Bengaluru: In an open letter due to be published this week, 239 scientists from 32 countries have written to the World Health Organization asking for a revision of transmission safety recommendations and guidelines of Covid-19 claiming that recent evidence suggested it could be an airborne infection, according to The New York Times.
The letter is reportedly due to be published in the journal Clinical Infectious Diseases this week.
Airborne transmission, and more specifically aerosol transmission, implies that the virus lingers in the air, especially indoors, and can infect those nearby.
Recent evidence suggests that aerosol transmission of Covid-19 may pose a higher risk than was earlier believed. Experts caution that emerging evidence of transmission, especially from environments like those of meat processing plants, which have recently emerged as infection hotspots, suggests that aerosol transmission may be more common than we think.
Since the beginning of the outbreak, the WHO has maintained that the SARS-CoV-2 virus “is not airborne”.
The global health organisation has said that the virus spreads mainly through larger respiratory droplets and the risk of ‘aerosolization’ — where particles smaller than 5 micrometers are released — increases during certain medical procedures, but outside this context, it claimed the evidence was unclear.
Thus, it places the risk of transmission by aerosols only under certain conditions, such as inside operation theatres.
WHO’s most recent advisory on transmission safety was issued on 29 June, just three days after aerosol experts had flagged suspicions of aerosol transmission and stated that the research the WHO was using to stress on transmission through respiratory particles was outdated.
However, not all experts agree.
“We must wait for evidence to show airborne or aerosol viral transmission is common before guidelines should be revised,” S.K. Satheesh, meteorologist and Chair at Divecha Centre for Climate Change at the Indian Institute of Science (IISc) in Bengaluru, told ThePrint.
Airborne transmission and aerosolization
Whether there is airborne transmission and how much of it occurs, has been the subject of much debate. Definitions of what constitutes ‘airborne’ transmission vary widely and the term has been used interchangeably with aerosol transmission. However, there is a distinction between the two.
Airborne transmission occurs when suspended particles of varying sizes, including droplets, are dispersed by air. On the other hand, ‘aerosols’ are much smaller particles than cough droplets and can pass through most masks other than a secure N95 respirator class of masks.
The open letter flags potentially higher risk of aerosol transmission of Covid-19.
Emerging evidence from studying early outbreaks, super-spreader events, and local clusters have provided several insights into transmission routes.
In one of the most widely cited super spreader cases in China, an infected but then-asymptomatic individual managed to pass on the virus to nine others in a restaurant selectively, even though over 70 people had dined there. Retrospective studies showed that air currents from an indoor air conditioner carried the virus from the index patient to very specific people who were in the path of the air currents, but not those who were seated next to them.
Yet another super spreader event was the Washington choir group in the US. Analysis showed that one infected singer spread the virus to 32 others and potentially infected 20 more, from transmission that by aerosolization, which in turn occurred through the process of singing.
Many studies have also identified airborne transmission as the primary pathway of the spread of the virus. Airborne transmission includes transmission by both larger droplets as well as aerosols, and is well established in medical literature.
Droplets are thought to be anywhere between 5 and 1,000 micrometers in size, while aerosols are less than 5 micrometers. A micrometer is one-thousandth of a millimetre. Humans produce respiratory droplets that range from 0.1 micrometer to 1,000 micrometers in size. The SARS-CoV-2 virus is considered to be approximately 0.1 micrometer in size.
Larger particles are thought to spread for shorter distances and settle on surfaces, while smaller ones can disperse in the air outdoors and spread much more quickly indoors. It is now thought that the risk of airborne transmission is higher in an indoor setting, which can be mitigated to an extent by extended physical distancing and good ventilation.
Furthermore, studies suggest that risk of aerosolisation or atomisation occurs not only only during surgical procedures but also from just speech or exhalation. Other respiratory diseases causing viruses, like the influenza A virus, have exhibited evidence of transmission via aerosols.
In an article published in the journal Science in June, aerosol researchers alleged that WHO’s recommendations are based on research carried out in the 1930s, where the technology to study sub-micron aerosols were unavailable. The authors compared the spread of the virus to that of a cigarette smoke in indoor and outdoor settings, recommending indoor mask usage where necessary.
However, the WHO’s technical lead on infection control, Dr. Benedetta Allegranzi, disagreed with the need to revise safety guidelines, telling The New York Times that the evidence for airborne transmission is not substantive yet.
She clarified that WHO has stated that it’s possible but not yet supported by evidence.
Hitchhiking through the air
Viruses can also attach themselves to other aerosol particles, such as those found in polluted areas, to travel even farther. This increases their dispersion, as they move with the particles in the air. This has been demonstrated in some studies, which have found that people in higher areas of pollution tend to be more severely affected by Covid-19.
“Virus transmission by sticking to airborne particles is well known since the 1950s,” said Satheesh. “This is especially true for particles near the coast which tend to be moist. Most particles over land are from pollution and tend to be dry, so we don’t yet know how effective the virus is in latching on to them and dispersing, and how long the virus can survive on these particles.”
Residence time of the SARS-CoV-2, or how long the virus can remain suspended in the atmosphere, is currently unknown. Since wind currents disperse particles of all sizes, outdoor locations are considered to be much safer.
A revision of guidelines would imply changes in masking policy around the world. Even simple cloth masks can provide protection against droplet transmission, especially from an infected person, but only masks that can filter out sub-micron particles, like the N95 respirator, will be effective against aerosolized transmission. It might also lead to stricter physical distancing measures, especially indoors.
“Aerosol transmission cannot be ruled out,” Satheesh said, while also emphasising that the evidence is still not substantive enough to revise guidelines yet.