“Solid and consistent” evidence; Coronavirus is transmitted through the air
A group of researchers published in The Lancet that there is "strong and consistent" evidence that the SARS-CoV-2 virus, which causes COVID-19, is predominantly transmitted through the air. This means that it's no longer enough to just wash our hands frequently and maintain a distance of two meters; it's also important to be mindful of the air we breathe because someone can potentially become infected by inhaling aerosols.
Aerosols are tiny particles, solid or liquid, smaller than 100 microns, that remain suspended in the air for a period of time, from minutes to hours, depending on various factors. If these aerosols contain coronavirus, inhaling them can cause infection. If an infectious virus is primarily airborne, someone can become infected by inhaling these aerosols, which are produced when an infected person exhales, speaks, shouts, sings, or sneezes. Therefore, airborne control measures should include: ventilation, air filtration, reducing crowding and the amount of time people spend in an indoor space, wearing masks whenever indoors (even if not 6 feet or 2 meters away from others), attention to mask quality and fit, and a higher level of PPE (Personal Protective Equipment) for healthcare and other personnel when working in contact with potentially infectious individuals.
Although other routes of transmission "may also contribute", the research team suggests that airborne transmission is dominant, and presents ten pieces of evidence that support this hypothesis:
- Super-spreader events, such as the Skagit Valley choir outbreak in Washington State a year ago, in which 53 people were infected from a single infected case. "The high incidence of such events strongly suggests the predominance of aerosol transmission."
- Transmission of the virus "between people in adjacent rooms, but never in the presence of one another," as has been observed in quarantine hotels .
- Asymptomatic or presymptomatic transmission of the coronavirus from people who "don't cough or sneeze" accounts for at least a third, and perhaps as much as 59% , of all transmission worldwide. Infected people only need to speak to spread the virus.
- Transmission "is greater indoors than outdoors, and is substantially reduced with indoor ventilation."
- Infections have been documented in healthcare organizations where strict precautions against droplet exposure but not aerosol exposure have been implemented.
- Viable SARS-CoV-2 has been detected in the air, the authors point out, recalling how in laboratory experiments the virus remained infectious in the air for up to 3 hours.
- The virus has been detected in air filters and ducts in hospital buildings with Covid-19 patients, where it could only have arrived "through aerosols".
- Studies have been conducted, involving infected caged animals, which were connected to uninfected animals, caged separately, through an air duct, and which have shown that the transmission of the coronavirus "can only be adequately explained by aerosols."
- No study has provided solid or consistent evidence to refute the hypothesis of airborne transmission of SARS-CoV-2.
- "There is limited evidence" to support other dominant transmission routes.
The first months of the coronavirus pandemic were marked by uncertainty about how it spread. Initially thought to be like the flu (through large droplets of secretions expelled when coughing or sneezing), it was advised that only people with symptoms should wear masks. It was also thought that, like the flu, it could be transmitted by touching contaminated surfaces, and thorough disinfection of surfaces, including everyday clothing and shoes, was recommended.
According to this study published in The Lancet, the main route of transmission is respiratory, through the microscopic aerosols formed when speaking, singing, or shouting, and the contribution of transmission through contact with contaminated surfaces is much smaller than previously thought. With this new information, our daily activities should be geared towards reducing the likelihood of aerosol transmission, by wearing two masks, avoiding crowds in enclosed, unventilated spaces for more than 15 minutes, and avoiding sharing enclosed spaces with people who do not live in the same household.
Ramona Ávila Núñez, PhD
Reference:
Trisha Greenhalgh, Jose L Jimenez, Kimberly A Prather, Zeynep Tufekci, David Fisman, Robert Schooley. Ten scientific reasons in support of airborne transmission of SARS-CoV-2. The Lancet (2021). Published: April 15, 2021 DOI:https://doi.org/10.1016/S0140-6736(21)00869-2