Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses.pdf

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Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses, a Systematic
Review
Tommaso Celeste Bulfone, MS,
a
Mohsen Malekinejad, MD, DrPh,
b
George W. Rutherford,
MD, AM,
b,c
Nooshin Razani MD, MPH
b,c,*
Francisco. 2121 Berkeley Way, Room 5302 Berkeley, CA 94720-7360, USA.
b. Department of Epidemiology and Biostatistics, University of California, San Francisco.
550 16th St 2nd floor, San Francisco, CA 94158, USA
c. Department of Pediatrics, University of California, San Francisco
Corresponding author: Nooshin Razani, MD 550 16th St 2nd floor, San Francisco, CA
94158,
nooshin.razani@ucsf.edu,
415-722-1915
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This systematic review found that while outdoor environments do seem at lower risk for
transmission of SARS-CoV-2 and other respiratory viruses than indoor environments, there are data
showing that infection transmission is possible outdoors, thus warranting further rigorous
investigation.
© The Author(s) 2020. Published by Oxford University Press for the Infectious Diseases Society of
America. All rights reserved. For permissions, e-mail: journals.permissions@oup.com.
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Summary:
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a. Joint Medical Program, University of California, Berkeley - University of California, San
Abstract
Background
While risk of outdoor transmission of respiratory viral infections is hypothesized to be low,
there is limited data of SARS-CoV-2 transmission in outdoor compared to indoor settings.
Methods
We conducted a systematic review of peer-reviewed papers indexed in PubMed, EMBASE
cases of human transmission of SARS-CoV-2. Reports of other respiratory virus transmission
were included for reference.
Results
Five identified studies found that a low proportion of reported global SARS-CoV-2 infections
have occurred outdoors (<10%) and the odds of indoor transmission was very high compared
to outdoors (18.7 times; 95% CI 6.0, 57.9). Five studies described influenza transmission
outdoors and two described adenovirus transmission outdoors. There was high heterogeneity
draw conclusions about outdoor transmission risks. In general, factors such as duration and
frequency of personal contact, lack of personal protective equipment and occasional indoor
infection.
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Conclusion
Existing evidence supports the wide-held belief that the the risk of SARS-CoV-2
transmission is lower outdoors but there are significant gaps in our understanding of specific
pathways.
Keywords:
coronaviruses, SARS-CoV-2, COVID-19, transmission, outdoor
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gathering during a largely outdoor experience were associated with outdoor reports of
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in study quality and individual definitions of outdoor settings which limited our ability to
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and Web of Science and pre-prints in Europe PMC through August 12
th
, 2020 that described
Background
Recommendations about methods to curb transmission of the severe acute respiratory
syndrome coronavirus type 2 (SARS-CoV-2) beyond wearing masks and maintaining social
distance have varied, especially regarding outdoor transmission.[1] This variability reflects a
general lack of information on how SARS-CoV-2 is transmitted outdoors.
Outdoor spaces generally allow for more physical distancing, which mitigates the risk of
virus transmission through larger respiratory droplets [2]. Outdoor spaces allow for airflow,
ventilation, and lack of recycled air, which all minimize the theoretical risk of aerosol
transmission through smaller respiratory droplets. While aerosol spread in community
settings is controversial, emerging data suggest that indoor recycled air can spread SARS-
CoV-2 — with examples of spreading events in a restaurant in Guangzhou [3], at an indoor
choir practice in Skagit, Washington, USA [4], at a South Korean call center [5], at meat-
ventilation, aerosolized droplets have the capacity to linger for longer before being inhaled or
falling to a surface, which could result in fomite transmission [8]. In enclosed environments,
viral particles [9]. Outdoor environments also generally have fewer high touch surfaces that
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may harbor the virus. UV light, present outdoors from sunlight, results in a ten-fold decrease
in virus survival on surfaces [10]. Finally, indoor environments may increase host
susceptibility; the low indoor humidity has been associated with slower host ciliary clearance
and complications such as pneumonia, and lack of sunlight has been associated with lower
vitamin D levels [11]. For these reasons, the risk of virus transmission in outdoor locations
has been hypothesized to be lower than in indoor spaces.
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low humidity, air conditioning, and low UV light may all contribute to longer survival of
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packing plants in the USA [6] and in a nursing home in the Netherlands [7]. In areas with low
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We sought to quantify the risk of SAR-CoV-2 transmission in outdoor settings. We
conducted a systematic review of the literature on transmission of SARS-CoV-2 to better
understand the risks of outdoor transmission. Where data was available, we estimated the risk
of outdoor compared to indoor transmission. Anticipating a paucity of data on SARS-CoV-2,
we chose a broad search strategy that included other human beta coronaviruses and
Methods
Search strategy and selection criteria
Data for this review were identified by searches of PubMed, EMBASE, Web of Science, as
well as preprints available in Europe PMC [12]. Details of our search strategies and eligibility
criteria can be found in our protocol published on August 3
rd
, 2020 on PROSPERO (ID:
data on SARS-CoV-2, the search was repeated to include most recent literature on August
12
th
, 2020.
Exposures and outcomes
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The exposure of interest - outdoor gatherings - was defined as persons congregating outdoors
for work, social or recreational activities (Supplementary Material 1 for our full search
strategy). The outcome of interest included cases of transmission of SARS-CoV-2 or other
respiratory viruses identified by a case report, illness, or mortality. We also included
secondary outcomes of clusters or outbreaks of cases. Our search included any viral infection
that can be spread by respiratory droplets and, in addition to SARS-CoV-2, included the other
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183826). The search was conducted on June 17
th
, 2020, and because of the rapidly expanding
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respiratory viruses.
two recognized human beta-human coronaviruses viruses (SARS-CoV-1 and Middle East
Respiratory Syndrome), human influenza viruses, adenoviruses, rhinoviruses, human
metapneumoviruses, and respiratory syncytial virus.
We included studies (experimental or observational with empirical data collection) that
described human-to-human transmission of respiratory viruses between humans in an outdoor
setting, any review of these studies, and any study (experimental or observational) that
We excluded reviews of previously published data, studies of exclusively indoor outbreaks,
outdoor outbreaks within animal populations or between animals and humans, and outbreaks
where the site of transmission was not listed or was unclear. We also excluded studies limited
to built environments (homes, apartment buildings, military barracks), hospitals, or forms of
transportation (airplanes, trains, buses, cars, ships).
After removing duplicate records, one author (TCB) reviewed all downloaded citations based
on their titles and pre-specified inclusion criteria. A second co-author (MM) reviewed a 5%
Two authors (TCB and NR) then independently screened the titles, abstracts and descriptor
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terms and compared and discussed discrepancies until consensus was reached; a third author
(MM) served as an arbiter when needed. Two authors (TCB and NR) then independently
inspected the full texts of the remaining studies for relevance based on exposure, design and
outcome measures to select the included papers, and discussed discrepancies until consensus
was reached with a third author (MM) serving as arbiter. We used Endnote X9.3.2 (Clarivate
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random sample of the excluded titles (rejected from initial search results) for quality control.
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Data Selection and Extraction
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compared respiratory viral transmission among humans in an outdoor versus indoor settings.

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