Recently, the World Health Organization (WHO) acknowledged the possibility of airborne transmission of the SARS-CoV-2 virus, the causative agent of COVID-19. This announcement has brought in its wake many questions regarding the effectiveness of the best practices to be followed for prevention and control of infection spread. Measures such as social distancing and use of masks are now being questioned.
So, what exactly does airborne transmission mean?
Airborne route of transmission is different from transmission through respiratory droplets. Respiratory droplets are large (>5 µm) and settle on surfaces rapidly after being released. These droplets can carry pathogens that may spread diseases through the droplet mode of infection either through direct person-to-person interaction or through indirect interaction with contaminated surfaces also known as ‘fomites’. On the other hand, smaller particles (<5 µm) expelled during aerosol/droplet generating activities such as coughing, sneezing, talking, laughing, and certain medical procedures are too light to settle down rapidly and ride on air currents, thus, lingering in the air for extended periods of time and travelling larger distances before settling down. Droplets generated by such activities have a large water content that starts to evaporate rapidly after being released, resulting in dehydrated droplets which are much smaller in size and therefore take longer to fall to the ground or to settle down on surfaces. The reduced water content in such smaller droplets compromises the ability of pathogens to survive owing to greater exposure to environmental stress factors. This, fortunately results in relatively fewer airborne pathogens. Some examples of pathogens that are notorious for being airborne are tuberculosis bacteria, anthrax bacteria, measles virus, SARS-CoV virus (that causes SARS), and influenza virus.
There is also accumulating evidence suggesting the possibility of airborne transmission of SARS-CoV-2, although conclusive studies are yet to be reported. The lack of conclusive studies is attributed to the difficulties faced while trying to verify the viability (the ability of the virus particle to cause an infection) of the virus particles present in airborne aerosols/droplets. Although many studies have found the presence of the virus particles in the aerosolized droplets, the viability of these virus particles is still being debated since PCR (polymerase chain reaction), the technique used to detect viral particles confirms the presence of the viral nucleic acid which is only part of the virus while infectivity requires the intact virus particle. In addition to viability of the viral particles, sufficient viral dose is also an important determining factor for the occurrence of a viral infection. The probability of the presence of a considerable viral dose in dispersed aerosolized droplets has also been found to be quite low. In spite of the lack of conclusive evidence for airborne transmission of the SARS-CoV-2 virus, it is important to acknowledge the airborne transmission of the disease as a possible mode of transmission in addition to the well-established direct and indirect droplet modes of transmission. The crisis challenging the world in the wake of the COVID-19 pandemic requires immediate awareness and acknowledgement of any possible means of transmission of the disease to ensure effective containment of the spread of the disease.
While the risk of aerosols transmitting infections is definitely higher in healthcare set-ups due to certain aerosol-generating procedures (AGPs) such as suctioning or tracheal intubation, the risk of aerosols transmitting infectious respiratory diseases through regular activities such as laughing, talking loudly, singing, coughing, sneezing, or even breathing cannot be completely undermined. This is especially true of small, enclosed, and crowded spaces with poor ventilation, and has led to several super-spreading events across the world. Studies have highlighted the significance of air circulation with regular introduction of fresh air and good ventilation in reducing the presence of viral particles in the air. Therefore, ensuring proper ventilation (placement of supply and exhaust vents), avoiding recirculation of indoor air, use of air purifiers, and air sterilizers would definitely go a long way in reducing the spread of COVID-19 in indoor spaces.
Notably, however, the limitations of the airborne mode of transmission such as viability of the virus and dose of the virus can actually be used to our advantage. For example, masks could substantially reduce the volume of aerosols/droplets generated through regular activities such as talking, breathing, laughing etc. and although the smaller aerosolized particles travel longer distances on air currents, physical distancing could definitely ensure that the dose of the viral particles inhaled is very low as compared to that in direct person-to-person transmission.
The significance of the best practices and guidelines already in place for the prevention of COVID-19, therefore, remain un-impacted irrespective of the possibility of airborne transmission of the disease. The appropriate use of masks, good respiratory and hand hygiene, frequent disinfection and sanitization of frequently touched surfaces, maintaining a physical distance of at least 6 feet, and avoiding crowded and enclosed spaces continue to be indispensable cornerstone practices for preventing or at least reducing the spread of COVID-19.
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