The question of whether SARS-CoV-2 is mainly transmitted by droplets or aerosols has been highly controversial. We sought to explain this controversy through a historical analysis of transmission research in other diseases. For most of human history, the dominant paradigm was that many diseases were carried by the air, often over long distances and in a phantasmagorical way. This miasmatic paradigm was challenged in the mid to late 19th century with the rise of germ theory, and as diseases such as cholera, puerperal fever, and malaria were found to actually transmit in other ways. Motivated by his views on the importance of contact/droplet infection, and the resistance he encountered from the remaining influence of miasma theory, prominent public health official Charles Chapin in 1910 helped initiate a successful paradigm shift, deeming airborne transmission most unlikely. This new paradigm became dominant. However, the lack of understanding of aerosols led to systematic errors in the interpretation of research evidence on transmission pathways. For the next five decades, airborne transmission was considered of negligible or minor importance for all major respiratory diseases, until a demonstration of airborne transmission of tuberculosis (which had been mistakenly thought to be transmitted by droplets) in 1962. The contact/droplet paradigm remained dominant, and only a few diseases were widely accepted as airborne before COVID-19: those that were clearly transmitted to people not in the same room. The acceleration of interdisciplinary research inspired by the COVID-19 pandemic has shown that airborne transmission is a major mode of transmission for this disease, and is likely to be significant for many respiratory infectious diseases.
Practical Implications
Since the early 20th century, there has been resistance to accept that diseases transmit through the air, which was particularly damaging during the COVID-19 pandemic. A key reason for this resistance lies in the history of the scientific understanding of disease transmission: Transmission through the air was thought dominant during most of human history, but the pendulum swung too far in the early 20th century. For decades, no important disease was thought to be airborne. By clarifying this history and the errors rooted in it that still persist, we hope to facilitate progress in this field in the future.
The COVID-19 pandemic motivated an intense debate over the modes of transmission of the SARS-CoV-2 virus, involving mainly three modes: First, impact of “sprayborne” droplets on eyes, nostrils, or mouth, that otherwise fall to the ground close to the infected person. Second, by touch, either by direct contact with an infected person, or indirectly by contact with a contaminated surface (“fomite”) followed by self-inoculation by touching the interior of the eyes, nose, or mouth. Third, upon inhalation of aerosols, some of which can remain suspended in the air for hours (“airborne transmission”).1, 2
Public health organizations including the World Health Organization (WHO) initially declared the virus to be transmitted in large droplets that fell to the ground close to the infected person, as well as by touching contaminated surfaces. The WHO emphatically declared on March 28, 2020, that SARS-CoV-2 was not airborne (except in the case of very specific “aerosol-generating medical procedures”) and that it was “misinformation” to say otherwise.3 This advice conflicted with that of many scientists who stated that airborne transmission was likely to be a significant contributor. e.g. Ref.4-9 Over time, the WHO gradually softened this stance: first, conceding that airborne transmission was possible but unlikely;10 then, without explanation, promoting the role of ventilation in November 2020 to control spread of the virus (which is only useful for controlling airborne pathogens);11 then declaring on April 30, 2021, that transmission of SARS-CoV-2 through aerosols is important (while not using the word “airborne”).12 Although a high-ranking WHO official admitted in a press interview around that time that “the reason we’re promoting ventilation is that this virus can be airborne,” they also stated that they avoided using the word “airborne.”13 Finally in December 2021, WHO updated one page in its website to clearly state that short- and long-range airborne transmission are important, while also making clear that “aerosol transmission” and “airborne transmission” are synonyms.14 However, other than that web page, the description of the virus as “airborne” continues to be almost completely absent from public WHO communications as of March 2022.
The Centers for Disease Control and Prevention (CDC) in the United States followed a parallel path: first, stating the importance of droplet transmission; then, in September 2020, briefly posting on its website an acceptance of airborne transmission that was taken down three days later;15 and finally, on May 7, 2021, acknowledging that aerosol inhalation is important for transmission.16 However, CDC frequently used the term “respiratory droplet,” generally associated with large droplets that fall to the ground quickly,17 to refer to aerosols,18 creating substantial confusion.19 Neither organization highlighted the changes in press conferences or major communication campaigns.20 By the time these limited admissions were made by both organizations, the evidence for airborne transmission had accumulated, and many scientists and medical doctors were stating that airborne transmission was not just a possible mode of transmission, but likely the predominant mode.21 In August 2021, the CDC stated that transmissibility of the delta SARS-CoV-2 variant approached that of chickenpox, an extremely transmissible airborne virus.22 The omicron variant that emerged in late 2021 appeared to be a remarkably fast spreading virus, exhibiting a high reproductive number and a short serial interval.23
The very slow and haphazard acceptance of the evidence of airborne transmission of SARS-CoV-2 by major public health organizations contributed to a suboptimal control of the pandemic, whereas the benefits of protection measures against aerosol transmission are becoming well established.24-26 Quicker acceptance of this evidence would have encouraged guidelines that distinguished rules for indoors and outdoors, greater focus on outdoor activities, earlier recommendation for masks, more and earlier emphasis on better mask fit and filter, as well as rules for mask-wearing indoors even when social distancing could be maintained, ventilation, and filtration. Earlier acceptance would have allowed greater emphasis on these measures, and reduced the excessive time and money spent on measures like surface disinfection and lateral plexiglass barriers, which are rather ineffective for airborne transmission and, in the case of the latter, may even be counterproductive.29, 30
Why were these organizations so slow, and why was there so much resistance to change? A previous paper considered the issue of scientific capital (vested interests) from a sociological perspective.31 Avoiding costs associated with measures needed to control airborne transmission, such as better personal protective equipment (PPE) for healthcare workers32 and improved ventilation33 may have played a role. Others have explained the delay in terms of perception of hazards associated with N95 respirators32 that have, however, been disputed34 or because of poor management of emergency stockpiles leading to shortages early in the pandemic. e.g. Ref.35
An additional explanation not offered by those publications, but which is entirely consistent with their findings, is that the hesitancy to consider or adopt the idea of airborne transmission of pathogens was, in part, due to a conceptual error that was introduced over a century ago and became ingrained in the public health and infection prevention fields: a dogma that transmission of respiratory diseases is caused by large droplets, and thus, droplet mitigation efforts would be good enough. These institutions also displayed a reluctance to adjust even in the face of evidence, in line with sociological and epistemological theories of how people who control institutions can resist change, especially if it seems threatening to their own position; how groupthink can operate, especially when people are defensive in the face of outsider challenge; and how scientific evolution can happen through paradigm shifts, even as the defenders of the old paradigm resist accepting that an alternative theory has better support from the available evidence.36-38 Thus, to understand the persistence of this error, we sought to explore its history, and of airborne disease transmission more generally, and highlight the key trends that led to droplet theory becoming predominant.
Come from https://www.safetyandquality.gov.au/sub-brand/covid-19-icon
Post time: Sep-27-2022