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COVID-19 updated guidance: CDC acknowledges airborne transmission

M3 India Newsdesk Oct 10, 2020

Now it is official. On 5th October 2020, the US Centers for Disease Control and Prevention (CDC), published an updated guidance in its “How COVID-19 spreads”- website, accepting that COVID-19 virus can be spread by airborne transmission. CDC recommends more study to find out the magnitude of airborne contribution.

For our comprehensive coverage and latest updates on COVID-19 click here.

The agency clarified-

“CDC continues to believe, based on current science that people are more likely to become infected the longer and closer they are to a person with COVID-19. Today’s update acknowledges the existence of some published reports showing limited, uncommon circumstances where people with COVID-19 infected others who were more than 6 feet away or shortly after the COVID-19-positive person left an area. In these instances, transmission occurred in poorly ventilated and enclosed spaces that often involved activities that caused heavier breathing, like singing or exercise. Such environments and activities may contribute to the build up of virus-carrying particles."

Available data indicate that it is much more common for the COVID-19 virus to spread through close contact with a person who has COVID-19 than through airborne transmission.

CDC noted that respiratory droplets can also land on surfaces and objects. It is possible that a person could get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or eyes. According to CDC, spread from touching surfaces is not thought to be a common way that COVID-19 spreads

CDC’s recommendations remain the same based on existing science and after a thorough technical review of the guidance.

People can protect themselves from COVID-19 virus by staying at least 6 feet away from others, wearing a mask that covers their nose and mouth, washing their hands frequently, cleaning touched surfaces often and staying home when sick.

The CDC admitted that airborne transmission can occur under special circumstances. It acknowledged that pathogens that are mainly transmitted through close contact (i.e., contact transmission and droplet transmission) can sometimes also be spread via airborne transmission under special circumstances.

There are several well-documented examples in which SARS-CoV-2 appears to have been transmitted over long distances or times.

In its SCIENTIFIC BRIEF: SARS-CoV-2 and Potential Airborne Transmission, published on the same day (5th October 2020), CDC stated thus (verbatim):

“The principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory droplets carrying infectious virus.

Respiratory droplets are produced during exhalation (e.g. breathing, speaking, singing, coughing, sneezing) and span a wide spectrum of sizes that may be divided into two basic categories based on how long they can remain suspended in the air:

  • Larger droplets some of which are visible and that fall out of the air rapidly within seconds to minutes while close to the source, and
  • Smaller droplets and particles (formed when small droplets dry very quickly in the airstream) that can remain suspended for many minutes to hours and travel far from the source on air currents."

Airborne transmission can occur under special circumstances

According to CDC, these airborne transmission events appear uncommon and have typically involved the presence of an infectious person producing respiratory droplets for an extended time (>30 minutes to multiple hours) in an enclosed space. Enough virus was present in the space to cause infections in people who were more than 6 feet away or who passed through that space soon after the infectious person had left.

Airborne transmission of the novel virus appears to have occurred in enclosed spaces within which an infectious person either exposed susceptible people at the same time or to which susceptible people were exposed shortly after the infectious person had left the space. It may occur due to prolonged exposure to respiratory particles, often generated with expiratory exertion (such as shouting, singing, exercising) that increased the concentration of suspended respiratory droplets in the air space. It may also occur due to inadequate ventilation or air handling that allowed a build-up of suspended small respiratory droplets and particles.

Controversy between some experts and the CDC

There was an unseemly controversy between some experts and the CDC on the role of aerosols in spreading the virus. Such finely articulated/orchestrated controversy may leave the common man confused and bewildered!

Scientific American (1 October 2020) aptly stated that in one corner, we have scientists, epidemiologists, infectious-disease physicians, clinicians, engineers—many different experts in the medical community, that is—arguing that the spread of COVID-19 by aerosols (that is, tiny droplets that can remain airborne long enough to travel significantly farther than the six-foot separation we’ve been told to observe) is both real and dangerous. In the other, it’s the Centers for Disease Control (CDC) and Prevention and the World Health Organization (WHO), which until very recently have allowed only that aerosol spread is possible, not necessarily likely.

Beginning of the dispute over airborne spread

The dispute over the importance of airborne viral transmission began a few months ago. On 1st April 2020, in response to Dr. Kelvin Droegemeier, Office of Science and Technology Policy, Executive Office of the President, Dr. Harvey V. Fineberg, Chair, Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats, The National Academies of Sciences, Engineering and Medicine wrote that currently available research supports the possibility that SARS-CoV-2 could be spread via bio-aerosols generated directly by patients’ exhalation. It was then a new discovery.

Dr. Fineberg clarified-

“One must be cautious in imputing the findings with one respiratory virus to another respiratory virus, as each virus may have its own effective infectious inoculum and distinct aerosolization characteristics. Studies that rely on PCR to detect the presence of viral RNA may not represent viable virus in sufficient amounts to produce infection. Nevertheless, the presence of viral RNA in air droplets and aerosols indicates the possibility of viral transmission via these routes.”

The specialists led by Dr. Fineberg concluded that while the current SARS-CoV-2- specific research is limited, the results of available studies are consistent with aerosolisation of virus from normal breathing.

One of the profoundly healthy, recent developments is that highly qualified specialists from many disciplines with commendable insights on the behaviour of aerosols entered the field. With their theoretical knowledge and access to technologies of indescribable sensitivity, they studied the microscopic aerosols that carry the virus and photographed them while in motion. The telling videos they made may convince anyone of the need to comply with various recommendations.

Open letter by specialists

In an unprecedented move, on 6 July 2020, two hundred and thirty nine scientist-signatories including many from environmental engineering in addition to epidemiology, infectious disease and public health from around the globe published an open letter to "the medical community and relevant national and international bodies,” in the journal, Clinical Infectious Diseases.

Quoting from several latest scientific papers, the scientists asserted thus (verbatim):

“Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in micro-droplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1–2 m from an infected individual (...). For example, at typical indoor air velocities [.], a 5-μm droplet will travel tens of meters, much greater than the scale of a typical room, while settling from a height of 1.5 m to the floor."

“Several retrospective studies conducted after the severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) epidemic demonstrated that airborne transmission was the most likely mechanism explaining the spatial pattern of infections [.]. Retrospective analysis has shown the same for severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) [....].”

The scientists pleaded with the agencies to implement the many measures they recommended to mitigate airborne transmission risk. They pointed out that the current guidance from numerous international and national bodies focuses on hand washing, maintaining social distancing, and droplet precautions.

“Most public health organisations, including the World Health Organization (WHO) [.], do not recognise airborne transmission except for aerosol-generating procedures performed in healthcare settings. Hand washing and social distancing are appropriate but, in our view, insufficient to provide protection from virus-carrying respiratory micro-droplets released into the air by infected people. This problem is especially acute in indoor or enclosed environments, particularly those that are crowded and have inadequate ventilation [.] relative to the number of occupants and extended exposure periods” they cautioned.

WHO on aerosol transmission

On 9 July 2020, three days after the scientists published their open letter, WHO stated thus, on aerosol transmission (verbatim):

“Some medical procedures can produce very small droplets (called aerosolised droplet nuclei or aerosols) that are able to stay suspended in the air for longer periods of time. When such medical procedures are conducted on people infected with COVID-19 in health facilities, these aerosols can contain the COVID-19 virus. These aerosols may potentially be inhaled by others if they are not wearing appropriate personal protective equipment. Therefore, it is essential that all health workers performing these medical procedures take specific airborne protection measures, including using appropriate personal protective equipment. Visitors should not be permitted in areas where such medical procedures are being performed.”

“There have been reported outbreaks of COVID-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing. In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out. More studies are urgently needed to investigate such instances and assess their significance for transmission of COVID-19.”

Close contact more important

Based on considerations of the epidemiology of SARS-CoV-2, CDC asserted that most infections are spread through close contact, not airborne transmission.

CDC found that diseases that are spread efficiently through airborne transmission tend to have high attack rates because they can quickly reach and infect many people shortly. The agency knows that a significant proportion of SARS-CoV-2 infections (estimated 40-45%) occur without symptoms and that infection can be spread by symptomless people.

CDC asserted-

“Thus, were SARS-CoV-2 spread primarily through airborne transmission like measles, experts would expect to have observed considerably more rapid global spread of infection in early 2020 and higher percentages of prior infection measured by sero-surveys. Available data indicate that SARS-CoV-2 has spread more like most other common respiratory viruses, primarily through respiratory droplet transmission within a short range (e.g., less than six feet). There is no evidence of efficient spread (i.e., routine, rapid spread) to people far away or who enter a space hours after an infectious person was there."

Prevention of COVID-19 by airborne transmission

CDC stated that the existing interventions to prevent the spread of SARS-CoV-2 appear sufficient to address transmission both through close contact and under the special circumstances favourable to potential airborne transmission.

“Among these interventions, which include social distancing, use of masks in the community, hand hygiene, and surface cleaning and, disinfection, ventilation and avoidance of crowded indoor spaces are especially relevant for enclosed spaces, where circumstances can increase the concentration of suspended small droplets and particles carrying infectious virus,” the agency clarified.

“At this time, there is no indication of a general community need to use special engineering controls, such as those required to protect against airborne transmission of infections, like measles or tuberculosis, in the healthcare setting,” CDC added.

CDC listed several critical questions that need to be answered to refine guidance for prevention of COVID-19.

“SARS-CoV-2 is a new virus, and we are still learning about how it behaves,” CDC concluded with characteristic humility.

What are the takeaways for the general public?

National agencies keep close watch on enforcing mitigatory measures in light of recent developments. They may relax or tighten mitigatory measures based on many considerations. Sadly, people tend to lower the bar every time authorities declare some relaxation or other. Strict compliance with safety measures at individual level is needed to vanquish the virus.


Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

Dr. K S Parthasarathy is a freelance science journalist and a former Secretary of the Atomic Energy Regulatory Board. 


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