Healthcare Hygiene magazine September 2020 September 2020 | Page 20

TRANSMISSION DYNAMICS AND COVID-19
Brown and
Mitchell (2020)
insist that their
model “will
reduce the
complexity of
some infection
prevention
measures,
which, under
the current
paradigm, are
connected
to specific
microorganisms’
transmission
routes and
must change
to address
new diseases
that workers
encounter.”
•
to workers’ job tasks, opportunistically transmissible
through others.”
What is intriguing about what Brown and Mitchell
(2020) propose is that the two-route options reject
the traditional size and distance limitations on particle
movement and re-allocate elements of the conventional
routes, rather than entire routes themselves,
into new categories.
As the experts explain, “When it comes to protecting
workers, saying that a disease is transmissible by a
certain route is an incomplete way of thinking – and
certainly not consistent with effective, exposure-focused
practices for controlling other occupational
hazards. Rather, we should think about tasks as
posing exposure hazard(s) and choose appropriate
controls to interrupt those mechanisms of exposure.
For occupational infection prevention, it makes
more sense to forego the conventional mechanisms
of transmission and instead think about exposures
falling into two re-defined categories: ‘contact,’
which generally accounts for mucocutaneous and
percutaneous exposures to body fluids and other
infectious materials and particles; and ‘aerosol,’ which
generally accounts for exposures to infectious particles
suspended in the air.”
The researchers emphasize that the concepts of
contact and aerosol exposure “need better definitions
than what the prominent infection prevention guidelines
and other literature have proposed. For contact
transmission, the HICPAC guideline’s definitions of
direct and indirect contact are acceptable as elements
of contact transmission, but are not fully sufficient
on their own.”
Rather than merely adopting the existing transmission
modes from the conventional framework
or joining the existing droplet and contact modes,
the model proposed by Brown and Mitchell (2020)
requires “parsing out parts of droplet transmission to
new contact and aerosol categories where they more
realistically fit. Conventional droplet transmission …
actually represents a form of contact transmission
… They continue to pose a contact exposure hazard
even after their trajectory directs them on to objects
or environmental surfaces, until they are no longer
viable (i.e. infectious) or unless they are aerosolized.”
The researchers also address aerosol exposure via
droplets that are suspended in the air and remain
for some time after their generation: “All particles
in air, regardless of their size, that are not causing
a contact exposure at any specific point in time are
at that point part of the aerosol exposure hazard.
They can potentially be breathed into a worker’s
respiratory system and, depending on particle size,
settle along the tract anywhere from the nasal mucosa
(larger particles) to much deeper in the lungs (smaller
particles). Because this description of aerosols is
broader, it allows for the elimination altogether of the
airborne transmission category, which conventionally
represents ‘dissemination of either airborne droplet
nuclei or small particles in the respirable size range
containing infectious agents that remain infective
over time and distance.’”
The researchers’ proposed model combines all
pathogenic particles that can be aerosolized, regardless
of the distance between the source and susceptible
individual, which, according to Brown and Mitchell
(2020), also eliminates uncertainties that entangle
existing definitions of droplet and airborne transmission.
They say their definitions of contact and aerosol categories
better align with the ways in which healthcare
personnel are exposed to pathogenic microorganisms.
Brown and Mitchell (2020) insist that their model “will
reduce the complexity of some infection prevention
measures, which, under the current paradigm, are
connected to specific microorganisms’ transmission
routes and must change to address new diseases that
workers encounter. Although changing controls to
align with evolving hazards is certainly appropriate,
having to do so based on exposure hazards associated
with job tasks and work environments is a simpler and
potentially more effective strategy.”
Facing the future, Dooley and Frieden (2020)
observe, “Although every healthcare facility must
implement the full hierarchy of source, environmental/
engineering, administrative, and PPE controls, the
scale of this epidemic necessitates thinking beyond
individual healthcare facilities. Especially in areas with
many cases, most persons with known or suspected
COVID-19 could ideally be channeled to designated
facilities. Infection control procedures would still be
needed in all facilities, but enhanced efforts could
concentrate on COVID-19-designated facilities,
and reusable PPE could be safely used, maintained,
and disinfected. Critical supplies, equipment, and
treatments could be allocated to designated facilities
more efficiently. This would require participation
by most or all hospitals in a geographic area, with
centralized coordination, but might ultimately reduce
this epidemic’s toll on patients, healthcare workers,
and society.”
References:
Brown CK and Mitchell AH. Realigning the conventional routes
of transmission: an improved model for occupational exposure
assessment and infection prevention. J Hosp Infect. Vol. 195, No.
1. Pp 17-23. May 2020. https://doi.org/10.1016/j.jhin.2020.03.011
Dooley SW and Frieden TR. Commentary: We Must Rigorously
Follow Basic Infection Control Procedures to Protect Our Healthcare
Workers from SARS-CoV-2. Infect Control Hosp Epidemiol. DOI:
10.1017/ice.2020.394
National Institute for Occupational Safety and Health (NIOSH).
Hierarchy of Control. Accessible at: https://www.cdc.gov/niosh/topics/
hierarchy/default.html
20 september 2020 • www.healthcarehygienemagazine.com