COVID-19 Airborne Transmission: WHO To Study And Summarise Evidence

Scientists have challenged WHO's view that aerosol transmission of COVID-19 is only possible in healthcare settings

The World Health Organization (WHO) has acknowledged the emerging evidence of SARS-CoV-2 spreading through airborne transmission after a group of scientists wrote an open letter asking the organisation to update its guidelines to prevent airborne transmission

So far it has maintained that the virus that causes COVID-19 is majorly transmitted through droplets.

In a press briefing on Tuesday, July 7, Maria Van Kerkhove, technical lead on the Covid-19 pandemic at the WHO discussed the possibility of airborne transmission as a mode of transmission of COVID-19 "The possibility of airborne transmission in public settings - especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out," Kerkhove has said.

Currently, the WHO calls airborne transmission a rare occurrence. The WHO believes that SARS-CoV-2 primarily spreads through respiratory droplets that have a 5-10µm diameter and are released when a person is in close proximity (1m- 6 feet) of a person exhibiting COVID-19 symptoms. The droplets are released when the infected person is coughing or sneezing and last in the air for 10-15 minutes. A person can get infected if they inhale the same air in that short period.

This decision to summarise and scrutinise theories of the virus spreading through aerosols comes after 239 scientists from 32 countries wrote an open letter to the WHO asking it to heed the evidence that smaller particles can also transmit the virus in poorly ventilated spaces. The New York Times first discussed this letter which was published in the journal Clinical Infectious Diseases. All the scientists appealed to international bodies to identify the potential of the airborne spread of the disease. They have also asked for the updation of the current guidelines. This warrants that wearing masks would become a necessity even indoors in enclosed spaces.

Airborne Transmission

The emerging evidence that the scientists discuss highlights that the virus spreads through droplet nuclei which are smaller in size (< 5µm) and stays for longer in air. The virus can spread through aerosols if the person is in the vicinity of an already infected person who is not wearing a mask or following social distancing norms.

The WHO believes that the current evidence suggesting airborne transmission as a major mode of transmission is unconvincing. Clinical institutional procedures related to intubation and ventilation are the only areas believed to be sources of aerosol transmission by the WHO.

Studies conducted by some of the 239 signatories suggest otherwise. They say that aerosols released during normal exhalatation of air, talking to another person, contain microdroplets of the virus which could possibly travel further than the 1-2m prescribed for social distancing and stay in the air for longer.

One of the most widely referenced studies providing evidence about aerosol transmission is the cluster outbreak found in three non-associated families eating at a restaurant in Guangzhou, China. The ten infected patrons were sitting back to back and the researchers believe that the virus spread through aerosols due to the poor ventilation in the restaurant.

The scientists also commented on the WHO's heavy dependence on hand hygiene for infection control even after the USA Centres of Disease Control and Prevention revised its guidelines stating that it is easier for the virus to transmit through human contact than through surfaces.

If the WHO chooses to change its guidelines, it will have to focus on the ways to prevent transmission in indoor settings. Providing sufficient and effective ventilation (supplying clean outdoor air, minimizing recirculating air) in public buildings, workplace environments, schools, hospitals, and aged care homes,carrying out airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights, and avoidong overcrowding in closed spaces are some of the mitigation measures to curb the airborne transmission, according to the scientists.

This letter penned by Lidia Morawska, air quality expert from the Queensland University of Technology and Donald Milton, environmental health expert, University of Maryland is a call for the WHO to focus on airborne transmission as this could be one of the methods due to which the virus would proliferate and increase cases. All the signatories believe that the WHO needs to review its guidelines pertaining to airborne transmission.

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