COVID-19 Airborne Transmission

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TOPIC: General Studies 2

• Global crisis – COVID-19

In News: The World Health Organization has acknowledged that evidence is emerging about the airborne spread of the novel coronavirus. In the brief, the WHO has formally acknowledged the possibility that the novel coronavirus can remain in the air in crowded indoor spaces, where “short-range aerosol transmission… cannot be ruled out”.

The updated brief has come three days after a group of 239 scientists from 32 countries published a commentary titled ‘It is Time to Address Airborne Transmission of COVID-19’, in which they issued an “appeal to the medical community and to the relevant national and international bodies to recognize the potential for airborne spread of COVID-19”.

The Brief

Some outbreaks that have occurred in crowded indoor spaces suggest “the possibility of aerosol transmission”, although “combined with droplet transmission”. Such situations have arisen “during choir practice, in restaurants or in fitness classes”.

According to the WHO, “in these events, short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out”.

There is, however, a caveat — the WHO does not think that even in these situations, the virus was transmitted exclusively by the aerial route. The WHO brief still says that “the detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters”.

As per the WHO, airborne transmission may be possible in specific circumstances and settings. These include settings in which procedures that generate aerosols are performed; endotracheal intubation; bronchoscopy; open suctioning; administration of nebulised treatment; manual ventilation before intubation; turning a patient to the prone position; disconnecting a patient from the ventilator; non-invasive positive-pressure ventilation; tracheostomy; and cardiopulmonary resuscitation.

Evidence for aerosol transmission in research 

A. The scientists, led by Lidia Morawska of the International Laboratory for Air Quality and Health, WHO Collaborating Centre, Queensland University of Technology, Brisbane, Australia, appealed to “the medical community and the relevant national and international bodies to recognise the potential for airborne spread of COVID-19. There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several metres, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.

B. One of the first studies, published in Nature, was conducted in Renmin Hospital and Wuchang Fangcang Field Hospital in Wuhan. 

• It investigated the aerodynamic nature of the virus SARS-CoV-2 by measuring its viral RNA in aerosols. 
• This study found that the concentration of the virus in aerosols detected in isolation wards and ventilated patient rooms was “very low”, but it was “higher in the toilet areas used by the patients”.

C. In April, a correspondence published on NEJM by researchers from the US National Institute of Allergy and Infectious Diseases in the United States evaluated the stability of SARS-CoV-2 (and SARS-CoV-1, which causes SARS) in aerosols and on various surfaces.

• It found that SARS-CoV-2 “remained viable in aerosols” throughout the duration of the experiment that lasted for three hours.

What are aerosols? How different are they from respiratory droplets?

Aerosols are minute particles that are expelled under pressure, as in the case of fine mist from a jar of perfume, or a can of roach repellent. However, aerosol is a term used to broadly refer to particles suspended in the air; they could include fine dust, mist, or smoke. In the context of transmission of viruses, as in this case, aerosols are read as micro droplets, much smaller (5 microns or less) than respiratory droplets, and take a longer time to drop to the floor. They will be expelled by people breathing, laughing or singing, as against respiratory droplets that are expelled with forceful acts such as sneezing or coughing.

As they remain suspended in the air for longer, an individual who is COVID-19 positive is likely to infect people standing even at a distance of 1-2 m in a small, poorly ventilated room. This poses the risk that people sharing such environments can potentially inhale these viruses, resulting in infection and disease.

The Way Forward

Urgent high-quality research is needed to 

• Elucidate the relative importance of different transmission routes
• The role of airborne transmission in the absence of aerosol generating procedures
• The dose of virus required for transmission to occur
• The settings and risk factors for super spreading events
• The extent of asymptomatic and pre-symptomatic transmission

To avoid transmission apart from hand washing and physical distancing, one should avoid crowded places, close-contact settings and confined and enclosed spaces with poor ventilation, and wear fabric masks when in closed, overcrowded spaces to protect others; and ensure good environmental ventilation in all closed settings and appropriate environmental cleaning and disinfection. Providing sufficient and effective ventilation as far as possible in public buildings, schools and hospitals, avoiding overcrowding in public buildings and transportation systems are recommended, besides, supplementing general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.

Findings around the role of blood-borne transmission of SARS-COV-2 remain uncertain. Currently, there is no evidence for intrauterine transmission of SARS-CoV-2 from infected pregnant women to their fetuses, although data remain limited. Low viral titters in plasma and serum also suggested that risk of blood-borne transmission may be low.

Connecting the Dots:

1. Could Covid-19 virus be airborne? Discuss the impact it would have if this is true.

Note:

Fomite transmission refers to transmission through infected surfaces — such as doorknobs, elevator buttons, handrails, phones, switches, pens, keyboards and, if not disinfected, even a doctor’s stethoscope.

Superemitters: Aerosol emission during speech has been correlated with loudness of vocalization, and certain persons, who release an order of magnitude more particles than their peers, have been referred to as superemitters and have been hypothesized to contribute to superspeading events.

Respiratory Droplets: If the droplets particles are larger than 5-10 microns in diameter, they are referred to as respiratory droplets; if they are smaller than 5 microns in diameter, they are referred to as droplet nuclei.