a growing body of evidence indicates that many respiratory pathogens – including SARS-CoV-2– spread through virus-laden microscopic respiratory aerosols. In this Review, Chia Wang and colleagues discuss the recent research regarding airborne transmission of respiratory viruses and how an improved understanding of aerosol transmission will allow for better-informed controls to reduce and mitigate airborne transmission. Until recently, most respiratory pathogens were assumed to spread through large droplets expirated from an infectious person or transferred from contaminated surfaces. This understanding has largely guided public health recommendations in mitigating viral spread. However, several respiratory pathogens, including influenza and the common cold, are also known to spread through infectious respiratory aerosols, which can float and travel in air flows at far greater distances and for much longer, infecting those that inhale them. According to a growing body of evidence, much of which gained from studying the spread of COVID-19, airborne transmission may be a more dominant mode of respiratory virus transmission than previously thought. Here, Wang et al. highlight how infectious aerosols are generated, travel throughout an environment and deliver their viral payloads to hosts. The authors also outline ways to mitigate aerosol transmission at long and short ranges, including improvements to ventilation and airflows, air filtration, UV disinfection and personal face mask fit and design.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Leah Grout, Research Fellow, Department of Public Health, University of Otago
This study highlights the role of airborne transmission of SARS-CoV-2 and the need to reevaluate aerosol transmission for other respiratory viruses. During the COVID-19 pandemic, the World Health Organization, the US Centers for Disease Control and Prevention, and others were slow to acknowledge airborne transmission of the virus, possibly in an effort to prevent public panic. This study recommends that protective measures including the ventilation and filtration of indoor environments, along with mask use and physical distancing, be used to reduce airborne transmission.
To date, the issue of ventilation has not been adequately addressed in the New Zealand setting, and 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.
Dr Joel Rindelaub, Aerosol Chemist, University of Auckland
This paper highlights how gaps in the knowledge of aerosol behaviour influenced early statements from health officials about the SARS-COV-2 pandemic, such as that the virus was not airborne.
Aerosol scientists have long pushed for greater recognition of COVID-19’s aerosol transmission potential, with experts publicly questioning the WHO’s stance on airborne transmission since at least April 2020. After a letter signed by 239 aerosol scientists was released in July 2020, the WHO finally recognised that 'short-range aerosol transmission… could not be ruled out’. It wasn’t until May 2021 that the CDC stated that aerosols were the main mode of transmission, 11 months after aerosol scientists had published the very same conclusion.
Limited expertise on the generation and transport of respiratory aerosols may have an impact on infections beyond COVID-19, calling for a review on the health recommendations for other respiratory viruses, such as influenza virus, human rhinovirus, and respiratory syncytial virus (RSV).
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.
Since we know that the smallest aerosol particles, the ones that will travel the farthest, are enriched with the virus, the current recommendations regarding 1 or 2 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 air flow are reduced compared to outdoor settings. The distance respiratory aerosol will travel is dependent on several factors, such as their size, the initial speed at which they are expelled, ambient air velocity, and other environmental factors. Thus, it may not be straightforward to recommend a universal distancing protocol. That being said, physical distancing of at least 2 meters along with focus on ventilation, airflows, air filtration, UV disinfection, and mask fit can be used to help prevent transmission of COVID-19.
