A group of 239 multidisciplinary scientists from around the globe have united in an appeal to the medical community and relevant national and international bodies to recognize the potential for airborne spread of SARS-CoV-2. Their letter has been published in the journal Clinical Infectious Diseases and could have important implications for the precautions needed to prevent the spread of COVID-19, especially for indoor environments.
Respiratory droplets vs airborne transmission
An important distinction lies between respiratory droplets and airborne transmission. This may seem like a semantic point, however when it comes to transmission of SARS-CoV-2 and the precautions that must be implemented to prevent the spread of the disease, it is a critical distinction for the public and authorities issuing guidance to understand fully.
Professor Jose Vazquez-Boland, Chair of Infectious Diseases at the University of Edinburgh, said, “A problem here is the potential conflict between the technical notion of airborne transmission and the perception of the general public about this term. A technical distinction is typically made between "transmission by respiratory droplets" expelled through coughing or sneezing, relatively heavy – thus reaching relatively short distances – and assumed by the World Health Organization (WHO) and other official bodies to be the main transmission mechanism for SARS-CoV-2; and "airborne transmission", which involves smaller particles suspended in the air (aerosols). The latter may travel longer distances but are more exposed to desiccation and WHO and other public health bodies interpret that, except in certain circumstances (aerosol-generating clinical procedures during patient care), they play a lesser role in COVID-19 transmission.”
Whilst transmission of SARS-CoV-2 via larger respiratory droplets is widely accepted, the role of airborne transmission (droplets below around 10 microns in diameter) has been debated.
Cath Noakes, Professor of Environmental Engineering for Buildings at the University of Leeds, and one of the letter’s signatories commented, “Transmission through small aerosols is likely in some circumstances, for example when there is a high rate of respiratory droplet generation coupled with a poorly ventilated environment. COVID-19 doesn't spread as easily as diseases like measles or tuberculosis, which are true airborne diseases and can spread by air over long distances. COVID-19 is more likely to be "opportunistically" airborne and therefore poses a risk to people who are in the same room for long periods of time.”
With the opening of public buildings like cinemas and restaurants, public transport and workplaces, this could have important implications for the spread of disease in these environments.
Determining airborne transmission
Most public health organizations, including WHO, do not recognize that airborne transmission plays an important role in the current pandemic, except in aerosol-generating procedures performed in healthcare settings. However, there is mounting evidence, some of which is presented in the letter, that suggests otherwise. Whilst WHO say that there is insufficient evidence to prove that airborne transmission of SARS-CoV-2 is happening, the scientists argue that there is insufficient evidence to prove that airborne transmission does not occur.
Dr Julian Tang, Associate Professor of Respiratory Sciences at the University of Leicester and one of the letter’s signatories commented, “We know that SARS-COV-2 RNA can be found in the air and on high level surfaces (like ventilation grills/blades). The only way this virus can get into the air or to high level surfaces is if an infected patient exhales them and the virus then stays suspended in the air (i.e. does not fall to the ground immediately under gravity) then gets carried upwards to high level surfaces by air movements. So far, we have yet to show that these viral RNA counts from air-samples also correlate to viable viruses (by viral culture) – but this does not by itself exclude airborne transmission, as the WHO claims.”
Coronaviruses are relatively fragile, enveloped viruses and the air sampling techniques used rather harsh. It is therefore important to consider that studies that have failed to isolate live virus from air samples have done so as an artifact of the sampling technique used, not due to an absence of infectious virus.
Tang continued, “Other studies using different, gentler, air-sampling methods are underway to demonstrate this. But the real question to WHO is – can you prove that none of those 11 million+ cases of COVID-19 acquired their infection via aerosols? Clearly, they cannot prove this either.”
Public understanding, indoor spaces and official guidance
As many countries relax lockdown measures, there is clear concern from the expert authors that the recommended precautions, such as 1.5 m social distancing, are insufficient to protect people from infection and risk generating a false sense of protection.
“For the public it may be difficult to differentiate between the different situations and technical definitions. Consequently, there is clearly a danger of people misinterpreting WHO guidance regarding airborne transmission and underestimating the general risk of contracting COVID-19 through respiratory/ mucosal exposure (and the importance of wearing protective masks),” concluded Vazquez-Boland.
The authors suggest a number of measures that could be taken to help protect people from an airborne transmission route, including:
- Sufficient and effective ventilation (supply clean outdoor air, minimize air recirculation) especially in public buildings, workplaces, schools, hospitals, and care homes for the elderly
- Supplement general ventilation with airborne infection controls such as local exhausts, high efficiency air filtration, and germicidal ultraviolet (UV) lights
- Avoiding overcrowding, especially in indoor areas like public buildings and on public transport
The authors therefore urge the acknowledgement of the role that airborne spread of SARS-CoV-2 may be playing to enable greater precautions to be implemented.
“We understand that all of this cannot and will not happen overnight (except perhaps the wearing of N95 masks in COVID-19 patient areas and the opening of windows). But having the airborne transmission risk acknowledged in the WHO guidelines will motivate and fund hospital teams, governments, NGOs, etc. to work towards these goals, which can also be applied to some forms of public transport – buses, coaches, trains, planes, cruise ships. All of the interventions can be combined to reduce the overall build-up and concentration of airborne SARS-CoV-2 in the air to reduce the risk of infection,” concluded Tang.