Healthcare Hygiene magazine September 2020 September 2020 | Page 18

TRANSMISSION DYNAMICS AND COVID-19
Transmissionbased
precautions,
as described in
the Healthcare
Infection Control
Practices
Advisory
Committee
(HICPAC)
isolation
guideline, aim
to interrupt
a pathogenic
microorganism’s
route(s) of
transmission
that may not
be completely
stopped using
standard
precautions
alone.
•
that UPPs will be accepted by patients and staff and
that it will ultimately result in a safer healthcare
environment for all.”
Providing additional food for thought, Brown
and Mitchell (2020) reviewed current recommended
infection prevention and control practices and offer
what they say is a framework for better protection and
controls from an occupational health point of view.
Their model outlines two exposure routes – contact
and aerosol – resulting from work activities and
environments, shifting the focus away from particular
pathogenic microorganisms’ typical methods for
spreading to patients or to other non-workers in
hospital and community settings.
The authors observe that “… infection prevention
measures are typically based on what we know
about patient and/or public exposures to infectious
diseases in hospital and community settings, respectively.
Control of other occupational hazards typically
follows the industrial hygiene hierarchy of controls
to eliminate or mitigate exposures at their source,
based largely on how workers’ job tasks contribute to
those exposures, before relying on other safeguards.
By contrast, infection prevention approaches often
focus on protecting susceptible individuals based on
a pathogenic micro-organism’s typical mechanism
of spread under hospital or community conditions.”
For decades, IP&C programs have followed
standard and transmission-based precautions to avoid
occupationally acquired infections (OAIs). As Brown
and Mitchell (2020) state, “Standard precautions,
which expand universal precautions indicated in
the Occupational Safety and Health Administration
(OSHA) Bloodborne Pathogens standard, are
designed to prevent not only OAIs among workers,
but also healthcare-associated infections (HAIs)
among patients. Transmission-based precautions, as
described in the Healthcare Infection Control Practices
Advisory Committee (HICPAC) isolation guideline, aim
to interrupt a pathogenic microorganism’s route(s) of
transmission that may not be completely stopped using
standard precautions alone. Like standard precautions,
transmission-based precautions are designed to prevent
HAIs, and they attempt to balance worker protections
with their possible adverse effects on patients (e.g.
anxiety, perceptions of stigma). Because of differences
both in how patients and workers are exposed to
pathogenic micro-organisms and in how they should
be protected, the conventional paradigm of contact,
droplet, and airborne transmission that is typically
applied to characterizing and preventing OAIs is not
optimal for addressing worker exposure hazards.”
As we know, conventional routes of transmission
categorize pathogen spread via contact, droplet, and
airborne transmission and as Brown and Mitchell
(2020) note, they “comprise the prevailing paradigm
are used almost universally to describe transmission
in all settings.” From their review of the literature,
the authors say there is “marked inconsistency with
which scientists, infection preventionists, clinicians, and
others view transmission mechanisms. Transmission
terminology is often used incorrectly and inconsistently,
with the contact, droplet, and airborne labels – and
sometimes a loosely defined ‘aerosol’ term – frequently
applied to similar scenarios. This suggests that the
definitions for the categories, if not the mechanisms
themselves, are perhaps not completely understood,
not universally accepted, or some combination of
those things.”
Brown and Mitchell (2020) say that droplet
and airborne transmission are “perhaps the most
controversial and easily conflated routes.” They
explain, “There is significant debate about how
to define them, including whether they should be
separate or combined, or whether they accurately
represent infectious particle movement at all. Most
notable are challenges to the commonly accepted
delineation of these two categories, which argue that
typical definitions impose an artificial dichotomy of
large (>5 μm) droplets versus small (≤5 μm) airborne
particles. This breakdown dates back 60 to 80 years to
studies suggesting that larger droplets may come into
contact with exposed mucosa of very nearby (<1 m)
individuals upon their initial generation or expulsion
(i.e. from an infectious patient), but generally settle
out of the air quickly without being inhaled into a
susceptible individual’s respiratory tract. By contrast,
smaller particles were thought to travel greater
distances from their sources and cause infection
upon being inhaled deeper into the lungs. Whereas
some of this may hold true, newer evidence suggests
that sources of what would typically be identified as
droplets and airborne particles may actually generate
particles of a range of sizes that can travel a range
of distances and infect susceptible individuals both
directly (through deposition on exposed mucosa,
non-intact skin, and at points along the respiratory
tract) and indirectly (through environmental surface
or fomite contamination).”
The authors point to newer aerosol science that
supports a more modern view of droplet and airborne
transmission, and take into consideration whether
routes should be regarded as mutually exclusive,
and if certain microorganisms must reach particular
sites within a susceptible host’s body in order to
cause infection. As they explain, “Older studies
often assumed a single mode of transmission for
a disease, but contemporary literature has trended
toward the idea that ascribing single routes may not
accurately characterize disease spread, including
during significant epidemics. For many pathogens,
‘transmission is not fully elucidated’ and ‘the relative
contribution of each mode may not be precisely
quantified.’ At a minimum, we must recognize that
many of the pathogens of concern for workers are
unlikely to be obligate spreaders via any single route
in an occupational context. They may be preferentially
spread via a certain route and, at least in part due
18 september 2020 • www.healthcarehygienemagazine.com