It’s not just SARS-CoV-2: Most respiratory viruses spread by aerosols
Conventional wisdom on viral disease transmission needs revision, international science team finds
Peer-Reviewed PublicationIMAGE: CARTOON PANEL SERIES DEPICTING AIRBORNE TRANSMISSION OF VIRUSES CREATED BY NATIONAL SUN YAT-SEN UNIVERSITY, TAIWAN. FULL SERIES AVAILABLE AT HTTPS://DRIVE.GOOGLE.COM/FILE/D/1KX8FKNP__5RZRRS8VZ3PM-RFXLOWAEK7/VIEW?USP=SHARING view more
CREDIT: NATIONAL SUN YAT-SEN UNIVERSITY, TAIWAN
SARS-CoV-2, the virus behind today’s global coronavirus pandemic, spreads primarily by inhalation of virus-laden aerosols at both short and long ranges —and a comprehensive new assessment of respiratory viruses finds that many others probably do as well. SARS-CoV, MERS-CoV, influenza, measles, and the rhinoviruses that cause the common cold can all spread via aerosols that can build up in indoor air and linger for hours, an international, interdisciplinary team of researchers has reported in a review published in Science Aug. 27.
Over the last century and at the beginning of this pandemic, it was widely believed that respiratory viruses, including SARS-CoV-2, mainly spread through droplets produced in coughs and sneezes of infected individuals or through touching contaminated surfaces. However, droplet and fomite transmission of SARS-CoV-2 fails to account for the numerous superspreading events observed during the COVID-19 pandemic, or the much higher transmission that occurs indoors versus. outdoors. Motivated by a desire to understand the factors leading to the COVID-19 pandemic, the researchers from Taiwan, the United States, and Israel sought to identify as clearly as possible how the coronavirus and other respiratory viruses spread. For example, the team reviewed numerous studies of superspreading events observed during the COVID pandemic, and found the studies consistently showed that airborne transmission is the most likely transmission route, not surface contacts or contact with large droplets. One common factor at these superspreading events was the shared air people inhaled in the same room. Many were linked to crowded locations, exposure durations of one hour or more, poor ventilation, vocalization, and lack of properly worn masks. The researchers also reviewed evidence collected from many other types of studies—air sampling, polymerase chain reaction (PCR)-based and/or cell culture studies, epidemiological analysis, laboratory and clinical studies, and modeling work—and concluded that airborne transmission is a major, or even dominant transmission pathway for most respiratory diseases, not just COVID-19.
CAPTION
Rendering of mutating virus cells
CREDIT
istock.com/wildpixel
“Transmission through inhalation of virus-laden aerosols has been long underappreciated. It is time to revise the conventional paradigms by implementing aerosol precautions to protect the public against this transmission route”, said Chia C. Wang, director of the Aerosol Science Research Center and an aerosol physical chemist at National Sun Yat-sen University, Taiwan, who led the review.
Prevailing paradigms about respiratory disease transmission date back as much as a century, the team noted. Airborne transmission was paternalistically dismissed in the early 1900s by prominent public health figure Charles Chapin due to a concern that mentioning transmission by air would scare people into inaction and displace hygiene practices. An unsupported assumption that erroneously equated infections at close range with droplet transmission has shaped the current paradigm for controlling respiratory virus transmission. However, “this assumption neglects the fact that aerosol transmission also occurs at short distances, because the concentration of exhaled aerosols is higher when one is closer to the infected person emitting them,” said Kim Prather, director of the National Science Foundation Center for Aerosol Impacts on Chemistry of the Environment at UC San Diego’s Scripps Institution of Oceanography at UC San Diego and an aerosol chemist who co-led the review.
Respiratory aerosols are formed by expiratory activities, such as breathing, talking, singing, shouting, coughing, and sneezing. Before COVID-19, the traditional size cut-off between aerosols which float like smoke and droplets which drop had been set at 5 µm, however, 100 µm is a more appropriate size distinction. This updated size better represents the largest particles that can remain suspended in still air for more than 5 seconds (from a height of 1.5 meters), travel beyond one meter from the infected person, and be inhaled. “The physical size predominantly determines how long they can stay suspended in the air, how far they can reach, whether they are inhalable, and how deep they can enter into the respiratory tract if inhaled. “The majority of aerosols produced by respiratory activities are smaller than 5 µm, which allows them to travel deep into the bronchiolar and alveolar regions and deposit there. Studies find that viruses are more enriched in aerosols smaller than 5 µm”, said Josué Sznitman, a pulmonary physiologist of Technion, Israel.
Another distinct behavior of aerosols that should be taken into serious consideration is their capacity to be influenced by airflow and ventilation. Ensuring sufficient ventilation rates, filtration, and avoiding recirculation help reduce airborne transmission of infectious virus-laden aerosols. “Monitoring CO2 with portable meters helps verify that ventilation is sufficient, and implementing portable HEPA (high efficiency particulate air) purifiers and upper room UV disinfection systems also help reduce the concentrations of virus-laden aerosols”, added Jose-Luis Jimenez, an atmospheric aerosol chemist of the University of Colorado Boulder. On the other hand, the plexiglass barriers commonly used to block droplet spray from coughs and sneezes in indoor spaces may “impede proper ventilation and create higher exposures for some people,” said Linsey Marr of Virginia Tech, who has studied airborne transmission of pathogens for years. “They are not recommended except for brief, face-to-face transactions, but even then, masks are better because they help remove aerosols, while barriers just divert them.”
With the surge in infections caused by the Delta variant and the increasing occurrence of “COVID-19 breakthrough cases” (infections among people who have been fully vaccinated), many governments and national disease control agencies have resumed universal masking in public. Universal masking is an effective and economic way to block virus-laden aerosols, reported in the review. However, “we need to consider multiple barriers to transmission such as vaccination, masking, and ventilation. One single strategy is unlikely to be strong enough to eliminate transmission of emerging SARS-CoV-2 variants. ”, added Seema S. Lakdawala, a virologist of the University of Pittsburgh.
As the evidence for airborne transmission of SARS-CoV-2 has increased over time and become particularly strong, agencies have taken notice. In April and May 2021, the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) acknowledged inhalation of virus-laden aerosols as a main route in spreading COVID-19 at both short and long ranges. This means that to mitigate transmission and end this pandemic, decision makers should consider implementing aerosol precautionary measures, including universal masking with attention to mask fit, improving ventilation rates in indoor spaces, avoiding recirculation of contaminated indoor air, installation of air filtration such as HEPA purifiers that can effectively remove airborne particles, and using UV disinfection lamps. “What are traditionally called droplet precautions are not replaced wholesale, but instead are modified, expanded and deployed in a more effective manner in accordance with actual transmission mechanisms”, noted by Zeynep Tufekci, a sociologist at the University of Columbia who studies societal challenges in COVID-19 pandemic. Having the correct mental model of transmission of this disease and other respiratory diseases will also allow ordinary people to make better decisions in everyday situations and administrators and officials to create better guidelines and working and socializing environments even after the pandemic, she added.
This pandemic vividly illuminates the importance of the long underestimated airborne transmission route and the necessity of preserving people’s right to breathe clean and pathogen-free air. “What we have learnt from this pandemic also lights up the ways for us to make appropriate changes to enter the post-epidemic era,” said Wang. As addressed at the end of this review, these aerosol precautionary measures will not only protect against airborne transmission of respiratory diseases, but also improve indoor air quality and result in health benefits extending well beyond the COVID-19 pandemic.
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JOURNAL
Science
METHOD OF RESEARCH
Literature review
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
It’s Not Just SARS-CoV-2: Most Respiratory Viruses Spread by Aerosols
ARTICLE PUBLICATION DATE
27-Aug-2021
COVID-19 is advancing knowledge of airborne transmission of respiratory viruses
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.
JOURNAL
Science
ARTICLE TITLE
Airborne transmission of respiratory viruses
ARTICLE PUBLICATION DATE
27-Aug-2021
COVID-19: Will telecommuting strategies stop the virus from circulating?
Peer-Reviewed PublicationHow can we best organise on-site workplace and school attendance periods and remote work to slow the circulation of Sars-CoV-2? Is it better to separate classes? Bring your whole team in at the same time? Set this up on daily or weekly schedules? The COVID-19 pandemic has forced most countries to impose contact limitations in workplaces, universities and schools. Scientists from the CNRS, Université Versailles-Saint-Quentin-en-Yvelines, ENS de Lyon and INRIA1, in collaboration with the Institut Pasteur and INSERM, have analysed the impact of two strategies, rotating and on-off strategies2, in stopping the epidemic as it reaches a community, whether in a school or in an office. Their results show that below a certain local reproduction number threshold3 in the community, the two strategies, combined with other health measures, effectively control the epidemic, although the ‘Rotating week-by-week’ strategy is the most effective of those studied. These results, published on 26 August 2021 in PLOS Computational Biology, offer new elements to guide public health decisions related to distance working, office or school.
Notes:
1 The French laboratories involved are at the Institut de Recherche en Informatique Fondamentale (CNRS/Université de Paris), LIP6 (CNRS/Sorbonne Université), GIPSA-Lab (CNRS/Université Grenoble Alpes), Département d’Informatique de l’ENS de Lyon, Centre de Recherche en Epidémiologie et Santé des Populations (INSERM/Université Paris-Saclay/UVSQ), Laboratoire Epidémiologie et Modélisation de la Résistance aux Antimicrobiens (Institut Pasteur/INSERM/UVSQ).
2 Rotation is intended to maintain a continuous presence in the workplace, while on-off is intended to maintain community cohesion. For example, for a school, in both cases an individual will only go to school one day in two or one week in two, but as part of a rotation strategy, the community is split in two: Cohort A goes to school while Cohort B stays at home, and vice versa; in an on-off strategy, everyone goes to school at the same time or stays at home at the same time.
3 The local reproduction number , or “local R0,” indicates the average number of new cases of a disease that a single infected and contagious person will generate in the community, on average, in a population without any immunity.
JOURNAL
PLoS Computational Biology
It’s not just SARS-CoV-2: Most respiratory viruses spread by aerosols
Conventional wisdom on viral disease transmission
needs revision, international science team finds
IMAGE: PHASES INVOLVED IN AIRBORNE TRANSMISSION OF RESPIRATORY VIRUSES view more
CREDIT: FROM WANG ET AL., ‘AIRBORNE TRANSMISSION OF RESPIRATORY VIRUSES’ (HTTPS://DOI.ORG/10.1126/SCIENCE.ABD9149). N.CARY/SCIENCE
SARS-CoV-2, the virus behind today’s global coronavirus pandemic, spreads primarily by inhalation of virus-laden aerosols at both short and long ranges —and a comprehensive new assessment of respiratory viruses finds that many others probably do as well. SARS-CoV, MERS-CoV, influenza, measles, and the rhinoviruses that cause the common cold can all spread via aerosols that can build up in indoor air and linger for hours, an international, interdisciplinary team of researchers has reported in a review published in Science today.
Over the last century and at the beginning of this pandemic, it was widely believed that respiratory viruses, including SARS-CoV-2, mainly spread through droplets produced in coughs and sneezes of infected individuals or through touching contaminated surfaces. However, droplet and fomite transmission of SARS-CoV-2 fails to account for the numerous superspreading events observed during the COVID-19 pandemic, or the much higher transmission that occurs indoors versus. outdoors. Motivated by a desire to understand the factors leading to the COVID-19 pandemic, the researchers from Taiwan, the United States, and Israel sought to identify as clearly as possible how the coronavirus and other respiratory viruses spread. For example, the team reviewed numerous studies of superspreading events observed during the COVID pandemic, and found the studies consistently showed that airborne transmission is the most likely transmission route, not surface contacts or contact with large droplets. One common factor at these superspreading events was the shared air people inhaled in the same room. Many were linked to crowded locations, exposure durations of one hour or more, poor ventilation, vocalization, and lack of properly worn masks. The researchers also reviewed evidence collected from many other types of studies—air sampling, polymerase chain reaction (PCR)-based and/or cell culture studies, epidemiological analysis, laboratory and clinical studies, and modeling work—and concluded that airborne transmission is a major, or even dominant transmission pathway for most respiratory diseases, not just COVID-19.
“Transmission through inhalation of virus-laden aerosols has been long underappreciated. It is time to revise the conventional paradigms by implementing aerosol precautions to protect the public against this transmission route”, said Chia C. Wang, director of the Aerosol Science Research Center and an aerosol physical chemist at National Sun Yat-sen University, Taiwan, who led the review.
Prevailing paradigms about respiratory disease transmission date back as much as a century, the team noted. Airborne transmission was paternalistically dismissed in the early 1900s by prominent public health figure Charles Chapin due to a concern that mentioning transmission by air would scare people into inaction and displace hygiene practices. An unsupported assumption that erroneously equated infections at close range with droplet transmission has shaped the current paradigm for controlling respiratory virus transmission. However, “this assumption neglects the fact that aerosol transmission also occurs at short distances, because the concentration of exhaled aerosols is higher when one is closer to the infected person emitting them”, said Kim Prather, director of the National Science Foundation Center for Aerosol Impacts on Chemistry of the Environment at UC San Diego’s Scripps Institution of Oceanography at UC San Diego and an aerosol chemist who co-led the review.
Respiratory aerosols are formed by expiratory activities, such as breathing, talking, singing, shouting, coughing, and sneezing. Before COVID-19, the traditional size cut-off between aerosols which float like smoke and droplets which drop had been set at 5 µm, however, 100 µm is a more appropriate size distinction. This updated size better represents the largest particles that can remain suspended in still air for more than 5 seconds (from a height of 1.5 meters), travel beyond one meter from the infected person, and be inhaled. “The physical size predominantly determines how long they can stay suspended in the air, how far they can reach, whether they are inhalable, and how deep they can enter into the respiratory tract if inhaled. “The majority of aerosols produced by respiratory activities are smaller than 5 µm, which allows them to travel deep into the bronchiolar and alveolar regions and deposit there. Studies find that viruses are more enriched in aerosols smaller than 5 µm”, said Josué Sznitman, a pulmonary physiologist of Technion, Israel.
Another distinct behavior of aerosols that should be taken into serious consideration is their capacity to be influenced by airflow and ventilation. Ensuring sufficient ventilation rates, filtration, and avoiding recirculation help reduce airborne transmission of infectious virus-laden aerosols. “Monitoring CO2 with portable meters helps verify that ventilation is sufficient, and implementing portable HEPA (high efficiency particulate air) purifiers and upper room UV disinfection systems also help reduce the concentrations of virus-laden aerosols”, added Jose-Luis Jimenez, an atmospheric aerosol chemist of the University of Colorado Boulder. On the other hand, the plexiglass barriers commonly used to block droplet spray from coughs and sneezes in indoor spaces may “impede proper ventilation and create higher exposures for some people,” said Linsey Marr of Virginia Tech, who has studied airborne transmission of pathogens for years. “They are not recommended except for brief, face-to-face transactions, but even then, masks are better because they help remove aerosols, while barriers just divert them.”
With the surge in infections caused by the Delta variant and the increasing occurrence of “COVID-19 breakthrough cases” (infections among people who have been fully vaccinated), many governments and national disease control agencies have resumed universal masking in public. Universal masking is an effective and economic way to block virus-laden aerosols, reported in the review. However, “we need to consider multiple barriers to transmission such as vaccination, masking, and ventilation. One single strategy is unlikely to be strong enough to eliminate transmission of emerging SARS-CoV-2 variants. ”, added Seema S. Lakdawala, a virologist of the University of Pittsburgh.
As the evidence for airborne transmission of SARS-CoV-2 has increased over time and become particularly strong, agencies have taken notice. In April and May 2021, the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) acknowledged inhalation of virus-laden aerosols as a main route in spreading COVID-19 at both short and long ranges. This means that to mitigate transmission and end this pandemic, decision makers should consider implementing aerosol precautionary measures, including universal masking with attention to mask fit, improving ventilation rates in indoor spaces, avoiding recirculation of contaminated indoor air, installation of air filtration such as HEPA purifiers that can effectively remove airborne particles, and using UV disinfection lamps. “What are traditionally called droplet precautions are not replaced wholesale, but instead are modified, expanded and deployed in a more effective manner in accordance with actual transmission mechanisms”, noted by Zeynep Tufekci, a sociologist at the University of Columbia who studies societal challenges in COVID-19 pandemic. Having the correct mental model of transmission of this disease and other respiratory diseases will also allow ordinary people to make better decisions in everyday situations and administrators and officials to create better guidelines and working and socializing environments even after the pandemic, she added.
This pandemic vividly illuminates the importance of the long underestimated airborne transmission route and the necessity of preserving people’s right to breathe clean and pathogen-free air. “What we have learnt from this pandemic also lights up the ways for us to make appropriate changes to enter the post-epidemic era,” said Wang. As addressed at the end of this review, these aerosol precautionary measures will not only protect against airborne transmission of respiratory diseases, but also improve indoor air quality and result in health benefits extending well beyond the COVID-19 pandemic. To promote some of the key concepts of airborne transmission of respiratory viruses to the general public, including school kids, a series of comics entitled "The Journey of Virosols" made by Aerosol Science Research Center at National Sun Yat-sen University, Taiwan is also released as the supplemental multimedia along with the review.
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Images: https://drive.google.com/drive/folders/1os0Yt-tvmHDvJpE70t2M7oKfDvGGftEZ?ths=true
CAPTION
2_Cast_Virosols versus Droplets
CREDIT
Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan
CAPTION
4_How can viruses be spread through aerosol transmission?
CREDIT
Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan
CAPTION
Plexiglass barriers may trap higher concentration of aerosols
CREDIT
Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan
CAPTION
Virus-laden aerosols can enter and deposit in the lower respiratory tract
CREDIT
Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan
CAPTION
How to stop airborne transmission of viruses
CREDIT
Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan
JOURNAL
Science
METHOD OF RESEARCH
Systematic review
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Airborne transmission of respiratory viruses
ARTICLE PUBLICATION DATE
27-Aug-2021
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