Reducing transmission of SARS-CoV-2 Kimberly A. Prather1, Chia C. Wang2,3, Robert T. Schooley4 See all authors and affiliations Science 27 May 2020: eabc6197 DOI: 10.1126/science.abc6197 Article Figures & Data Info & Metrics eLetters PDF Abstract Masks and testing are necessary to combat asymptomatic spread in aerosols and droplets SIGN UP FOR THE SCIENCE eTOC Get the latest issue of Science delivered right to you! Email Respiratory infections occur through the transmission of virus-containing droplets (>5 to 10 μm) and aerosols (≤5 μm) exhaled from infected individuals during breathing, speaking, coughing, and sneezing. Traditional respiratory disease control measures are designed to reduce transmission by droplets produced in the sneezes and coughs of infected individuals. However, a large proportion of the spread of coronavirus disease 2019 (COVID-19) appears to be occurring through airborne transmission of aerosols produced by asymptomatic individuals during breathing and speaking (1–3). Aerosols can accumulate, remain infectious in indoor air for hours, and be easily inhaled deep into the lungs. For society to resume, measures designed to reduce aerosol transmission must be implemented, including universal masking and regular, widespread testing to identify and isolate infected asymptomatic individuals. Humans produce respiratory droplets ranging from 0.1 to 1000 μm. A competition between droplet size, inertia, gravity, and evaporation determines how far emitted droplets and aerosols will travel in air (4, 5). Respiratory droplets will undergo gravitational settling faster than they evaporate, contaminating surfaces and leading to contact transmission. Smaller aerosols (≤5 μm) will evaporate faster than they can settle, are buoyant, and thus can be affected by air currents, which can transport them over longer distances. Thus, there are two major respiratory virus transmission pathways: contact (direct or indirect between people and with contaminated surfaces) and airborne inhalation. In addition to contributing to the extent of dispersal and mode of transmission, respiratory droplet size has been shown to affect the severity of disease. For example, influenza virus is more commonly contained in aerosols with sizes below 1 μm (submicron), which lead to more severe infection (4). In the case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it is possible that submicron virus-containing aerosols are being transferred deep into the alveolar region of the lungs, where immune responses seem to be temporarily bypassed. SARS-CoV-2 has been shown to replicate three times faster than SARS-CoV-1 and thus can rapidly spread to the pharynx from which it can be shed before the innate immune response becomes activated and produces symptoms (6). By the time symptoms occur, the patient has transmitted the virus without knowing. Identifying infected individuals to curb SARS-CoV-2 transmission is more challenging compared to SARS and other respiratory viruses because infected individuals can be highly contagious for several days, peaking on or before symptoms occur (2, 7). These “silent shedders” could be critical drivers of the enhanced spread of SARS-CoV-2. In Wuhan, China, it has been estimated that undiagnosed cases of COVID-19 infection, who were presumably asymptomatic, were responsible for up to 79% of viral infections (3). Therefore, regular, widespread testing is essential to identify and isolate infected asymptomatic individuals.
