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Infectious disease expert dismisses letter warning of airborne spread
The letter, signed by more than 230 scientists, states COVID-19 can spread through the air and urges countries to update infection control guidance.
An open letter claims airborne transmission appears to be ‘the only plausible explanation’ for several super-spreader events.
According to the letter, there is ‘significant potential’ for coronavirus exposure via microscopic respiratory droplets (microdroplets) at a distance of up to ‘tens of metres’ in some indoor settings.
It claims airborne transmission appears to be ‘the only plausible explanation’ for several super-spreader events and that authorities ‘must address every potentially important pathway to slow the spread of COVID-19’.
‘Studies … have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1–2 m from an infected individual,’ the letter states.
‘This problem is especially acute in indoor or enclosed environments, particularly those that are crowded and have inadequate ventilation relative to the number of occupants and extended exposure periods.
‘There is every reason to expect … that transmission via airborne microdroplets is an important pathway.’
Proof that coronavirus could spread in such a way – akin to highly infectious diseases like measles – would have major implications on infection control procedures in place to protect health workers and the general public.
However, Professor Lyn Gilbert, Chair of the Infection Control Expert Group (ICEG) that has helped guide Australia’s public health response to COVID-19, told newsGP the letter does not contain much new information.
‘The clinical significance, if any, of airborne transmission is minimal – but probably not zero,’ she said.
‘It depends on many variables, including temperature, humidity, air currents, ventilation, size of the indoor space, individual variations in susceptibility [ie small infective inoculum], variations in viral load, and the type of activity performed by an infected person – for example, shouting, singing, vigorous activity or panting.
‘The epidemiology of COVID-19, in common with other respiratory viruses, suggests that by far the most transmission relates to close contact and fomite or surface contamination from droplets.’
Guidelines developed by the Communicable Diseases Network Australia (CDNA) and endorsed by the Australian Health Protection Principle Committee (AHPPC) reflect this belief and currently indicate airborne protection is only required where clinicians are performing aerosol-generating procedures.
These guidelines closely match others used by other global health bodies, such as the World Health Organization (WHO); however, the signatories to the open letter believe these safeguards are inadequate.
‘The current guidance from numerous international and national bodies focuses on hand washing, maintaining social distancing, and droplet precautions,’ the letter states.
‘Hand washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people.
‘We are advocating for the use of preventive measures to mitigate … airborne transmission.’
These measures would include:
- providing sufficient and effective ventilation (supply clean outdoor air, minimise recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and aged care homes
- supplementing general ventilation with airborne infection controls such as local exhaust, high-efficiency air filtration, and germicidal ultraviolet lights
- avoid overcrowding, particularly in public transport and public buildings.
The letter’s corresponding author, Professor Lidia Morawska, an expert in atmospheric sciences and environmental engineering at Queensland University of Technology, told
The LA Times she and her co-signatories are ‘100% sure’ about the risk of airborne transmission.
However, Clinical Professor Daniel Diekama, an infectious diseases physician, epidemiologist, and clinical microbiologist at the University of Iowa,
is not convinced.
While not discounting there is some evidence of spreading via microdroplets, he believes a major problem affecting the discussion is ‘the false dichotomy’ between droplet and airborne transmission used in healthcare settings.
‘This dichotomy divides application of transmission-based precautions between those pathogens spread via respiratory droplets – all of which must absolutely fall to the ground within six feet [1.8 m] of the source – and those pathogens which become airborne,’ he said.
‘[Airborne] meaning they travel long distances on air currents, remain in the air for very long periods of time, and most importantly can cause infection after their airborne sojourns if they find the right mucosal surface.
‘But we know, and WHO experts know, that there is no such dichotomy – it’s more of a continuum. At the very least, there is a middle category, let’s call it small particle aerosol transmission [SPAT].
‘It’s important to distinguish SPAT from “classic airborne transmission” [CAT]. The CAT pathogens [TB, measles, VZV] have very different transmission dynamics than SPAT pathogens.
‘The distinction is important because for most healthcare epidemiologists, using the term “airborne” implies a common set of one-size fits all interventions to prevent transmission, interventions that require resource-intensive engineering controls and PPE requirements.
‘It is not at all clear that such interventions are required to prevent transmission of SPAT pathogens. In fact, most evidence and real world experience suggests that they are not.’
It also appears unlikely the WHO will change any guidance based on the latter.
Dr Benedetta Allegranzi, the global health body’s technical lead on infection control, recently
told The New York Times evidence for the virus spreading by the air is unconvincing.
‘Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence,’ she said.
‘There is a strong debate on this.’
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