In News

  • The World Health Organization has recently acknowledged the emerging evidence of the airborne spread of the novel coronavirus.
  • This comes after 239 scientists from 32 countries wrote an open letter to the World Health Organization (WHO), which stated that the virus causing Covid-19 can remain airborne for a period of time and thus transmit itself.
  • Scientists claim that historically, there has been an opposition in the medical profession to the notion of aerosol transmission, and the criteria for proof has been set very high, as that can lead to panic.

The transmission debate

  • A respiratory infection such as Covid-19 is transmitted through droplets of different sizes.
  • 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.
  • As per WHO, Covid-19 virus is primarily transmitted between people through respiratory droplets and contact routes.
  • In other words, the WHO’s viewpoint is that droplets containing the virus, produced during speech, coughing, sneezing etc, are larger than 5-10 microns in diameter and fall on the ground due to gravity after travelling less than 1 metre.
  • The 239 scientists, on the other hand, claim that the virus can be present in droplet nuclei (less than 5 microns in diameter) that do travel distances longer than 1 metre, and can remain in the air for a longer time.
  • Thus the scientists suggest that aerosol transmission too can happen. If this can be established, it will mean that the risk of transmission is higher than previously thought.

Settings where WHO acknowledges airborne transmission

  • Even the WHO has acknowledged in the past that airborne transmission may be possible in specific circumstances and settings.
  • These include settings in which procedures that generate aerosols are performed:
    • Endotracheal intubation (inserting a tube for ventilation)
    • Administration of nebulised treatment (treatment used for inhaling medicinal drugs)
    • Manual ventilation before intubation
    • Turning a patient to the prone position (chest down and back up)
    • Disconnecting a patient from the ventilator

Various studies studying aerosol transmission

Study published in the journal Nature:

  • One of the first studies, published in Nature, was conducted in Renmin Hospital in Wuhan. It investigated the aerodynamic nature (motion in air) of the virus SARS-CoV-2 by measuring its viral RNA in aerosols.
  • The 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. Further, levels of airborne viral RNA in most public areas was undetectable.
  • Although the study has not yet found the infectivity of the virus detected in these hospital areas, it states that SARS-CoV-2 may have the potential to be transmitted through aerosols.

Study published in The New England Journal of Medicine (NEJM):

  • A study published in 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 (capable of working) in aerosols throughout the duration of the experiment that lasted for three hours.
  • The results indicate that aerosol and fomite transmission of SARS-CoV-2 is possible since the virus can remain viable and infectious in aerosols for hours. Fomite transmission refers to the transmission of infectious diseases by objects.
  • However, WHO did not agree with the findings published in NEJM. It stated that the findings do not reflect a clinical setting in which aerosol-generating procedures are performed and that the study was based on experimentally generated aerosols.

Study by Centers for Disease Control and Prevention (CDC):

  • The US Centers for Disease Control and Prevention (CDC), has published a study titled “High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice”.
  • The researchers studied super-spreading events in the study. Super-spreading events occur when a single person infects a large number of people in one setting.
  • The study found that the 2.5-hour choir practice, which was attended by 61 persons and included a symptomatic patient, led to 32 confirmed and 20 probable secondary Covid-19 cases.
  • The study noted that the act of singing itself might have contributed to transmission through the emission of aerosols, which is affected by the loudness of vocalization.
  • It further stated, that certain persons, known as super-emitters, who release more aerosol particles during speech than others, might have contributed to the studied event and previously reported COVID-19 super spreading events.

Impact of aerosol transmission

  • Any change in the WHO’s assessment of risk of transmission could affect its current advice on keeping 1-metre (3.3 feet) of physical distancing.
  • Governments, which rely on the agency for guidance policy, may also have to adjust public health measures aimed at reducing the spread of the virus and a comprehensive package of interventions would be required to stop the transmission.
  • It would mean that wearing a mask will become more important than ever, more so, in certain settings where physical distancing is not possible.
  • It might be possible that N-95 masks, which are used by clinicians in hospital settings, could now be recommended to prevent aerosol transmission, subject to availability, and depending on the health condition of a person.

Way Ahead

  • WHO has stated that 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.
  • It further stated that initial findings need to be interpreted carefully. Researchers will look at the specific settings in which the virus was found airborne, the duration for which the virus was found staying in the air, and, most importantly, whether the virus continues to be infectious throughout this duration.
  • The WHO would also publish a scientific brief summarising the state of knowledge on modes of transmission of the virus in the coming days.