SCIENTIFIC evidence accumulated after a year of the Covid-19 pandemic now points towards the predominant long-range (over five feet) airborne nature of its transmission.
To date, transmission of this nature received only cursory attention from authorities, policymakers worldwide and more so by the general public. Therefore, our existing pandemic management measures focussing solely on droplet and fomite transmission, although extensive, might be sub-optimal.
On April 15, medical journal The Lancet has published the article titled Ten scientific reasons in support of the airborne transmission of SARS-CoV-2. Around the same time, two other top medical journals – The British Medical Journal (BMJ) and Journal of the American Medical Association (Jama) published articles in a similar vein.
The following is a summary of scientific reasons or facts listed by Professor Greenhalgh and her colleagues in The Lancet, which strongly point towards the important (if not predominant) airborne nature of Covid-19 transmission.
The first factor is the presence of super spreaders and super spreading events, where one infected individual infects many others present in the same premises – and not necessary within five feet of the infected. Only airborne transmission can adequately explain the frequency of such occurrences.
Second, in a New Zealand study by Eichler and team, the researchers (while methodologically combining genomic sequence analysis and epidemiologic investigation) showed the possibility of airborne transmission between people in adjacent quarantine rooms who were never in direct contact with each other.
Third, according to the latest estimates, asymptomatic individuals are responsible for transmitting close to 60% of all infections. Asymptomatic individuals do not sneeze or cough much, and it is difficult to explain transmission unless we consider airborne transmission.
The smallest particles (droplets less than 5μm in size), laden with the virus and capable of travelling long distances in the air are can be emitted while talking or even just breathing.
Fourth, it is well-established that the risk of Covid-19 transmission is significantly lower outdoors than indoors and in well-ventilated versus poorly-ventilated indoor settings – which is consistent with airborne transmission.
Fifth, spread of Covid-19 was reported in hospital environments with protective measures explicitly designed to address concerns about large droplets but not the possibility of airborne transmission.
Six, scientist detected viable Covid-19 virus in air samples in laboratory conditions for up to three hours. However, detecting airborne virus outside of laboratory settings is challenging due to mechanical damage during sampling. Authors also stress that other infections such as measles and tuberculosis – whose primary airborne route of transmission is undeniable by now – "have never been cultivated from room air".
Seven, in Uppsala University Hospital, Sweden, scientists detected SARS-CoV-2 N and E genes inside the ventilation HEPA filters located far outside the range to be explained by large-droplet transmission. In the said Swedish study, researchers could extract only genetic material rather than viable virus due to sampling challenges described earlier.
Eight, another scientific experiment supporting airborne mode of Covid-19 transmission has been conducted in the Netherlands. Jasmin Kutter and her colleagues demonstrated transmission to two of four indirect recipient ferrets and the virus to all four while eliminating the possibility of direct transmission between animals' enclosures.
Ninth, to the best of Professor Greenhalgh and her other reputable colleagues' knowledge, no study so far "has provided strong or consistent evidence to refute the hypothesis of airborne SARS-CoV-2 transmission" while empirical evidence of otherwise is piling up continuously.
Tenth, the authors demonstrate inconclusiveness of arguments that to date have been put forward in favour of predominant close-contact (large droplets and fomites) mode of Covid-19 transmission. A higher risk of close-contact infection does not prove that the Covid-19 virus predominantly travels in droplets and does not contradict the hypothesis about its airborne transmission.
How convincing does all of this sound? It must be compelling enough that even the World Health Organisation (WHO), on April 30 updated the English version of its Q&A section with regards to Covid-19 spread between people by adding a para that underscores the possibility of long-range virus transmission in poorly ventilated and/or crowded indoor settings.
Note that previously WHO and the US Centre for Disease Control and Prevention – although not outright denying the possibility of large-distance airborne nature of Covid-19 transmission, but solely emphasized the close-contact or short-range modes of transmission (large droplets and fomites).
The other two articles in BMJ and Jama echo the proposition about SARS-CoV-2 likely having a predominant airborne transmission route. They also discuss the implications of this scientific finding for the effective non-pharmaceutical intervention.
Wearing masks, physical distancing, limiting indoor occupancy and crowded outdoor activities remain relevant measures to curb infection – either from direct contact with surfaces or droplets or from inhaling droplet nuclei. However, when we consider accumulated scientific evidence about Covid-19 airborne transmission, few concerns immediately arise about the sufficiency of existing efforts.
First, the engineering control of indoor air quality – ventilation and filtration – in our work environment, educational institutions, shopping malls, other recreational public spaces and even our homes become essential.
A pertinent question is about the current conditions of our ventilation and filtration systems in all these spaces nationwide. Except for hospitals, they might be set for bare minimums and not designed for effective infection control.
The airborne nature of Covid-19 transmission would call for a thorough revision and upgrade of ventilation and filtration standards and systems nationwide. Public spaces deserve immediate attention.
As an immediate solution, increasing the humidity level in public spaces would significantly reduce travel distance of droplet nuclei laden with the virus.
Additionally, Internet of Things devices measuring CO2 concentration can be placed strategically to alert about presence of areas with dangerous levels and thus calling for sanitation. In public spaces, it is also possible to use a gaseous disinfectant (similar to those used on airplanes) released periodically throughout the day.
In the long run, authorities must make ventilation and filtration standards to combat airborne infections effectively part of development standards. Allen and Ibrahim discuss at length these targets in various settings grounded in the basics of exposure science in the aforementioned Jama article.
Also, reducing taxes for anyone who is in manufacturing, installing, and maintaining high-efficiency ventilation and filtration systems will increase general public access to it nationwide.
The revision and upgrading of ventilation and air conditioning systems might seem a daunting task associated with many fixed costs. However, the tremendous savings and other benefits that can be realised from improved public health and reduced sick leave for other respiratory viruses nationwide might far outweigh the costs.
Additionally, the use of "smart" systems tracing the occupancy or CO2 concentration may significantly reduce the cost of operating such systems.
Such an indoor air control system overhaul nationwide would also increase our resilience and reduce costs associated with possible future pandemics and air quality-related disasters. But it is also crucially important now when we do not know how long more we will have to combat Covid-19 and its possible variants.
The second big concern is the quality of masks used – high filtration efficiency and a tight fit are simply crucial for adequate protection. The airborne nature of Covid-19 transmission also requires tailoring intervention strategies and tightening SOPs for locations where masks are not worn all the time, like in restaurants. For waiters, wearing high-efficiency masks and for patrons to always wear masks except when eating is probably mandatory.
The accumulated empirical evidence of airborne Covid-19 transmission discussed here should not only make governments and health leaders heed the science and re-focus their efforts but also make the general public more cautious and vigilant in compliance with the SOPs. Don't let your guard down. This virus might be more subtle and ubiquitous than we think it is.
Dr Rais Hussin and Dr Margarita Peredaryenko are part of the research team at Emir Research, an independent think tank focused on strategic policy recommendations based on rigorous research.