Most Respiratory Viruses Actually Spread by Aerosols

Stages involved in the transmission of respiratory viruses through the air. Credit: Van Wang et al., ‘Airborne Transmission of Respiratory Viruses’ (https://doi.org/10.1126/science.abd9149). N.CARY/SCIENCE

Conventional wisdom about the transmission of viral diseases needs to be reconsidered, the international scientific team believes.

SARS-CoV-2, the virus behind the current global coronavirus pandemic, spreads primarily through inhalation of virus-laden aerosols at both short and long distances – and a comprehensive new assessment of respiratory viruses finds that many others likely do as well. SARS-CoV, MERS-CoV, the flu, measles and the rhinoviruses that cause the common cold can all spread through aerosols that can accumulate in indoor air and linger for hours, an international, interdisciplinary team of researchers has reported in a published review. . in Science on August 27, 2021.

Over the past century and at the start of this pandemic, it was widely believed that respiratory viruses, including SARS-CoV-2, spread primarily through droplets produced when infected individuals cough and sneeze or touch contaminated surfaces. However, the droplet and fomite transmission of SARS-CoV-2 does not take into account the numerous superdissemination events observed during the COVID-19 pandemic, or the much higher transmission that occurs indoors than outdoors.

Motivated by a desire to understand the factors leading to the COVID-19 pandemic, 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 that the studies consistently showed that airborne transmission is the most likely transmission route, no surface contact or contact with large droplets. A common factor in these superspreading events was the shared air that people breathed in the same room.

Many were linked to crowded locations, exposure times of an hour or more, poor ventilation, vocalization and the lack of properly worn masks. The researchers also reviewed the evidence gathered 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 an important or even dominant route of transmission for most respiratory diseases, not just COVID-19.

Rendering of mutating virus cells. Credit: istock.com/wildpixel

“Inhalation transmission of virus-laden aerosols has long been undervalued. It’s time to rethink conventional paradigms by taking aerosol precautions to protect the public from 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.

The prevailing paradigms about the transmission of respiratory diseases date back a century, the team noted. Airborne transmission was paternalistically rejected by the prominent public health figure Charles Chapin in the early 1900s over concerns that mentioning airborne transmission would deter people into inaction and crowd out hygiene practices. An unsupported assumption that short-range infections are mistakenly equated with droplet transmission has shaped the current paradigm for controlling respiratory virus transmission. “However, this assumption ignores 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 who is radiating them,” said Kim Prather, director of the National Science Foundation Center for Aerosol. Effects on Environmental Chemistry 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 exhalation activities, such as breathing, talking, singing, yelling, coughing, and sneezing. Before COVID-19, the traditional boundary between aerosols that float like smoke and droplets that fall was set at 5m, but 100m is a more appropriate size distinction. This updated size better represents the largest particles that can linger in still air for more than 5 seconds (from a height of 1.5 meters), travel further than a meter from the infected person, and be inhaled. “Physical size mainly determines how long they can remain in the air, how far they can reach, whether they are inhalable and how deep they can enter the airways when inhaled. “Most aerosols produced by respiratory activities are smaller than 5 m, allowing them to travel deep into the bronchiolar and alveolar regions and deposit there. Studies show that viruses are more enriched in aerosols smaller than 5 µm,” says Josué Sznitman, a pulmonary physiologist from Technion, Israel.

Source: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan Credit: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan Credit: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan

Source: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan Credit: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan Credit: Chia C. Wang, Aerosol Science Research Center, National Sun Yat-sen University, Taiwan

Another distinctive behavior of aerosols that should be seriously considered is their ability to be affected by airflow and ventilation. By ensuring adequate ventilation rates, filtration and avoiding recirculation, the transmission of infectious virus-laden aerosols through the air is reduced. “Monitoring CO2 with portable meters helps verify that ventilation is adequate, and implementing portable High Efficiency Particulate Air (HEPA) and UV disinfection systems in the upper room also helps reduce concentrations of virus-laden aerosols” , added Jose-Luis Jimenez, an atmospheric aerosol chemist from the University of Colorado Boulder. On the other hand, the plexiglass barriers commonly used to block droplet spray from coughing and sneezing in indoor spaces can “impede proper ventilation and cause higher exposure for some people,” said Virginia Tech’s Linsey Marr, who explains the transmission of airborne pathogens. year. “They’re not recommended except for short, face-to-face transactions, but even then, masks are better because they help remove aerosols, while barriers just distract them.”

With the increase in infections caused by the Delta variant and the increasing incidence 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 economical way to block virus-laden aerosols, stated in the review. “However, we need to consider multiple barriers to transmission, such as vaccination, masking and ventilation. It is unlikely that a single strategy is strong enough to eliminate the transmission of emerging SARS-CoV-2 variants,” added Seema S. Lakdawala , a virologist from the University of Pittsburgh.

As the evidence for airborne transmission of SARS-CoV-2 has increased over time and has become particularly strong, agencies have noted this. In April and May 2021, the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) recognized inhalation of virus-laden aerosols as the primary route in both short- and long-range spread of COVID-19. This means that to reduce transmission and end this pandemic, decision makers should consider taking aerosol precautions, including universal masking with attention to mask fit, improving indoor ventilation rates, avoiding recirculation of polluted indoor air, installation of air filtration such as HEPA air purifiers that can effectively remove particles in the air, and using UV disinfection lamps. “What is traditionally called drip precautions are not being widely replaced, but are instead being adapted, expanded and deployed in a more effective way according to actual transmission mechanisms,” said Zeynep Tufekci, a sociologist at Columbia University who studies societal challenges. studies in Covid19 pandemic. Having the right mental model for the transmission of this disease and other respiratory diseases will also enable ordinary people to make better decisions in everyday situations and administrators and officials to create better guidelines and work and social environments, even after the pandemic, she added.

This pandemic vividly highlights the importance of the long-underrated airborne transmission route and the need to preserve people’s right to breathe clean and pathogen-free air. “What we’ve learned from this pandemic also illuminates the ways we can make the right changes to move into the post-epidemic era,” Wang said. As discussed at the end of this review, these aerosol precautions will not only protect against airborne transmission of respiratory diseases, but also improve indoor air quality and result in health benefits that go well beyond the COVID-19 pandemic.

Reference: “Airborne Transmission of Respiratory Viruses” By Kimberly A. Prather, Josué Sznitman, Jose L. Jimenez, Seema S. Lakdawala, Zeynep Tufekci, and Linsey C. Marr, Aug. 27, 2021, Science.
DOI: 10.1126/science.abd9149