The locations where mass infections were confirmed so far are places where the following three conditions were met simultaneously: (1) closed space with poor ventilation, (2) crowded with many people and (3) conversations and vocalization in close proximity (within arm’s reach of one another). It is believed that more people were infected in such places. Therefore, we ask that you predict locations and settings where these three conditions could occur simultaneously and avoid them.
We do not have enough scientific evidence yet on how significantly such actions can reduce the risk of spreading infection. However, since places with poor ventilation and crowded places are increasing infections, we ask that you take precautions even before scientific evidence for clear standards is found.” (page 2)
This narrative assumes the possibility of airborne transmission and asks citizens to share the uncertainty and act in a precautionary way.
The droplet-but-not-airborne narrative emphasised randomised control trial evidence (see appendix on bmj.com);22 it drew implicitly on the hierarchy of evidence —a formalisation of the assumed superiority of randomised trials,23 which “… typically serve[s] the needs and realities of clinical medicine, but not necessarily public policy” (page 665).24 It did not acknowledge the hierarchy of controls —a more public health-oriented framework which favours system-level interventions aimed at pathogen elimination, followed by environmental controls aimed at making air and water safe, and both of these over interventions to influence human behaviour.5 Since randomised trials are not considered appropriate in the science of building design and ventilation,25 this mindset led policymakers to reject a wealth of wider evidence which we discuss in the next section.

“Covid is unequivocally airborne”

Aerosol scientists study how fluids and particles travel in the air. Some had specialised in how respiratory pathogens—including tuberculosis, influenza and other coronaviruses such as SARS and MERS—travel. They had shown, using laboratory studies, real-world case studies and computer modelling, that these pathogens are transmitted by aerosols and require airborne mitigation measures (Table 1, column b),26 27 and that coughs and sneezes generate turbulent gas clouds different-sized particles which can travel long distances.28 29
From early 2020, evidence accumulated from a range of study designs to support the hypothesis that, like most other respiratory pathogens—and perhaps more so than other coronaviruses—SARS-CoV-2 is transmitted through the air (Box 2).18 30-32

Box 2: 10 streams of evidence in support of airborne transmission of SARS-CoV-2