A new review article published in The Lancet has presented 10 key scientific reasons why SARS-CoV-2, the virus that causes COVID-19, is predominantly spread though the air. The research adds to a growing chorus of experts saying the evidence for airborne transmission is overwhelming and the sooner global health authorities admit this, the sooner more effective measures to better protect the public can be implemented.
Perhaps one of the most vociferous debates over the past 12 months has been over exactly how most people catch COVID-19. As the pandemic spread across the globe in early 2020 the general perspective from most public health experts was that SARS-CoV-2 primarily spread by droplet transmission.
This belief hinged on a traditional binary between droplet and aerosol viral transmission. Aerosol particles have classically been defined as smaller than 5 micrometres (m). They can remain suspended in the air for extended periods of time and can travel significant distances from a source.
Respiratory droplets, on the other hand, are larger particles, often propelled from a source by coughing or sneezing. These particles fall to the ground in seconds and typically dont travel further than six feet (1.8 m) from a source.
The general presumption from early last year was that SARS-CoV-2 primarily spreads via respiratory droplets, and this led to public health advice recommending basic measures such as social distancing, hand washing and frequent cleaning of surfaces. However, as 2020 progressed, more and more case studies presented scenarios whereby large numbers of people were infected in superspreading events despite being significant distances away from a viral source.
A new review published in The Lancet, led by Trish Greenhalgh from the University of Oxford, is arguing there is consistent and strong evidence to suggest SARS-CoV-2 is predominantly transmitted through airborne routes. The researchers lay out 10 streams of evidence from the past year that overwhelmingly support this hypothesis. The review also claims respiratory droplet transmission of this novel virus is based on flawed and outdated models of viral transmission.
The assessment references a large volume of evidence from the past 12 months, including numerous cases studies documenting long-range transmission of the virus between people in adjacent hotel rooms and superspreading events in indoor venues that cannot be explained by droplet transmission. The researchers argue particles as large as 100 m are known to remain suspended in the air for extended periods of time and the old fixed definition of aerosol particles as less than 5 m has led to misunderstandings of how SARS-CoV-2 is spread.
The flawed assumption that transmission through close proximity implies large respiratory droplets or fomites was historically used for decades to deny the airborne transmission of tuberculosis and measles, the researchers write in the study. This became medical dogma, ignoring direct measurements of aerosols and droplets which reveal flaws such as the overwhelming number of aerosols produced in respiratory activities and the arbitrary boundary in particle size of 5 m between aerosols and droplets, instead of the correct boundary of 100 m.
The researchers are far from alone in their call for widespread acknowledgment of airborne SARS-CoV-2 transmission. In early February the editors of the prestigious science journal Nature criticized public health bodies and the World Health Organization for failing to effectively communicate the predominance of airborne transmission.
The editorial recognized a growing acceptance of COVID-19 being spread through the air while suggesting continued recommendations for surface disinfection and other droplet transmission prevention measures are confusing the public and leading to huge investments in expensive disinfection efforts that shift resources away from measures such as improving ventilation in indoor spaces.
This lack of clarity about the risks of fomites compared with the much bigger risk posed by transmission through the air has serious implications, the journal editors write. People and organizations continue to prioritize costly disinfection efforts, when they could be putting more resources into emphasizing the importance of masks, and investigating measures to improve ventilation. The latter will be more complex but could make more of a difference.
The Centers for Disease Control and Prevention (CDC) in the United States is a useful case in point. The CDC still suggests COVID-19 is primarily spread through respiratory droplets. Its current advice claims direct contact is the most common vector for infection, although its information has more recently been updated to note, COVID-19 can sometimes be spread by airborne transmission.
A recent case study published by the CDC described a COVID-19 cluster last year in an Australian church. The study reports 12 people were infected across two days of church services. The primary case patient was a member of the church choir and all the epidemiological evidence points to airborne spread as the best explanation. Nevertheless, the study also notes, this investigation only provides circumstantial evidence of airborne transmission.
Co-author on the new Lancet article Zeynep Tufekci, a writer and sociologist from the University of North Carolina, Chapel Hill, says many of our current precautions based on the droplet transmission hypothesis are still effective. Distancing and masks, for example, are vital tools to prevent infection, but she argues some key public health policies are diverting resources from implementing more useful measures.
Even after a whole year, we still see the widespread practice of unnecessary levels of cleaning to the detriment of public health, use of plexiglass indoors that is far from sufficiently protective and, depending on air flows, may even be contraindicated, instead of attention to ventilation and aerosol risks, says Tufekci. We cannot fix this situation without accurately informing the public so that people feel empowered to make decisions to better protect themselves across different contexts, and adjusting guidelines globally to fit the best available evidence.
A recent editorial published in The BMJ argues traditional scientific definitions of viral transmission need urgent revision. Co-authored by Linsey Marr, an expert in airborne transmission of viruses, the article agrees many of our current infection control measures are useful and shouldnt change even with a broader agreement over the predominance of airborne transmission.
However, a big problem with the current focus on droplet transmission is a lack of emphasis regarding indoor ventilation. Marr and colleagues suggest more attention needs to immediately be paid to ventilation and air filtration technologies for indoor spaces. This will help future-proof our indoor spaces from this and other viruses that may arise.
Covid-19 may well become seasonal, and we will have to live with it as we do with influenza, the researchers write in The BMJ. So governments and health leaders should heed the science and focus their efforts on airborne transmission. Safer indoor environments are required, not only to protect unvaccinated people and those for whom vaccines fail, but also to deter vaccine resistant variants or novel airborne threats that may appear at any time.
The new study was published in The Lancet.
Source: University of Colorado Boulder
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