Health desk – 18 April 2021: If an infectious virus spreads predominantly through large respiratory droplets that fall quickly, the key control measures are reducing direct contact, cleaning surfaces, physical barriers, physical distancing, use of masks within droplet distance, respiratory hygiene, and wearing high-grade protection only for so-called aerosol-generating health-care procedures.but if an infectious virus is mainly airborne, an individual could potentially be infected when they inhale aerosols produced when an infected person exhales, speaks, shouts, sings, sneezes, or coughs. Reducing airborne transmission of virus requires measures to avoid inhalation of infectious aerosols, including ventilation, air filtration, reducing crowding and time spent indoors, use of masks whenever indoors, attention to mask quality and fit, and higher-grade protection for health-care staff and front-line workers. Airborne transmission of respiratory viruses is difficult to demonstrate directly.
Ten streams of evidence collectively support the hypothesis that SARS-CoV-2 is transmitted primarily by the airborne route.
- superspreading events account for substantial SARS-CoV-2 transmission; Detailed analyses of human behaviours and interactions, room sizes, ventilation, and other variables in choir concerts, cruise ships, slaughterhouses, care homes, and correctional facilities, among other settings, have shown patterns .
- long-range transmission of SARS-CoV-2 between people in adjacent rooms but never in each other’s presence has been documented in quarantine hotels.
- asymptomatic or presymptomatic transmission of SARS-CoV-2 from people who are not coughing or sneezing is likely to account for at least a third, and perhaps up to 59%, of all transmission globally and is a key way SARS-CoV-2 has spread around the world, supportive of a predominantly airborne mode of transmission.
- transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation.
- nosocomial infections have been documented in health-care organisations, where there have been strict contact-and-droplet precautions and use of personal protective equipment (PPE) designed to protect against droplet but not aerosol exposure. 6. viable SARS-CoV-2 has been detected in the air. In laboratory experiments, SARS-CoV-2 stayed infectious in the air for up to 3 h with a half-life of 1·1 h.
- SARS-CoV-2 has been identified in air filters and building ducts in hospitals with COVID-19 patients; such locations could be reached only by aerosols.
- studies involving infected caged animals that were connected to separately caged uninfected animals via an air duct have shown transmission of SARS-CoV-2 that can be adequately explained only by aerosols.
- no study to our knowledge has provided strong or consistent evidence to refute the hypothesis of airborne SARS-CoV-2 transmission. Some people have avoided SARS-CoV-2 infection when they have shared air with infected people, but this situation could be explained by a combination of factors, including variation in the amount of viral shedding between infectious individuals by several orders of magnitude and different environmental (especially ventilation) conditions.
- There is limited evidence to support other dominant routes of transmission—ie, respiratory droplet or fomite. Ease of infection between people in close proximity to each other has been cited as proof of respiratory droplet transmission of SARS-CoV-2. However, close-proximity transmission in most cases along with distant infection for a few when sharing air is more likely to be explained by dilution of exhaled aerosols with distance from an infected person.