Dear Editor
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the aetiological
agent of coronavirus disease 2019 (COVID-19), has led to a global pandemic defying
the geographical borders and putting the lives of billions at risk. The commonly evident
symptoms include fever, altered sense of smell and/or taste, cough, sputum expectoration,
sore throat, dyspnoea, fatigue and myalgia; whereas the uncommon symptoms include
confusion, dizziness, headache, conjunctivitis, rhinorrhoea, nasal congestion, hemoptysis,
chest pain, bronchial breath sounds, tachypnoea, crackles/rales on auscultation, cutaneous
manifestations, cyanosis, and gastrointestinal symptoms. Throughout the world, mitigation
strategies have lingered on the adoption of social distancing, face masks, hand hygiene
and environmental disinfection. However, these precautionary measures are still not
completely reliable until the modes of transmission of the SARS-CoV-2 remain unraveled.
The transmission of respiratory pathogens have been associated with three primary
modes known as “contact,” “droplet,” and “airborne” transmission. These modes are
also being speculated in the context of SARS-CoV-2, but the existing research-based
literature and the consequent guidance from the leading public health agencies are
still paradoxical. The contact transmission can occur directly by physical touch or
indirectly via fomites containing settled droplets. The droplet transmission involves
large droplets more than 20 μm in diameter resulting from a violent expiratory event
and deposited upon the conjunctiva or mucus membranes of a susceptible host directly
[1] apart from being captured by inspiratory air flows and deposited along the respiratory
tract.
The airborne or aerosol transmission occurs via small respiratory droplets or droplet
nuclei, less than 10 μm in diameter, which remains airborne for sufficient time to
transmit the pathogen and may get deposited deep into the respiratory tract, including
alveolar region [1]. For the distinction of the transmission modes, droplet sizes
need to be carefully interpreted; however, the propositions on the size-related terms
are also paradoxical. An example of dichotomy is the postulation from the World Health
Organization that respiratory droplets are more than 5 to 10 μm in diameter, whereas
droplet nuclei are less than 5 μm in diameter. However, it is also being speculated
that the particles of various sizes but indistinct behavior are produced in continuum
during the respiratory activities of the infected person and particles as large as
50 μm can also remain airborne and travel the considerable distance as per the factors
including force and volume of exhalation, airflow, temperature and humidity [2].
Although the contact and droplet routes are still being advocated as the main modes
by leading public health agencies, including WHO, the airborne transmission had been
recognized only for aerosol-generating procedures within the healthcare settings.
But considering the emerging evidence of the presence of viable SARS-CoV-2 even in
the absence of aerosol-generating procedures, the airborne transmission should also
be recognized as an important mode of transmission of the SARS-CoV-2 [3]. Furthermore,
it should not be neglected that the aerosols are generated even from activities such
as exhalation, coughing, sneezing and talking by the infected individuals [2]. The
indoor, as well as outdoor airborne transmission, have been elucidated by various
empirical and laboratory studies conducted in countries including China, Italy, Singapore
and USA [4].
The median estimates of the half-lives of the SARS-CoV-2 and SARS-CoV in aerosols
are almost the same i.e. approximately 1.1–1.2 hours (3 hours viability), indicating
that both viruses have similar stability characteristics for the plausible aerosol
transmission and superspreading [5]. This time is enough for the exposure, inhalation
and infection by the virus, which may occur near or far from the actual source, even
beyond 1 to 2 m from an infected individual [6]. Recently, even the timeline of 16
hours has been reported for the virus to retain infectivity in laboratory-created
aerosols [6].
Considering the airborne transmission, precautions such as hand washing and social
distancing are appropriate but insufficient. As the viable SARS-CoV-2 has been isolated
in air samples even 2 to 4.8 m away from the source, social distancing by currently
recommended parameters of 6 feet would not be effective, especially in an indoor setting
[3]. Additional mitigation measures should include the provision of effective ventilation,
local exhausts, high-efficiency air filtration, germicidal ultraviolet light, avoidance
of overcrowding in public places [6], toilet flushing with a closed lid, minimal use
of central air conditioner, universal use of proper facial masks (N95 respirators
or surgical or cloth masks as per the availability) fitting tightly to the face apart
from proper personal protective equipment (PPE) including the N95 particle protective
mask in the laboratory and healthcare settings [4] (Figure 1
). From the scientific community throughout the globe, there is an urgent and critical
requirement of the research to unravel the paradox of SARS-CoV-2 transmission and
infectivity so that effective mitigation measures can be outlined. Whereas, the policymakers
and leading public health agencies need to upgrade the precautionary approach to interrupt
all the plausible modes of transmission until this uncertainty is deciphered.
Figure 1
Primary control measures to mitigate the transmission of SARS-CoV-2 as per the consideration
of aerosol transmission.
Figure 1
CRediT authorship contribution statement
Priyanka: Conceptualization, Writing - review & editing. Om Prakash Choudhary: Conceptualization,
Writing - review & editing. Indraj Singh: Writing - review & editing. Gautam Patra:
Writing - review & editing.
Declaration of Competing Interest
We declare that we have no competing interests.