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Aerosol transmission of COVID-19 by asymptomatic individuals possible: Professor Chia C. Wang, Kimberly A. Prather, and Robert T. Schooley publish an article in Science

Asymptomatic individuals infected with coronavirus disease 2019 can spread the disease by aerosol transmission! Associate Professor Chia C. Wang, the Director of the Aerosol Science Research Center at National Sun Yat-sen University, in collaboration with the Director of the NSF Center for Aerosol Impacts on Chemistry of the Environment, University of California San Diego, USA, Kimberly A. Prather, and a renown infectious disease specialist Robert T. Schooley, published a perspective article in Science, a top international journal, stating that SARS-CoV-2 virus causing COVID-19 disease can be transmitted in the air in the form of aerosols and droplets. The aerosol transmission may be the main route of transmission of the virus, as asymptomatic individuals are unaware of the fact that they can transmit the disease and do not wear a mask nor use other protection measures.

SARS-CoV-2 causing the COVID-19 disease to spread rapidly around the world, with more than 16 million cases worldwide and a fatality rate with over 650,000 deaths, implying that there remain unresolved issues regarding the control and prevention strategies of this disease. It is generally perceived and acknowledged by WHO that the two main virus transmission pathways are fomite and droplet transmission. Disease prevention guidelines such as social distancing and hands washing were established in line with the conventional understanding.

Director Wang emphasized that in the past, people used to believe that they could get infected only by inhaling respiratory droplets produced in coughs and sneezes. However, people actually release a significant amount of tiny droplets of size below 1 μm when they speak, sing, cough, and simply breathe. For people infected with COVID-19, their respiratory tracts and lungs contain a significant amount of SARS-CoV-2 virus, which can be released into the environment via these expiratory activities. Provided that these virus-laden aerosols retain their infectivity when being inhaled into a person’s respiratory tract or lungs, these viruses can replicate in the host’s organism and causes the infection and disease.

Director Wang indicates that WHO recommends keeping social distance to avoid inhaling respiratory droplets in the coughs and sneezes of the infected person. Tracing back the history, the size of the droplets was measured by W. F. Wells in the 1930s. However, 90 years ago, the measurement equipment and technologies were not advanced enough to precisely measure the size of droplets of size below 1 μm. With the advent of microparticle and nanoparticle measurement technologies, the experiments on respiratory aerosols conducted in the recent 20 years undisputedly confirm that the majority of exhaled aerosols are of size below 1 micrometer. These virus-laden aerosols, when released into the environment from the respiratory tract of the infected person, not only can float in the air for a prolonged period of time but also travel to distances longer than the social distance recommended by WHO.

In the article, the three authors indicated that a 100-μm droplet will settle to the ground from a height of 8 ft in just 4.6 s, whereas a 1-μm aerosol particle will take 12.4 hours. The latest research also shows that aerosols containing SARS-CoV-2 can survive for up to 16 hours in the air. These tiny virus-laden aerosols once inhaled into the human body, can travel deep into a person’s lungs and cause infection.

Director Wang says that many viral diseases, including severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and influenza A H1N1, that caused epidemics in the past, have been proven to be transmissible via the airborne transmission route. Since the outbreak of COVID-19 at the end of the year 2019, there are already sufficient proofs, including experiments, field measurements, and sample analyses, epidemiological statistics, and aerodynamic modeling, which consistently support that SARS-CoV-2, which causes the COVID-19 disease, can be transmitted by the airborne route. This Perspective article published in Science also mentions that the risk of airborne transmission of virus-laden aerosols is highest when neither asymptomatic individuals who are unaware of their infection status nor people in their surroundings (potential hosts) wear a mask nor implement any preventive measures, especially when they stay in a closed or semi-closed space with reduced ventilation and high concentrations of virus aerosols. On the contrary, the risk of infection can be substantially reduced if both the infected person and the healthy person wear masks. Moreover, the article also mentions that in areas with relatively severe air pollution, the viruses may “hitchhike” on the man-made pollution particles floating in the air (e,g. PM2.5) and thus facilitate the spread of the disease.

Furthermore, this article also discusses the strategies implemented by the Taiwanese government which led to the success of Taiwan’s epidemic-prevention measures. By making a comparison with the state of New York, U.S.A., which has a similar population size, the authors discuss the set of measures implemented by the Taiwanese government that effectively prevented a potentially large-scale outbreak on the island. Having lessons learned from the past experience of the SARS epidemic in 2002-2003, the Taiwanese government established the Central Epidemic Command Center right at the beginning of the outbreak to gain full control of the epidemic, detect and track infected patients and their close contacts using advanced technologies, request citizens to wear masks in public places and at the same time secure the steady supply and availability of masks. Director Wang said that owing to the effective preventive measures and policies implemented by the Taiwanese government together with the cooperations of Taiwanese citizens who are willing to wear masks, the epidemic was well under control and the risk of getting infected with COVID-19 by the airborne transmission route in the country is relatively low. However, one shouldn’t take this lightly. On the contrary, many other countries still do not have a sufficient supply of masks, and certain citizens had a cultural objection to wearing masks, which contributed to an increased risk of airborne transmission of the virus. The authors hope that with this article, they can help people from all over the world to face the epidemic head-on, prevent the spread of the virus by the airborne route, re-evaluate the outdated paradigm of disease-prevention measures, and better control the epidemic to prevent the further loss of lives.

Article in Science: https://science.sciencemag.org/content/early/2020/05/27/science.abc6197.full


This article has attracted broad global attention and its publication was covered by such mainstream media as CNN and Fox News in the USA and many other countries. Director Wang also received the acknowledgment of many scholars, including those of Columbia University in New York, and Oregon State University in the USA, and McGill University in Canada, proving the great value of the article for the promotion of epidemic prevention in the region.

Aerosol Science Research Center (ASRC) at NSYSU is the first and only aerosol-themed center in Asia. Led by Director Chia C. Wang (University of California, Berkeley, USA, Ph.D. in Chemistry), the research team of ASRC has been actively participating in the PM 2.5-preventive medicine and aerosol biomedicine research. ASRC also engages in studying various diseases caused by PM 2.5, such as respiratory and pulmonary diseases, cardiovascular diseases, children's cognitive development disorders, and the pathological mechanisms and new medical treatments, paving the important foundation for the PM 2.5 preventive medicine. One of the main research topics of ASRC includes the airborne transmission of bioaerosols, including pathogens such as viruses or bacteria that are transmitted in the air in the form of aerosols. ASRC aims to better understand the pathogenic mechanisms, to develop innovative prevention and treatment strategies with the ultimate goal to improve peoples’ quality of life and well-being and fulfill the University's social responsibility.
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