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Vol 66: JULY | JUILLET 2020 | Canadian Family Physician | Le Médecin de famille canadien
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Editor’s key points
There is growing advocacy for the
use of masks in the community
to prevent transmission of viral
respiratory infections. This
systematic review found limited
evidence that the use of masks might
prevent viral respiratory infections.
The use of masks by a group in
the community setting appears to
reduce influenzalike illness in those
wearing masks. The pooled analysis
showed a significant risk reduction
(number needed to treat [NNT] = 24).
Using masks within a family 1 to 3
days after someone has developed
symptoms of a viral respiratory
infection does not appear to
prevent transmission to family
members, no matter if the masks
are used by the sick individual, the
healthy family members, or both.
Surgical masks are likely superior
to cloth masks for preventing
influenzalike illness in health care
workers (NNT = 50) but the results
are drawn from a single trial. N95
masks are likely superior to surgical
masks for preventing influenzalike
illness (NNT = 100) and clinical
respiratory infections (NNT = 40) in
health care workers.
Masks for prevention of
viral respiratory infections
among health care workers
and the public
PEER umbrella systematic review
Nicolas Dugré PharmD MSc Joey Ton PharmD Danielle Perry RN
Scott Garrison MD PhD CCFP Jamie Falk PharmD James McCormack PharmD
Samantha Moe PharmD Christina S. Korownyk MD CCFP
Adrienne J. Lindblad ACPR PharmD Michael R. Kolber MD CCFP MSc
Betsy Thomas BScPharm Anthony Train MB ChB MSc CCFP G. Michael Allan MD CCFP
Abstract
Objective To determine the effect of mask use on viral respiratory infection risk.
Data sources MEDLINE and the Cochrane Library.
Study selection Randomized controlled trials (RCTs) included in at least 1 published
systematic review comparing the use of masks with a control group, either in
community or health care settings, on the risk of viral respiratory infections.
Synthesis In total, 11 systematic reviews were included and 18 RCTs of 26 444
participants were found, 12 in the community and 6 in health care workers.
Included studies had limitations and were deemed at high risk of bias. Overall,
the use of masks in the community did not reduce the risk of influenza,
confirmed viral respiratory infection, influenzalike illness, or any clinical
respiratory infection. However, in the 2 trials that most closely aligned with
mask use in real-life community settings, there was a significant risk reduction
in influenzalike illness (risk ratio [RR] = 0.83; 95% CI 0.69 to 0.99). The use of
masks in households with a sick contact was not associated with a significant
infection risk reduction in any analysis, no matter if masks were used by the
sick individual, the healthy family members, or both. In health care workers,
surgical masks were superior to cloth masks for preventing influenzalike illness
(RR = 0.12; 95% CI 0.02 to 0.98), and N95 masks were likely superior to surgical
masks for preventing influenzalike illness (RR = 0.78; 95% CI 0.61 to 1.00) and any
clinical respiratory infections (RR = 0.95; 95% CI 0.90 to 1.00).
Conclusion This systematic review found limited evidence that the use of
masks might reduce the risk of viral respiratory infections. In the community
setting, a possible reduced risk of influenzalike illness was found among mask
users. In health care workers, the results show no difference between N95
masks and surgical masks on the risk of confirmed influenza or other confirmed
viral respiratory infections, although possible benefits from N95 masks were
found for preventing influenzalike illness or other clinical respiratory infections.
Surgical masks might be superior to cloth masks but data are limited to 1 trial.
510 Canadian Family Physician | Le Médecin de famille canadien Vol 66: JULY | JUILLET 2020
RECHERCHE
Points de repère
du rédacteur
Le port du masque est de plus
en plus préconisé pour prévenir
la transmission des infections
respiratoires virales. Cette revue
systématique a trouvé un nombre
limité de données pour soutenir
que l’utilisation d’un masque
est susceptible de prévenir les
infections respiratoires virales.
Le port du masque par un groupe
dans un milieu communautaire
semble réduire les affections
pseudo-grippales chez les
personnes qui portent le masque.
L’analyse regroupée a démontré
une réduction significative du
risque (nombre de sujets à traiter
[NST] = 24). Porter un masque au
sein d’une famille de 1 à 3 jours
après que l’un des membres a
développé des symptômes d’une
infection respiratoire virale ne
semble pas prévenir la transmission
aux autres membres de la famille,
peu importe si le masque est porté
par la personne malade, par les
membres en santé de la famille ou
par tous.
Les masques chirurgicaux sont
probablement supérieurs aux
masques en tissu pour prévenir les
affections pseudo-grippales chez
les travailleurs de la santé
(NST = 50), mais ces résultats
proviennent d’un seul essai. Les
masques N95 sont probablement
supérieurs aux masques
chirurgicaux pour prévenir les
affections pseudo-grippales
(NST = 100) et les infections
respiratoires cliniques (NST = 40)
chez les travailleurs de la santé.
Les masques pour prévenir les
infections respiratoires virales
chez les travailleurs de la
santé et la population
Revue-cadre systématique du groupe PEER
Nicolas Dugré PharmD MSc Joey Ton PharmD Danielle Perry RN
Scott Garrison MD PhD CCFP Jamie Falk PharmD James McCormack PharmD
Samantha Moe PharmD Christina S. Korownyk MD CCFP
Adrienne J. Lindblad ACPR PharmD Michael R. Kolber MD CCFP MSc
Betsy Thomas BScPharm Anthony Train MB ChB MSc CCFP G. Michael Allan MD CCFP
Résumé
Objectif Déterminer les effets du port du masque sur le risque d’infections
respiratoires virales.
Sources des données MEDLINE et la Bibliothèque Cochrane.
Sélection des études Les essais contrôlés randomisés (ECR) inclus dans au moins 1
revue systématique publiée comparant le port du masque avec cette pratique dans
un groupe témoin, soit en milieu communautaire ou en milieu de soins de santé,
portant sur le risque d’infections respiratoires virales.
Synthèse Au total, 11 revues systématiques ont été incluses, et 18 ECR auprès de
26 444 participants ont été recensés, 12 dans la communauté et 6 chez des travailleurs
de la santé. Les études retenues comportaient certaines limites et étaient jugées à
risque élevé de biais. Dans l’ensemble, le port du masque dans la communauté n’a pas
réduit le risque de grippe, d’infections respiratoires virales confirmées, d’affections
pseudo-grippales ou de toute autre infection respiratoire clinique. Toutefois, dans 2
essais qui concordaient le plus étroitement avec le port du masque dans des milieux
communautaires de la vie réelle, il s’est produit une réduction significative du risque
d’affections pseudo-grippales (risque relatif [RR] = 0,83; IC à 95 de 0,69 à 0,99). Dans
les analyses, le port du masque dans les familles en contact avec un membre malade
n’était pas associé à une réduction significative du risque d’infection, que le masque
soit utilisé ou non par le mlde, pr les membres en snté de l fmille ou pr tous.
Chez les trvilleurs de la santé, les masques chirurgicaux étaient supérieurs aux
masques en tissu pour prévenir les affections pseudo-grippales (RR = 0,12; IC à 95 %
de 0,02 à 0,98), et les masques N95 étaient probablement supérieurs aux masques
chirurgicaux pour prévenir les affections pseudo-grippales (RR = 0,78; IC à 95 % de 0,61 à
1,00) et les autres infections respiratoires cliniques (RR = 0,95; IC à 95 % de 0,90 à 1,00).
Conclusion Cette revue systématique a dégagé des données probantes limitées selon
lesquelles le port du masque pourrait réduire le risque d’infections respiratoires
virales. Dans la communauté, une réduction possible du risque d’affections pseudo-
grippales a été observée chez les porteurs de masque. Chez les travailleurs de la santé,
les résultats n’ont démontré aucune différence entre les masques N95 et les masques
chirurgicaux quant au risque de grippe confirmée ou d’autres infections respiratoires
virales confirmées, quoique des bienfaits possibles puissent être attribués aux
masques N95 pour prévenir les affections pseudo-grippales ou d’autres infections
respiratoires cliniques. Les masques chirurgicaux pourraient être supérieurs aux
masques en tissu, mais les données proviennent de 1 seul essai.
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In the management of infectious disease, prevention
is clearly preferred to treatment. For viral respira-
tory infections, the list of preventive options includes
vaccines, physical distancing, isolation (of those who
are sick), quarantine (of those who are exposed), hand
hygiene, masks, and a host of other interventions. For
health care workers, masks are one part of personal
protective equipment (PPE), but the amount of PPE var-
ies based on the clinical environment, current risk level,
and local directives. As the coronavirus disease 2019
(COVID-19) pandemic continues to spread, so has advo-
cacy for public mask use, with rationale based on simple
precautionary principles and the potential for benefit
over harm.1,2
In observational studies, wearing masks is associated
with a lower risk of contracting viral respiratory infec-
tions.3,4 For example, in a 2011 Cochrane systematic
review published by Jefferson et al, the use of masks in
case-control studies was associated with an important
risk reduction (odds ratio = 0.32; 95% CI 0.26 to 0.39).3
However, the observational design is at high risk of con-
founding and mask use might simply be a surrogate
measure for comparing more careful versus less careful
people. Experimental laboratory-based studies of masks
and mask types seem to provide promising and impor-
tant information, but translation into meaningful clinical
differences is often lacking.5 To reduce confounding and
determine the true effects of masks on infection preven-
tion, randomized controlled trial (RCT) data are required.
While there are a number of recent systematic reviews
of RCTs, some meta-analyzed studies had differing
designs or settings, which led to increased heterogene-
ity, while others focused on a very specific question.6-9
Some meta-analyses also did not appear to account for
cluster-randomized designs.4,6,8
Trials are under way to determine if masks can
reduce the spread of COVID-19 (ClinicalTrials.gov:
NCT04296643 and NCT04337541); however, none were
published at the time of writing.10 This systematic review
examines if masks can reduce the risk of viral respira-
tory infections in members of the public or in health care
workers. In addition, this review will examine if the type
of mask influences the risk of viral respiratory infections.
—— Methods ——
We followed PRISMA (Preferred Reporting Items for
Systematic Reviews and Meta-Analyses) guidelines
for completion of this systematic review.11 Our search
was modified to improve efficiency, similar to our previ-
ous systematic reviews.12,13
Search
Two team members (J.T. and D.P.) performed a search
of MEDLINE via Ovid from inception to May 5, 2020.
This search was limited to systematic reviews and used
both key and MeSH terms related to masks and infec-
tious disease transmission. In addition to MEDLINE,
the search was carried out on the same date in the
Cochrane Library, using the same terminology and lim-
ited to Cochrane systematic reviews. A search for RCTs
published in MEDLINE or added to the MedRxiv preprint
database from January 1, 2020, to May 5, 2020, was also
performed to identify any new trials not captured by the
included systematic reviews. Full details of the search
strategy, including MeSH terms, are in Appendix 1, avail-
able from CFPlus.*
Study selection
For added efficiency, our modified approach involved
identifying systematic reviews of studies examining the
use of masks for the prevention of viral respiratory infec-
tions. Systematic reviews were included if they were
published in English and they reported at least 1 RCT
comparing any mask use, either alone or in combination
with other interventions, with a control group. Systematic
reviews were reviewed by 2 team members and disagree-
ments were resolved by consensus and consultation with
a third author when necessary. Once all relevant system-
atic reviews were located, RCTs from each were reviewed
and included if they studied mask use for the prevention
of viral respiratory infections, either in health care work-
ers or in people in the community. The same inclusion
criteria were applied to individual RCTs found in the addi-
tional search for RCTs that were published in 2020. We
excluded observational studies and laboratory or surro-
gate experimental studies.
Data extraction
Data extraction was performed by a single reviewer,
with a second author reviewing for accuracy. Extracted
data included country, setting, population enrolled,
population details (eg, age, sex), cluster details when
appropriate (eg, universities, schools, tents), number of
participants randomized, number of participants ana-
lyzed, duration of study, types of masks used, who was
directed to wear masks, direction on when to wear
masks (eg, all of the time, 5 hours per day), adherence to
mask use, and data on 4 outcomes (confirmed influenza,
any confirmed viral respiratory infection, influenzalike
illness, and any clinical respiratory infection).
Quality assessment
Risk of bias for each RCT was assessed by 2 reviewers
(N.D. and G.M.A.) using the Cochrane Collaboration risk-
of-bias assessment tool.14
*The full PRISMA flow diagram (Appendix ); forest plots for all
analyses (Appendix ); all original trial data, cluster sizes, intracluster
correlation coefficients, adjusted events, and sample sizes (Appendix
); and Table and Figure are available at www.cfp.ca. Go to the
full text of the article online and click on the CFPlus tab.
512 Canadian Family Physician | Le Médecin de famille canadien Vol 66: JULY | JUILLET 2020
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Analysis
We focused on 4 primary end points: confirmed influenza,
confirmed viral respiratory tract infection, influenzalike
illness (defined by RCT authors), and any clinical respira-
tory tract infection. For all RCTs, we extracted the num-
ber of events for each outcome when available and the
number of participants analyzed in both the interven-
tion group and the control group. In studies where the
participants were randomized individually, we applied
those values directly into our analysis. For cluster RCTs,
we adjusted those values to account for clustering by
dividing the numbers by the design effect, with the
design effect calculated using the following equation:
design effect = 1 + ([cluster size - 1] × ICC), where ICC is
the intracluster correlation coefficient.15 Cluster size was
calculated by dividing the number of patients by the
number of clusters. The ICC was identified in the trials
whenever possible: first, if the authors provided an ICC
they calculated for a specific outcome; or second, if the
authors provided an expected ICC when determining
sample size. If the trial did not provide an ICC, we used
the ICC estimate from a similar study. This approach
allowed all results to be analyzed in a similar fashion.
Meta-analyses were performed to calculate risk ratios
(RRs) by pooling adjusted events and numbers. When a
study had multiple arms used in the same analysis, we
divided the total number of participants in the control
group by the number of intervention groups to avoid
counting participants more than once. Subgroup analyses
were performed based on setting (community or clinical
setting), mask type, control group, and who wore masks
(eg, only sick people, only healthy people). Because simi-
lar designs and settings were pooled, fixed-effects models
were used. We also performed random-effects sensitivity
analyses in the comparisons of N95 and surgical masks,
as different approaches to wearing N95 masks were used
(such as fit-tested versus non–fit-tested masks, or masks
worn only for higher-risk clinical scenarios versus worn
all day). Absolute risks of events and numbers needed
to treat (NNTs) were calculated by pooling the unad-
justed event rates in the control groups (baseline risk)
and applying cluster-adjusted, meta-analyzed, relative-
effects estimates to attain absolute benefits.
—— Synthesis ——
The search found 544 publications, out of which 11 relevant
systematic reviews were identified. Three of these system-
atic reviews were published in 2020. Six systematic reviews
focused on the effect of masks on influenza incidence.
From these 11 systematic reviews, 18 unique RCTs
were identified, including a total of 26 444 participants.
No additional RCTs published in 2020 were found. The
full PRISMA flow diagram is available in Appendix 1.*
All 18 RCTs involved using masks to prevent the spread
of viral respiratory infections and were broken into
2 primary groups: community use (n = 12) and use by
health care workers (n = 6). Details of the RCTs con-
ducted in community settings are found in Table 1,16-27
available from CFPlus.* Details of the RCTs conducted
in health care settings are found in Table 2.28-33 All trials
were deemed at high risk of bias. Risk-of-bias assess-
ment for each RCT is available in Appendix 1.*
All primary results for both settings are available in
Table 3. Forest plots for all analyses are available
in Appendix 2 from CFPlus.* All original trial data, clus-
ter sizes, intracluster correlation coefficients, adjusted
events, and sample sizes are provided in Appendix 3
from CFPlus.*
Community setting
All 12 community trials were cluster RCTs. Nine of these
12 community RCTS involved an index case. In 7 of
those, the index case was identified after receiving a
diagnosis of influenza or influenzalike illness by a health
care professional.19,21-25,27 The patient’s family was then
subsequently enrolled in the trial. In the intervention
arms, mask use could be recommended for everyone,
just the sick person, just the healthy family members
at home, or both. In 1 RCT conducted during the Hajj
in Saudi Arabia, index cases were pilgrims presenting
with influenzalike illness and the enrolled contacts were
the individuals sleeping within 2 metres of an index
case in the accommodation tents.26 In the intervention
arm, both index cases and contacts had to wear masks.
In 1 trial, masks were given to 509 households in New
York, NY, and participants were told to start using masks
if 1 household member developed influenzalike illness
(masks for the ill person and the caretaker).20 In the
3 remaining trials, masks were used in a prespecified
healthy population group, either American university
students randomized by residence hall or Australian Hajj
pilgrims randomized by accommodation tent.16-18
The use of masks in community settings in general
did not reduce the risk of confirmed influenza (RR = 0.97;
95% CI 0.75 to 1.25; I2 = 0%) or confirmed viral respiratory
infection (RR = 1.28; 95% CI 0.87 to 1.89; I2 = 0%). Results
were not statistically significant in any subgroup analy-
sis (masks worn by all, just the sick person, or just the
healthy family members at home). The use of masks in
community settings did not result in a significant risk
reduction of influenzalike illness (RR = 0.91; 95% CI 0.80 to
1.03; I2 = 0%) or any clinical respiratory infection (RR = 1.06;
95% CI 0.82 to 1.36; I2 = 0%). However, for influenzalike ill-
ness, the use of masks by everyone for 6 weeks during
influenza season in 2 RCTs conducted in an American
university appeared to reduce risk (RR = 0.83; 95% CI 0.69
to 0.99; I2 = 0%) (Figure 1).16,17 From the unadjusted num-
bers in the trials, the pooled control event rate (risk of
influenzalike illness) was 24.7% over 6 weeks. Applying
the cluster-adjusted RR, mask use would reduce this to
20.5%, a 4.2% absolute risk reduction or an NNT of 24.
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Table 2. Study characteristics of mask use in health care workers to prevent viral respiratory tract infections
RCT (CLUSTER:
YES OR NO) COUNTRY,
SETTING POPULATION
(AGE)
SAMPLE SIZE
ENROLLED
(ANALYZED)
CLUSTERS
(RANDOM-
IZED) INTERVENTIONS
WHO
WORE
THE
MASKS MASK USE
RECOMMENDATION ADHERENCE
Jacobs et al,
(no) Japan,
tertiary
care
hospital
Health care
workers
(mean
.y)
() NA Mask (), no
mask () Health
care
workers
(not sick)
Health care
workers wear
masks while on
hospital
property and
performing
their roles
. of
participants self-
reported “full
compliance,”
with the
remaining
complying
to of the
time (applies to
both mask use
and nonuse)
Loeb et al,
(no) Canada,
tertiary
hospitals
Nurses
(mean y) () NA Surgical masks
(), N
masks ()
Nurses When caring for
patients with
febrile
respiratory
illness
All participants
allocated to
surgical masks
wore them when
caring for patients
admitted to unit
in droplet
precautions for
influenza
MacIntyre et
al,
(yes)
China,
hospital
(ED and
respiratory
wards)
Nurses,
doctors,
ward clerks
(mean y)
() Cluster: unit
of random-
ization was
hospital (
hospitals
involved,
per study
arm)
Surgical masks
(), N fit-
tested masks
(), N
masks not fit-
tested ()
Health
care
workers
Every shift
(given
surgical masks
daily or N
masks daily)
Worn on
working days:
surgical,
(h/day); N fit,
(. h/day);
N no fit,
(. h/day)
MacIntyre et
al,
(yes)
China,
hospitals
(ED and
respiratory
wards)
Nurses,
doctors
(mean
.y)
Surgical
mask
(),
targeted use
of N
(),
N
(),
total
()
wards at
sites Surgical masks
(), targeted
use of N
masks (),
N masks ()
Nurses,
doctors All the time
(surgical
masks), as
needed
(targeted N
masks), all the
time (N
masks)
for surgical
masks, for
N targeted
masks, for
N masks
MacIntyre et
al,
(yes)
Vietnam,
hospitals
(ED, ICU, ID
or
respiratory
wards,
pediatric
ward)
Nurse or
doctor
(mean y)
() wards at
sites Surgical masks
(), cloth
masks (-layer
cotton) (),
control ()—
“standard
practice” of
mask use
Nurses
or
doctors
Surgical masks:
all the time on
shift; cloth
masks: all the
time on shift;
control:
standard
practice
Surgical masks
., cloth
masks .,
standard
practice .
Radonovich
et al,
(yes)
US,
outpatient
sites
(clinics,
primary
care clinics,
EDs)
Health care
personnel
(mean y)
health
care
personnel
seasons
participants
(
seasons)
clusters
at sites Surgical masks
( person-
seasons), N
masks (
person-
seasons)—note
that person
could be in
different arms
each of the
seasons
Those
involved
in direct
patient
care
Whenever
positioned
within ft of a
patient with
suspected or
confirmed
respiratory
illness
N: .
always, .
sometimes;
surgical: .
always, .
sometimes
ED—emergency department, ICU—intensive care unit, ID—infectious disease, NA—not applicable, RCT—randomized controlled trial, US—United States.
514 Canadian Family Physician | Le Médecin de famille canadien Vol 66: JULY | JUILLET 2020
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Note that the definition of influenzalike illness in these tri-
als was broad: cough and at least 1 constitutional symp-
tom such as chills or fever.
Health care setting
Of the 6 RCTs examining the use of masks by health
care workers, only 2 had a control group assigned to
“no mask.”28,32 In these trials, masks did not reduce influ-
enzalike illness (RR = 0.26; 95% CI 0.01 to 6.42; 1 trial),
any clinical respiratory infection (RR = 0.74; 95% CI 0.36
to 1.54; I2 = 0%), confirmed influenza (RR = not estimable),
or confirmed viral respiratory infection (RR = 0.90; 95% CI
0.33 to 2.44; 1 trial), compared with no masks. However,
1 of the 2 trials was small (32 participants) and the over-
all number of events were low (only 62 cases of clini-
cal respiratory infections in a total of 1160 individuals),
leading to imprecision (see analysis 2 in Appendix 2
for forest plots*). Also, in the larger trial (n = 1038 for
the “masks” vs “no mask” comparison), there was a
high contamination rate, with 99% of the participants
assigned to the control group reporting use of some
kind of mask at some point.32
In the only trial comparing surgical masks to cloth
masks, results favoured surgical masks over cloth masks
for reduction in clinical or laboratory-confirmed viral
respiratory infections; however, results were not sta-
tistically significant (see analysis 3 in Appendix 2 for
forest plots*).32 Influenzalike illness risk was signifi-
cantly reduced with surgical masks compared with cloth
masks (RR = 0.12; 95% CI 0.02 to 0.98) but again event
rates were low, explaining the large CI and limiting the
certainty of this result. The event rate in the cloth mask
group (risk of influenzalike illness) was 2.3% over 4
weeks. Applying the cluster-adjusted RR, surgical mask
use would reduce this risk to 0.3%, a 2% absolute risk
reduction or an NNT of 50.
Four RCTs compared the use of surgical and N95
masks in health care workers.29-31,33 In these trials, we
Table 3. Outcomes for mask use to prevent viral respiratory tract infections
MASK USERS
CONFIRMED INFLUENZA CONFIRMED VIRAL
RESPIRATORY INFECTION INFLUENZALIKE ILLNESS ANY RESPIRATORY INFECTION
RCTS (N*) RR† (95% CI) RCTS (N*) RR† (95% CI) RCTS (N*) RR† (95% CI) RCTS (N*) RR† (95% CI)
Community
• Community
members () .
(.-.) () NA () .
(.-.) () NA
• Families—sick
wearing masks () NA () .
(.-.) () .
(.-.) () .
(.-.)
• Families—
healthy
wearing masks
() .
(.-.) () .
(.-.) () .
(.-.) () NA
• Families—
healthy and
sick wearing
masks
() .
(.-.) () NA () .
(.-.) () NA
• Communities
staying in
tents
() .
(.-.) () .
(.-.) () .
(.-.) () .
(.-.)
• Families—given
masks before
illness
() NA () NA () .
(.-.) () NA
Health care
workers
• Masks vs
nothing () Not
estimable () .
(.-.) () .
(.-.) () .
(.-.)
• N masks vs
surgical masks () .
(.-.) () .
(.-.) () .
(.-.) () .
(.-.)
• Surgical masks
vs cloth masks () .
(.-.) () .
(.-.) () .
(.-.) () .
(.-.)
NA—not applicable, RCT—randomized controlled trial, RR—risk ratio.
*The number of patients is modified (reduced) to account for clustering.
†Results calculated using a fixed-effects model.
‡One RCT had arms.
Two of the included RCTs had arms, so RCTs would be arms and RCTs would be arms.
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found no difference between surgical and N95 masks
for confirmed influenza (RR = 1.10; 95% CI 0.91 to 1.32;
I2 = 0%) or confirmed viral respiratory infections (RR = 0.95;
95% CI 0.83 to 1.07; I2 = 0%). N95 masks appeared to
reduce influenzalike illness (RR = 0.78; 95% CI 0.61
to 1.00; I2 = 0%) and any clinical respiratory infection risk
(RR = 0.95; 95% CI 0.90 to 1.00; I2 = 55%) (Figure 2, avail-
able from CFPlus*). The pooled control event rate for
influenzalike illness from the unadjusted numbers in the
trials was 4.6% over 4 to 12 weeks (1 study looking at
12 weeks each year for 4 years for a total of 48 weeks,
but using individual seasons as their unit of analysis) of
wearing surgical masks. Applying the cluster-adjusted
RR, N95 masks would reduce this risk to 3.6%, a 1%
absolute risk reduction or an NNT of 100. The pooled
control event rate for clinical respiratory infections from
the unadjusted numbers in the trials was 49.4% over 4
to 12 weeks of wearing surgical masks. Applying the
cluster-adjusted RR, N95 masks would reduce this risk
to 46.9%, meaning a 2.5% absolute risk reduction or an
NNT of 40. To account for differences in trials compar-
ing N95 masks with surgical masks, the random-effects
model demonstrated a more conservative estimate with
wider CIs, reducing the certainty of a positive effect
for both influenzalike illness (RR = 0.79, 95% CI 0.62 to
1.02; I2 = 0%) and for any clinical respiratory infections
(RR = 0.69, 95% CI 0.47 to 1.03; I2 = 55%) (see analyses 4
and 5 in Appendix 2 for full details*).
—— Discussion ——
Overall, we found limited evidence regarding the effect of
masks on viral respiratory infections both in the commu-
nity and in health care settings, and most of our analyses
showed no statistically significant differences. Particularly
in the community setting, we wanted to see if there was
any evidence of benefit from systematic use of masks
by the general public outside the home, but we found
no such evidence. Our review still identified 4 poten-
tially important results. First, the use of masks by a group
in the community setting appears to reduce influenza-
like illness in those wearing masks. While community
trials that most closely aligned with mask use in real-
life community settings16,17 did not show significant
effects individually, our pooled analysis showed a signifi-
cant risk reduction (NNT = 24). Although the same analy-
sis showed no significant risk reduction in confirmed
influenza or confirmed viral infection, we believe influ-
enzalike illness to be an important patient-oriented out-
come. Second, using masks within a family 1 to 3 days
after someone has developed symptoms of a viral respi-
ratory infection does not appear to prevent transmission
to family members, no matter if the masks are used by
the sick individual, the healthy family members, or both.
Third, surgical masks are likely superior to cloth masks
for preventing influenzalike illness in health care workers
(NNT = 50) but our results are drawn from a single study.32
Finally, N95 masks are likely superior to surgical masks
for preventing influenzalike illness (NNT = 100) and clinical
respiratory infections (NNT = 40) in health care workers.
There are many potential reasons why RCTs of masks
have historically struggled to find statistically significant
differences. The first reason might simply be that masks
do not prevent viral respiratory infection transmission.
Some have postulated this is because people are not
using them properly, are touching their face while wear-
ing one, or are wearing it below their nose. Some also
postulated that people using masks might feel protected
and might be less likely to follow other recommenda-
tions such as hand hygiene. A host of other reasons are
also mentioned; however, these reasons remain hypoth-
eses and are unproven. Second, many studies use a
cluster-randomized design, which reduces the power of
these studies and the ability to achieve statistical sig-
nificance if indeed there is a difference. Third, adher-
ence to wearing masks is generally poor. For example,
most community studies found that mask use averaged
5 hours or less per day or that 50% of participants or less
reported regular use. And even if the rate of adherence
was high, most studies had particular instructions about
when to wear masks. For example, all studies in health
care workers instructed participants in the mask group
to wear a mask when at work. These individuals could
therefore get infected outside work, while not wearing
Figure 1. Daily mask use compared with no mask use in the community to prevent influenzalike illness
STUDY OR SUBGROUP
MASKS CONTROL
WEIGHT, % RISK RATIO* (95% CI) RISK RATIO* (95% CI)EVENTS TOTAL EVENTS TOTAL
Aiello, 2010 99 378 177 552 73.7 0.82 (0.66-1.01)
Aiello, 2012 45 387 50 366 26.3 0.85 (0.58-1.24)
Total (95% CI) 765 918 100.0 0.83 (0.69-0.99)
Total events 144 227
Heterogeneity: c2
1 = 0.04 (P = .85); I2 = 0% 0.01 0.1 1 10 100
Test for overall effect: Z = 2.05 (P = .04) Favours masks Favours control
*Mantel-Haenszel fixed-effects method.
516 Canadian Family Physician | Le Médecin de famille canadien Vol 66: JULY | JUILLET 2020
RESEARCH
a mask, influencing the overall results. In other studies,
mask use in the control arm also occurred. For example,
in the Alfelali et al study,18 masks were used by 25% of
the individuals in the mask arm and 14% of the individu-
als in the no-mask arm, making any separation of effect
less likely. Fourth, in some studies, event rates (eg, influ-
enza cases) were low, with only a few cases in either
arm, reducing the ability of the studies to determine sta-
tistical significance if there is a difference. This might
explain why we found significant risk reduction in more
common outcomes such as influenzalike illness and any
clinical respiratory infection, but not in confirmed influ-
enza or confirmed viral infection. Fifth, many commu-
nity studies were designed to start mask use after the
index patient was seen by a health care provider. This
means patients might have already been sick at home
for 1 to 3 days, potentially transmitting infection to fam-
ily members and making mask introduction potentially
useless. Sixth, in health care workers, the comparison of
N95 and surgical masks might not reach clear statistical
significance simply because both interventions might be
beneficial and differences between the 2 might be small.
It might also be because in all but 1 study, N95 masks
and surgical masks were used either all the time at work
or when caring for patients with respiratory illness, not
only in particularly high-risk situations (eg, intubation)
where N95 masks might be more warranted.
Strengths and limitations
This review has some limitations and notable remaining
uncertainties. Our search strategy pertaining to articles
published before January 1, 2020, was limited to sys-
tematic reviews and might have kept us from finding
additional RCTs. Our review did not identify any study
examining if wearing masks in a large community such
as a city prevents the spread of infection to others. The
studies of sick individuals wearing masks to prevent
secondary infection of family members did not find ben-
efit but had many limitations as mentioned above; there-
fore, we do not yet know if wearing masks will reduce
transmission to others. Our review did not find any RCTs
investigating the use of cloth masks in the community.
Pertaining to the use of masks by health care work-
ers, we found no studies conducted in primary care and
almost no evidence comparing wearing a mask to not
wearing a mask. The last is not surprising, as having
a “no mask” group raises ethical issues. Regarding our
analysis, the most correct way to perform these meta-
analyses is debatable. We chose to pool event rates
adjusted only for clustering rather than pooling adjusted
effect estimates (eg, odds ratios). We did this to mini-
mize other adjustments and to avoid selecting results
with different levels of adjustment, thereby maintain-
ing consistency in our analysis. Also, we tried to pool
studies into similar clinical scenarios and believed fixed-
effects models for analysis were the most appropriate.
We did, however, perform random-effects sensitivity
analyses when some design heterogeneity remained (as
in the comparison of N95 and surgical masks in health
care workers). We looked at multiple outcomes and did
numerous analyses, therefore increasing the probabil-
ity that our positive results are owing to chance. Our
review focused on masks and did not account for the
benefits of other preventive interventions such as hand
hygiene or additional PPE.
Our review also had a number of strengths includ-
ing using only RCTs, adjustment for cluster design, and
pooling based on clinical similarities.
None of the studies in this review included patients
with COVID-19. Future research on masks for the pre-
vention of COVID-19 in health care and community set-
tings is very much needed. In addition, the effect of cloth
masks on community prevention of any viral respira-
tory illness should be studied, as no RCTs exist to assess
their benefit.
Conclusion
Our systematic review found limited evidence that the
use of masks might reduce the risk of viral respiratory
infections. In the community setting, we found no evi-
dence regarding the use of masks by the general public
outside the home, but found a possible reduction on the
risk of influenzalike illness when masks are used at least
a few hours a day by a population in a specific area. In
health care workers, the best available evidence shows
no difference between N95 masks and surgical masks
on the risk of confirmed influenza or other confirmed
viral respiratory infections, although our results suggest
a possible benefit from N95 masks for preventing influ-
enzalike illness or other clinical respiratory infections.
Surgical masks might be superior to cloth masks but
data are limited to 1 trial.
Dr Dugré is a pharmacist at the CIUSSS du Nord-de-l’Ile-de-Montréal in Quebec and
Clinical Assistant Professor in the Faculty of Pharmacy at the University of Montreal.
Dr Ton is a pharmacist and Clinical Evidence Expert at the College of Family Physicians
of Canada in Edmonton. Ms Perry is Knowledge Translation Expert at the Alberta
College of Family Physicians in Edmonton. Dr Garrison is Associate Professor in the
Department of Family Medicine at the University of Alberta in Edmonton. Dr Falk
is Associate Professor in the College of Pharmacy at the University of Manitoba in
Winnipeg. Dr McCormack is Professor in the Faculty of Pharmaceutical Sciences at
the University of British Columbia in Vancouver. Dr Moe is Clinical Evidence Expert
at the College of Family Physicians of Canada in Mississauga, Ont. Dr Korownyk is
Associate Professor in the Department of Family Medicine at the University of Alberta.
Dr Lindblad is Knowledge Translation and Evidence Coordinator at the Alberta College
of Family Physicians and Associate Clinical Professor in the Department of Family
Medicine at the University of Alberta. Dr Kolber is Professor in the Department of
Family Medicine at the University of Alberta. Ms Thomas is Knowledge Translation
Expert at the Alberta College of Family Physicians. Dr Train is Assistant Professor in
the Department of Family Medicine at Queen’s University in Kingston, Ont. Dr Allan
is Director of Programs and Practice Support at the College of Family Physicians of
Canada and Professor in the Department of Family Medicine at the University of Alberta.
Acknowledgment
We thank our peer reviewers for their valuable feedback.
Contributors
All authors were part of the Evidence Review Team and contributed to preparing the
manuscript for submission.
Competing interests
None declared
Vol 66: JULY | JUILLET 2020 | Canadian Family Physician | Le Médecin de famille canadien
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RESEARCH
Correspondence
Dr Nicolas Dugré; e-mail nicolas.dugre@umontreal.ca
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This article has been peer reviewed.
Cet article a fait l’objet d’une révision par des pairs.
Can Fam Physician ;:-
