A view of CDC′s Tom Harkin Global Communications Center, Atlanta, Georgia. Photo: CDC/Wikimedia Commons.
The US Centres for Disease Control and Prevention (CDC) said on September 21 that it had posted its guidance on possible transmission of the new coronavirus through airborne particles “in error” and that it will be updating its recommendations.
“A draft version of proposed changes to these recommendations was posted in error to the agency’s official website,” the CDC said. The organisation did not immediately respond to a request for comment from Reuters on when the guidance would be updated or to what effect.
The now-withdrawn guidance, posted on the agency’s website on September 18, had said the novel coronavirus is ferried through the air principally by smaller particles like aerosols. In doing so, it had (inadvertently or otherwise) implied that heavier fluid particles like droplets were a ‘secondary’ mode of transmission, so to speak.
This acknowledgment has important implications for policy. If, for example, aerosols are held to be the ‘primary’ mode, it would be unwise from a public health point of view to reopen businesses, schools and factories – as the Donald Trump administration in the US has been planning to.
This is because aerosols are less affected by gravity than droplets. As a result, the six-feet physical distancing guideline, drafted with droplets in mind, may prove ineffective because aerosols can stay afloat in the air for longer and are more easily propelled forward by gentle breezes.
In fact, without this new guidance in the picture, the CDC’s current position is that the novel coronavirus spreads from person to person mainly through respiratory droplets. These particles, given their relatively larger size, are more submissive to the force of gravity and settle down on surfaces in six feet or so after being exhaled, or could be inhaled by other people nearby.
As a result, if the droplet mode of transmission retains its central position in policymaking, then safety guidelines are expected to continue to emphasise the importance of physical distancing and disinfecting high-touch surfaces.
Indeed, R. Prasad of The Hindu speculated that since the primacy of aerosol transmission could throw a spanner in the Trump’s administration’s ‘unlock’ plans, the withdrawal could have been the result of political backlash as well.
“But with the Trump administration vetting papers before being published in the Morbidity and Mortality Weekly Report, and altering CDC testing guidelines for people who do not show symptoms,” Prasad wrote, “it is not clear if the quick removal of the revised guidance on virus spread was due to pressure from the Trump administration” (links added for reference).
Aside from its implications for public policy, the debate surrounding the relative likelihood of the two modes of transmission – aerosol v. droplet – has also been fuelled by the imperfect epidemiological definition of aerosols. That is, while epidemiologists understand droplets are larger and aerosols are smaller, there is no fixed line demarcating the two. Ronak Gupta, a student of fluid dynamics at the University fo British Columbia, Vancouver, wrote for The Wire Science in May:
The mode of droplet transmission is based on a classification system that has a cut-off between small and large droplets – derived largely from a 1930s’ study on tuberculosis. In this mode, a virus is transmitted either by direct contact with large droplets or indirectly by touching surfaces contaminated by such droplets. If a specific virus does spread this way, researchers invoke the system to suggest physical distancing measures, like the ‘six feet’ rule. Six feet is approximately how far large droplets can travel before falling down.
The WHO has said it is monitoring “emerging evidence” of possible airborne (i.e. aerosolised) transmission. The body has not changed its policy on this contested issue, an official said on September 21.
As The Wire Science has previously reported, “It took an unusually public outcry from over 200 experts for WHO to admit that aerosol transmission was even possible. One reason could be that [CDC and WHO] didn’t wish to create panic.”
But as the authors of this article – Asit Mishra and Amit Mishra – continued:
… the virus does not understand or follow our definitions. With reputed infectious disease experts like Dr Antony Fauci acknowledging the possibility of aerosolised transmission, we hope that progress towards a consensus will be quick. So while scientists work to determine the significance of aerosol transmission during the coronavirus pandemic, we must assume in the interest of safety that aerosol transmission is possible and viable.
With inputs from Reuters (reporting by Manas Mishra in Bengaluru; editing by Ramakrishnan M. And Shinjini Ganguli)