A cluster randomised trial of cloth
masks compared with medical masks
in healthcare workers
C Raina MacIntyre,
Tham Chi Dung,
Nguyen Tran Hien,
Phan Thi Nga,
Abrar Ahmad Chughtai,
Dominic E Dwyer,
To cite: MacIntyre CR,
Seale H, Dung TC, et al.
A cluster randomised trial of
cloth masks compared with
medical masks in healthcare
workers. BMJ Open 2015;5:
▸Prepublication history for
this paper is available online.
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Received 9 September 2014
Revised 25 March 2015
Accepted 26 March 2015
For numbered affiliations see
end of article.
Professor C Raina MacIntyre;
Objective: The aim of this study was to compare the
efficacy of cloth masks to medical masks in hospital
healthcare workers (HCWs). The null hypothesis is that
there is no difference between medical masks and
Setting: 14 secondary-level/tertiary-level hospitals in
Participants: 1607 hospital HCWs aged ≥18 years
working full-time in selected high-risk wards.
Intervention: Hospital wards were randomised to:
medical masks, cloth masks or a control group
(usual practice, which included mask wearing).
Participants used the mask on every shift for 4
Main outcome measure: Clinical respiratory illness
(CRI), influenza-like illness (ILI) and laboratory-
confirmed respiratory virus infection.
Results: The rates of all infection outcomes were
highest in the cloth mask arm, with the rate of ILI
statistically significantly higher in the cloth mask arm
(relative risk (RR)=13.00, 95% CI 1.69 to 100.07)
compared with the medical mask arm. Cloth masks
also had significantly higher rates of ILI compared with
the control arm. An analysis by mask use showed ILI
(RR=6.64, 95% CI 1.45 to 28.65) and laboratory-
confirmed virus (RR=1.72, 95% CI 1.01 to 2.94) were
significantly higher in the cloth masks group compared
with the medical masks group. Penetration of cloth
masks by particles was almost 97% and medical
Conclusions: This study is the first RCT of cloth
masks, and the results caution against the use of cloth
masks. This is an important finding to inform
occupational health and safety. Moisture retention,
reuse of cloth masks and poor filtration may result in
increased risk of infection. Further research is needed
to inform the widespread use of cloth masks globally.
However, as a precautionary measure, cloth masks
should not be recommended for HCWs, particularly in
high-risk situations, and guidelines need to be
Trial registration number: Australian New Zealand
Clinical Trials Registry: ACTRN12610000887077.
The use of facemasks and respirators for the
protection of healthcare workers (HCWs)
has received renewed interest following the
2009 inﬂuenza pandemic,
infectious diseases such as avian inﬂuenza,
Middle East respiratory syndrome corona-
Historically, various types of cloth/
cotton masks (referred to here after as ‘cloth
masks’) have been used to protect HCWs.
Disposable medical/surgical masks (referred
to here after as ‘medical masks’) were intro-
duced into healthcare in the mid 19th
century, followed later by respirators.
Compared with other parts of the world, the
use of face masks is more prevalent in Asian
countries, such as China and Vietnam.
In high resource settings, disposable
medical masks and respirators have long
since replaced the use of cloth masks in hos-
pitals. Yet cloth masks remain widely used
Strengths and limitations of this study
▪The use of cloth masks is widespread around
the world, particularly in countries at high-risk
for emerging infections, but there have been no
efficacy studies to underpin their use.
▪This study is large, a prospective randomised
clinical trial (RCT) and the first RCT ever con-
ducted of cloth masks.
▪The use of cloth masks are not addressed in
most guidelines for health care workers—this
study provides data to update guidelines.
▪The control arm was ‘standard practice’, which
comprised mask use in a high proportion of par-
ticipants. As such (without a no-mask control),
the finding of a much higher rate of infection in
the cloth mask arm could be interpreted as harm
caused by cloth masks, efficacy of medical
masks, or most likely a combination of both.
MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577 1
Open Access Research
globally, including in Asian countries, which have histor-
ically been affected by emerging infectious diseases, as
well as in West Africa, in the context of shortages of per-
sonal protective equipment (PPE).
It has been
shown that medical research disproportionately favours
diseases of wealthy countries, and there is a lack of
research on the health needs of poorer countries.
Further, there is a lack of high-quality studies around the
use of facemasks and respirators in the healthcare
setting, with only four randomised clinical trials (RCTs)
Despite widespread use, cloth masks are rarely
mentioned in policy documents,
and have never been
tested for efﬁcacy in a RCT. Very few studies have been
conducted around the clinical effectiveness of cloth
masks, and most available studies are observational or in
Emerging infectious diseases are not constrained
within geographical borders, so it is important for global
disease control that use of cloth masks be underpinned
by evidence. The aim of this study was to determine the
efﬁcacy of cloth masks compared with medical masks in
HCWs working in high-risk hospital wards, against the
prevention of respiratory infections.
A cluster-randomised trial of medical and cloth mask
use for HCWs was conducted in 14 hospitals in Hanoi,
Vietnam. The trial started on the 3 March 2011, with
rolling recruitment undertaken between 3 March 2011
and 10 March 2011. Participants were followed during
the same calendar time for 4 weeks of facemasks use
and then one additional week for appearance of symp-
toms. An invitation letter was sent to 32 hospitals in
Hanoi, of which 16 agreed to participate. One hospital
did not meet the eligibility criteria; therefore, 74 wards
in 15 hospitals were randomised. Following the random-
isation process, one hospital withdrew from the study
because of a nosocomial outbreak of rubella.
Participants provided written informed consent prior
to initiation of the trial.
Seventy-four wards (emergency, infectious/respiratory
disease, intensive care and paediatrics) were selected as
high-risk settings for occupational exposure to respira-
tory infections. Cluster randomisation was used because
the outcome of interest was respiratory infectious dis-
eases, where prevention of one infection in an individual
can prevent a chain of subsequent transmission in
Epi info V.6 was used to generate a
randomisation allocation and 74 wards were randomly
allocated to the interventions.
From the eligible wards 1868 HCWs were approached
to participate. After providing informed consent, 1607
participants were randomised by ward to three arms:
(1) medical masks at all times on their work shift; (2)
cloth masks at all times on shift or (3) control arm
(standard practice, which may or may not include mask
use). Standard practice was used as control because the
IRB deemed it unethical to ask participants to not wear
a mask. We studied continuous mask use (deﬁned as
wearing masks all the time during a work shift, except
while in the toilet or during tea or lunch breaks)
because this reﬂects current practice in high-risk settings
Figure 1 Consort diagram of
recruitment and follow-up (HCWs,
2MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577
The laboratory results were blinded and laboratory
testing was conducted in a blinded fashion. As facemask
use is a visible intervention, clinical end points could
not be blinded. Figure 1 outlines the recruitment and
Primary end points
There were three primary end points for this study, used in
our previous mask RCTs:
(1) Clinical respiratory illness
(CRI), deﬁned as two or more respiratory symptoms or
one respiratory symptom and a systemic symptom;
(2) inﬂuenza-like illness (ILI), deﬁned as fever ≥38°C plus
one respiratory symptom and (3) laboratory-conﬁrmed
viral respiratory infection. Laboratory conﬁrmation was by
nucleic acid detection using multiplex reverse transcript-
ase PCR (RT-PCR) for 17 respiratory viruses: respiratory
syncytial virus (RSV) A and B, human metapneumovirus
(hMPV), inﬂuenza A (H3N2), (H1N1)pdm09, inﬂuenza
B, parainﬂuenza viruses 1–4, inﬂuenza C, rhinoviruses,
severe acute respiratory syndrome (SARS) associated
coronavirus (SARS-CoV), coronaviruses 229E, NL63,
OC43 and HKU1, adenoviruses and human bocavirus
Additional end points included compliance
with mask use, deﬁned as using the mask during the shift
for 70% or more of work shift hours.
HCWs were cate-
gorised as ‘compliant’if the average use was equal or more
than 70% of the working time. HCW were categorised as
‘non-compliant’if the average mask use was less than 70%
of the working time.
Nurses or doctors aged ≥18 years working full-time were
eligible. Exclusion criteria were: (1) Unable or refused
to consent; (2) Beards, long moustaches or long facial
hair stubble; (3) Current respiratory illness, rhinitis
Participants wore the mask on every shift for four con-
secutive weeks. Participants in the medical mask arm
were supplied with two masks daily for each 8 h shift,
while participants in the cloth mask arm were provided
with ﬁve masks in total for the study duration, which
they were asked to wash and rotate over the study
period. They were asked to wash cloth masks with soap
and water every day after ﬁnishing the shifts.
Participants were supplied with written instructions on
how to clean their cloth masks. Masks used in the study
were locally manufactured medical (three layer, made of
non-woven material) or cloth masks (two layer, made of
cotton) commonly used in Vietnamese hospitals. The
control group was asked to continue with their normal
practices, which may or may not have included mask
wearing. Mask wearing was measured and documented
for all participants, including the control arm.
Data collection and follow-up
Data on sociodemographic, clinical and other potential
confounding factors were collected at baseline.
Participants were followed up daily for 4 weeks (active
intervention period), and for an extra week of standard
practice, in order to document incident infection after
incubation. Participants received a thermometer (trad-
itional glass and mercury) to measure their temperature
daily and at symptom onset. Daily diary cards were pro-
vided to record number of hours worked and mask use,
estimated number of patient contacts (with/without ILI)
and number/type of aerosol-generating procedures
(AGPs) conducted, such as suctioning of airways,
sputum induction, endotracheal intubation and bron-
choscopy. Participants in the cloth mask and control
group (if they used cloth masks) were also asked to
document the process used to clean their mask
We also monitored compliance with mask use by a pre-
viously validated self-reporting mechanism.
were contacted daily to identify incident cases of respira-
tory infection. If participants were symptomatic, swabs of
both tonsils and the posterior pharyngeal wall were col-
lected on the day of reporting.
Sample collection and laboratory testing
Trained collectors used double rayon-tipped, plastic-
shafted swabs to scratch tonsillar areas as well as the pos-
terior pharyngeal wall of symptomatic participants.
Testing was conducted using RT-PCR applying published
Viral RNA was extracted from each respira-
tory specimen using the Viral RNA Mini kit (Qiagen,
Germany), following the manufacturer’s instructions.
The RNA extraction step was controlled by ampliﬁcation
of a RNA house-keeping gene (amplify pGEM) using
real-time RT-PCR. Only extracted samples with the house
keeping gene detected by real-time RT-PCR were submit-
ted for multiplex RT-PCR for viruses.
The reverse transcription and PCRs were performed
in OneStep (Qiagen, Germany) to amplify viral target
genes, and then in ﬁve multiplex RT-PCR: RSVA/B,
inﬂuenza A/H3N2, A(H1N1) and B viruses, hMPV
(reaction mix 1); parainﬂuenza viruses 1–4 (reaction
mix 2); rhinoviruses, inﬂuenza C virus, SARS-CoV (reac-
tion mix 3); coronaviruses OC43, 229E, NL63 and
HKU1 (reaction mix 4); and adenoviruses and hBoV
(reaction mix 5), using a method published by others.
All samples with viruses detected by multiplex RT-PCR
were conﬁrmed by virus-speciﬁc mono nested or hemi-
nested PCR. Positive controls were prepared by in vitro
transcription to control ampliﬁcation efﬁcacy and
monitor for false negatives, and included in all runs
(except for NL63 and HKU1). Each run always included
two negatives to monitor ampliﬁcation quality. Specimen
processing, RNA extraction, PCR ampliﬁcation and PCR
product analyses were conducted in different rooms to
MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577 3
The ﬁltration performance of the cloth and medical
masks was tested according to the respiratory standard
The equipment used was a TSI 8110
Filter tester. To test the ﬁltration performance, the ﬁlter
is challenged by a known concentration of sodium chlor-
ide particles of a speciﬁed size range and at a deﬁned
ﬂow rate. The particle concentration is measured before
and after adding the ﬁlter material and the relative
ﬁltration efﬁciency is calculated. We examined the
performance of cloth masks compared with the per-
formance levels—P1, P2 (=N95) and P3, as used for
assessment of all particulate ﬁlters for respiratory protec-
tion. The 3M 9320 N95 and 3M Vﬂex 9105 N95 were
used to compare against the cloth and medical masks.
Sample size calculation
To obtain 80% power at two-sided 5% signiﬁcance level
for detecting a signiﬁcant difference of attack rate
between medical masks and cloth masks, and for a rate
of infection of 13% for cloth mask wearers compared
with 6% in medical mask wearers, we would need eight
clusters per arm and 530 participants in each arm, and
intracluster correlation coefﬁcient (ICC) 0.027, obtained
from our previous study.
The design effect (deff ) for
this cluster randomisation trial was 1.65 (deff=1+(m
−1)×ICC=1+(25−1)×0.027=1.65). As such, we aimed to
recruit a sample size of 1600 participants from up to 15
Descriptive statistics were compared among intervention
and control arms. Primary end points were analysed by
intention to treat. We compared the event rates for the
primary outcomes across study arms and calculated
p values from cluster-adjusted χ
also estimated relative risk (RR) after adjusting for clus-
tering using a log-binomial model under generalised
estimating equation (GEE) framework.
We checked for
variables which were unequally distributed across arms,
and conducted an adjusted analysis accordingly. We
ﬁtted a multivariable log-binomial model, using GEE to
account for clustering by ward, to estimate RR after
adjusting for potential confounders. In the initial
model, we included all the variables that had p value
less than 0.25 in the univariable analysis, along with the
main exposure variable (randomisation arm). A back-
ward elimination method was used to remove the vari-
ables that did not have any confounding effect.
As most participants in the control arm used a mask
during the trial period, we carried out a post-hoc ana-
lysis comparing all participants who used only a medical
mask (from the control arm and the medical mask arm)
with all participants who used only a cloth mask ( from
the control arm and the cloth arm). For this analysis,
controls who used both types of mask (n=245) or used
N95 respirators (n=3) or did not use any masks (n=2)
were excluded. We ﬁtted a multivariable log-binomial
model, to estimate RR after adjusting for potential con-
founders. As we pooled data of participants from all
three arms and analysed by mask type, not trial arm, we
did not adjust for clustering here. All statistical analyses
were conducted using STATA V.12.
Owing to a very high level of mask use in the control
arm, we were unable to determine whether the differ-
ences between the medical and cloth mask arms were
due to a protective effect of medical masks or a detri-
mental effect of cloth masks. To assist in interpreting
the data, we compared rates of infection in the medical
mask arm with rates observed in medical mask arms
from two previous RCTs,
in which no efﬁcacy of
medical masks could be demonstrated when compared
with control or N95 respirators, recognising that sea-
sonal and geographic variation in virus activity affects
the rates of exposure (and hence rates of infection out-
comes) among HCWs. This analysis was possible because
the trial designs were similar and the same outcomes
were measured in all three trials. The analysis was
carried out to determine if the observed results were
explained by a detrimental effect of cloth masks or a
protective effect of medical masks.
A total of 1607 HCWs were recruited into the study. The
participation rate was 86% (1607/1868). The average
number of participants per ward was 23 and the mean
age was 36 years. On average, HCWs were in contact
with 36 patients per day during the trial period (range
0–661 patients per day, median 20 patients per day).
The distribution of demographic variables was generally
similar between arms (table 1). Figure 2 shows the
primary outcomes for each of the trial arms. The rates
of CRI, ILI and laboratory-conﬁrmed virus infections
were lowest in the medical mask arm, followed by the
control arm, and highest in the cloth mask arm.
Table 2 shows the intention-to-treat analysis. The rate
of CRI was highest in the cloth mask arm, followed by
the control arm, and lowest in the medical mask arm.
The same trend was seen for ILI and laboratory tests
conﬁrmed viral infections. In intention-to-treat analysis,
ILI was signiﬁcantly higher among HCWs in the cloth
masks group (RR=13.25 and 95% CI 1.74 to 100.97),
compared with the medical masks group. The rate of
ILI was also signiﬁcantly higher in the cloth masks arm
(RR=3.49 and 95% CI 1.00 to 12.17), compared with the
control arm. Other outcomes were not statistically signiﬁ-
cant between the three arms.
Among the 68 laboratory-conﬁrmed cases, 58 (85%)
were due to rhinoviruses. Other viruses detected were
hMPV (7 cases), inﬂuenza B (1 case), hMPV/rhinovirus
co-infection (1 case) and inﬂuenza B/rhinovirus
co-infection (1 case) (table 3). No inﬂuenza A or RSV
infections were detected.
Compliance was signiﬁcantly higher in the cloth mask
arm (RR=2.41, 95% CI 2.01 to 2.88) and medical masks
4MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577
arm (RR=2.40, 95% CI 2.00 to 2.87), compared with the
control arm. Figure 3 shows the percentage of partici-
pants who were compliant in the three arms. A post-hoc
analysis adjusted for compliance and other potential con-
founders showed that the rate of ILI was signiﬁcantly
higher in the cloth mask arm (RR=13.00, 95% CI 1.69 to
100.07), compared with the medical masks arm (table 4).
There was no signiﬁcant difference between the medical
mask and control arms. Hand washing was signiﬁcantly
protective against laboratory-conﬁrmed viral infection
(RR=0.66, 95% CI 0.44 to 0.97).
In the control arm, 170/458 (37%) used medical
masks, 38/458 (8%) used cloth masks, and 245/458
(53%) used a combination of both medical and cloth
masks during the study period. The remaining 1%
either reported using a N95 respirator (n=3) or did not
use any masks (n=2).
Table 5 shows an additional analysis comparing all par-
ticipants who used only a medical mask ( from the
control arm and the medical mask arm) with all partici-
pants who used only a cloth mask (from the control arm
and the cloth arm). In the univariate analysis, all out-
comes were signiﬁcantly higher in the cloth mask group,
compared with the medical masks group. After adjusting
for other factors, ILI (RR=6.64, 95% CI 1.45 to 28.65)
and laboratory-conﬁrmed virus (RR=1.72, 95% CI 1.01
to 2.94) remained signiﬁcantly higher in the cloth masks
group compared with the medical masks group.
Table 6 compares the outcomes in the medical mask
arm with two previously published trials.
that while the rates of CRI were signiﬁcantly higher in
one of the previously published trials, the rates of
laboratory-conﬁrmed viruses were not signiﬁcantly differ-
ent between the three trials for medical mask use.
On average, HCWs worked for 25 days during the trial
period and washed their cloth masks for 23/25 (92%)
days. The most common approach to washing cloth
masks was self-washing (456/569, 80%), followed by
combined self-washing and hospital laundry (91/569,
16%), and only hospital laundry (22/569, 4%). Adverse
events associated with facemask use were reported in
40.4% (227/562) of HCWs in the medical mask arm
and 42.6% (242/568) in the cloth mask arm ( p value
0.450). General discomfort (35.1%, 397/1130) and
breathing problems (18.3%, 207/1130) were the most
frequently reported adverse events.
Table 1 Demographic and other characteristics by arm of randomisation
(% and 95% CI)
(% and 95% CI)
(% and 95% CI)
Gender (male) 112/580
19.3 (16.2 to 22.8)
23.4 (20.0 to 27.1)
24.5 (20.6 to 28.7)
Age (mean) 36 (35.6 to 37.3) 35 (34.6 to 36.3) 36 (35.1 to 37.0)
Education (postgraduate) 114/580
19.7 (16.5 to 23.1)
17.4 (14.3 to 20.8)
17.0 (13.7 to 20.8)
Smoker (current/ex) 78/580
13.4 (10.8 to 16.5)
13.9 (11.1 to 17.0)
14.4 (11.3 to 18.0)
Pre-existing illness* 66/580
11.4 (9.0 to 14.2)
12.3 (9.8 to 15.3)
10.3 (7.8 to 13.4)
Influenza vaccination (yes) 21/580
3.6 (2.4 to 5.4)
3.7 (2.4 to 5.6)
3.3 (2.0 to 5.3)
Staff (doctors) 176/580
30.3 (26.6 to 34.3)
29.0 (25.3 to 32.9)
29.3 (25.1 to 33.7)
Number of hand washings per day
14 (13.8 to 15.4) 11 (10.9 to 11.9) 12 (11.5 to 12.7)
Number of patients had contact with
(median and range)‡
21 (0 to 540) 21 (0 to 661) 18 (3 to 199)
*Includes asthma, immunocompromised and others.
†‘Hand wash’variable was created by taking average of the number of hand washes performed by a healthcare worker (HCW) over the trial
period. The variable was log transformed for the multivariate analysis.
‡‘Number of patients had contact with’variable was created by taking average of the number of patients in contact with a HCW over the trial
period. Median and range is presented in the table.
Figure 2 Outcomes in trial arms (CRI, clinical respiratory
illness; ILI, influenza-like illness; Virus, laboratory-confirmed
MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577 5
Laboratory tests showed the penetration of particles
through the cloth masks to be very high (97%) com-
pared with medical masks (44%) (used in trial) and 3M
9320 N95 (<0.01%), 3M Vﬂex 9105 N95 (0.1%).
We have p r o v ided t h e ﬁrst clinical efﬁcacy data of cloth
masks, which suggest HCWs should not use cloth masks as
protection against respiratory infection. Cloth masks
resulted in signiﬁcantly higher rates of infection than
medical masks, and also performed worse than the control
arm. The controls were HCWs who observed standard prac-
tice, which involved mask use in the majority, albeit with
lower compliance than in the intervention arms. The
control HCWs also used medical masks more often than
cloth masks. When we analysed all mask-wearers including
controls, the higher risk of cloth masks was seen for
laboratory-conﬁrmed respiratory viral infection.
The trend for all outcomes showed the lowest rates of
infection in the medical mask group and the highest
rates in the cloth mask arm. The study design does not
allow us to determine whether medical masks had efﬁ-
cacy or whether cloth masks were detrimental to HCWs
by causing an increase in infection risk. Either possibil-
ity, or a combination of both effects, could explain our
results. It is also unknown whether the rates of infection
observed in the cloth mask arm are the same or higher
than in HCWs who do not wear a mask, as almost all
participants in the control arm used a mask. The phys-
ical properties of a cloth mask, reuse, the frequency and
effectiveness of cleaning, and increased moisture reten-
tion, may potentially increase the infection risk for
HCWs. The virus may survive on the surface of the face-
and modelling studies have quantiﬁed the con-
tamination levels of masks.
repeated use and improper dofﬁng is possible. For
example, a contaminated cloth mask may transfer patho-
gen from the mask to the bare hands of the wearer. We
also showed that ﬁltration was extremely poor (almost
0%) for the cloth masks. Observations during SARS sug-
gested double-masking and other practices increased the
risk of infection because of moisture, liquid diffusion
and pathogen retention.
These effects may be asso-
ciated with cloth masks.
We have previously shown that N95 respirators provide
superior efﬁcacy to medical masks,
but need to be
worn continuously in high-risk settings to protect HCWs.
Although efﬁcacy for medical masks was not shown, efﬁ-
cacy of a magnitude that was too small to be detected is
The magnitude of difference between cloth
masks and medical masks in the current study, if
explained by efﬁcacy of medical masks alone, translates
to an efﬁcacy of 92% against ILI, which is possible, but
not consistent with the lack of efﬁcacy in the two previous
Further, we found no signiﬁcant difference in
rates of virus isolation in medical mask users between the
three trials, suggesting that the results of this study could
be interpreted as partly being explained by a detrimental
effect of cloth masks. This is further supported by the
fact that the rate of virus isolation in the no-mask control
group in the ﬁrst Chinese RCT was 3.1%, which was not
signiﬁcantly different to the rates of virus isolation in the
medical mask arms in any of the three trials including
this one. Unlike the previous RCTs, circulating inﬂuenza
and RSV were almost completely absent during this study,
Table 2 Intention-to-treat analysis
Medical mask* 28/580 (4.83) Ref 1/580 (0.17) Ref 19/580 (3.28) Ref
Cloth masks†43/569 (7.56) 1.57 (0.99 to 2.48) 13/569 (2.28) 13.25 (1.74 to 100.97) 31/569 (5.45) 1.66 (0.95 to 2.91)
Control‡32/458 (6.99) 1.45 (0.88 to 2.37) 3/458 (0.66) 3.80 (0.40 to 36.40) 18/458 (3.94) 1.20 (0.64 to 2.26)
Bold typeface indicates statistically significant.
*p Value from cluster adjusted χ
tests is 0.510 and intracluster correlation coefficients is 0.065.
†p Value from cluster adjusted χ
tests is 0.028 and intracluster correlation coefficients is 0.029.
‡p Value from cluster adjusted χ
tests is 0.561 and intracluster correlation coefficients is 0.068.
CRI, clinical respiratory illness; ILI, influenza-like illness; RR, relative risk.
Table 3 Type of virus isolated
Study arm hMPV Rhino
B virus & rhino Total
Medical masks arm 1 16 1 1 0 19
Cloth mask arm 4 26 0 0 1 31
Control arm 2 16 0 0 0 18
Total 7 58 1 1 1 68
hMPV, human metapneumovirus; Rhino, rhinoviruses.
6MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577
with rhinoviruses comprising 85% of isolated pathogens,
which means the measured efﬁcacy is against a different
range of circulating respiratory pathogens. Inﬂuenza and
RSV predominantly transmit through droplet and
contact routes, while Rhinovirus transmits through mul-
tiple routes, including airborne and droplet routes.
The data also show that the clinical case deﬁnition of ILI
is non-speciﬁc, and captures a range of pathogens other
than inﬂuenza. The study suggests medical masks may be
protective, but the magnitude of difference raises the pos-
sibility that cloth masks cause an increase in infection risk
in HCWs. Further, the ﬁltration of the medical mask used
in this trial was poor, making extremely high efﬁcacy of
medical masks unlikely, particularly given the predomin-
ant pathogen was rhinovirus, which spreads by the air-
borne route. Given the obligations to HCW occupational
health and safety, it is important to consider the potential
risk of using cloth masks.
In many parts of the world, cloth masks and medical
masks may be the only options available for HCWs.
Cloth masks have been used in West Africa during the
Ebola outbreak in 2014, due to shortages of PPE, (per-
sonal communication, M Jalloh). The use of cloth masks
is recommended by some health organisations, with
In light of our study, and the obligation to
ensure occupational health and safety of HCWs, cloth
masks should not be recommended for HCWs, particu-
larly during AGPs and in high-risk settings such as emer-
gency, infectious/respiratory disease and intensive care
wards. Infection control guidelines need to acknowledge
the widespread real-world practice of cloth masks and
should comprehensively address their use. In addition,
other important infection control measure such as hand
hygiene should not be compromised. We conﬁrmed the
protective effects of hand hygiene against laboratory-
conﬁrmed viral infection in this study, but mask type was
an independent predictor of clinical illness, even
adjusted for hand hygiene.
A limitation of this study is that we did not measure
compliance with hand hygiene, and the results reﬂect
self-reported compliance, which may be subject to recall
or other types of bias. Another limitation of this study is
the lack of a no-mask control group and the high use of
masks in the controls, which makes interpretation of the
results more difﬁcult. In addition, the quality of paper
and cloth masks varies widely around the world, so the
results may not be generalisable to all settings. The lack
of inﬂuenza and RSV (or asymptomatic infections)
during the study is also a limitation, although the pre-
dominance of rhinovirus is informative about pathogens
transmitted by the droplet and airborne routes in this
setting. As in previous studies, exposure to infection
outside the workplace could not be estimated, but we
would assume it to be equally distributed between trial
arms. The major strength of the randomised trial study
design is in ensuring equal distribution of confounders
and effect modiﬁers (such as exposure outside the work-
place) between trial arms.
Cloth masks are used in resource-poor settings because
of the reduced cost of a reusable option. Various types of
cloth masks (made of cotton, gauze and other ﬁbres)
have been tested in vitro in the past and show lower ﬁltra-
tion capacity compared with disposable masks.
tection afforded by gauze masks increases with the
ﬁneness of the cloth and the number of layers,
ing potential to develop a more effective cloth mask, for
example, with ﬁner weave, more layers and a better ﬁt.
Cloth masks are generally retained long term and
reused multiple times, with a variety of cleaning
methods and widely different intervals of cleaning.
Further studies are required to determine if variations in
frequency and type of cleaning affect the efﬁcacy of
Table 4 Multivariable cluster-adjusted log-binomial model to calculate RR for study outcomes
RR (95% CI)
RR (95% CI)
RR (95% CI)
Medical masks arm Ref Ref Ref
Cloth mask arm 1.56 (0.97 to 2.48) 13.00 (1.69 to 100.07) 1.54 (0.88 to 2.70)
Control arm 1.51 (0.90 to 2.52) 4.64 (0.47 to 45.97) 1.09 (0.57 to 2.09)
Male 0.67 (0.41 to 1.12) 1.03 (0.34 to 3.13) 0.65 (0.34 to 1.22)
Vaccination 0.83 (0.27 to 2.52) 1.74 (0.24 to 12.56) 1.27 (0.41 to 3.92)
Hand washing 0.91 (0.66 to 1.26) 0.94 (0.40 to 2.20) 0.66 (0.44 to 0.97)
Compliance 1.14 (0.77 to 1.69) 1.86 (0.67 to 5.21) 0.86 (0.53 to 1.40)
Bold typeface indicates statistically significant.
CRI, clinical respiratory illness; ILI, influenza-like illness; RR, relative risk.
Figure 3 Compliance with the mask wearing—mask wearing
more than 70% of working hours.
MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577 7
Pandemics and emerging infections are more likely to
arise in low-income or middle-income settings than in
wealthy countries. In the interests of global public
health, adequate attention should be paid to cloth mask
use in such settings. The data from this study provide
some reassurance about medical masks, and are the ﬁrst
data to show potential clinical efﬁcacy of medical masks.
Medical masks are used to provide protection against
droplet spread, splash and spray of blood and body
ﬂuids. Medical masks or respirators are recommended
by different organisations to prevent transmission of
Ebola virus, yet shortages of PPE may result in HCWs
being forced to use cloth masks.
In the interest of
providing safe, low-cost options in low income countries,
there is scope for research into more effectively
designed cloth masks, but until such research is carried
out, cloth masks should not be recommended. We also
recommend that infection control guidelines be
updated about cloth mask use to protect the occupa-
tional health and safety of HCWs.
Faculty of Medicine, School of Public Health and Community Medicine,
University of New South Wales, Sydney, Australia
National Institute of Hygiene and Epidemiology, Hanoi, Vietnam
Institute for Clinical Pathology and Medical Research, Westmead Hospital
and University of Sydney, Sydney, New South Wales, Australia
Beijing Centers for Disease Control and Prevention, Beijing, China
Acknowledgements The authors would like to thank the staff members from
the National Institute of Hygiene and Epidemiology, Hanoi, Vietnam, who were
involved with the trial. They thank as well to the staff from the Hanoi hospitals
who participated. They also acknowledge the support of 3M for testing of
filtration of the facemasks. 3M was industry partner in the ARC linkage project
Table 5 Univariate and adjusted analysis comparing participants who used medical masks and cloth masks*
RR (95% CI)
RR (95% CI)
Medical mask (35/750, 4.67%) Ref Ref
Cloth mask (46/607, 7.58%) 1.62 (1.06 to 2.49) 1.51 (0.97 to 2.32)
Male 0.60 (0.32 to 1.12) 0.58 (0.31 to 1.08)
Vaccination 0.66 (0.17 to 2.62) 0.68 (0.17 to 2.67)
Hand washing 0.81 (0.58 to 1.15) 0.84 (0.59 to 1.20)
Compliance 1.01 (1.00 to 1.03) 1.01 (1.00 to 1.02)
Medical mask (2/750, 0.27%) Ref Ref
Cloth mask (13/607, 2.14%) 8.03 (1.82 to 35.45) 6.64 (1.45 to 28.65)
Male 0.95 (0.27 to 3.35) 0.92 (0.26 to 3.22)
Vaccination 1.87 (0.25 to 13.92) 1.97 (0.27 to 14.45)
Hand washing 0.56 (0.24 to 1.27) 0.61 (0.23 to 1.57)
Compliance 1.04 (1.01 to 1.08) 1.04 (1.00 to 1.08)
Medical mask (22/750, 2.93%) Ref Ref
Cloth mask (34/607, 5.60%) 1.91 (1.13 to 3.23) 1.72 (1.01 to 2.94)
Male 0.64 (0.30 to 1.33) 0.61 (0.29 to 1.27)
Vaccination 0.97 (0.24 to 3.86) 1.03 (0.26 to 4.08)
Hand washing 0.61 (0.41 to 0.93) 0.65 (0.42 to 1.00)
Compliance 1.00 (0.99 to 1.02) 1.0 (0.99 to 1.02)
Bold typeface indicates statistically significant.
*The majority (456/458) of HCWs in the control arm used a mask. Controls who exclusively used a medical mask were categorised and
analysed with the medical mask arm participants; and controls who exclusively wore a cloth mask were categorised and analysed with the
cloth mask arm.
CRI, clinical respiratory illness; HCWs, healthcare workers; ILI, influenza-like illness; RR, relative risk.
Table 6 A comparison of outcome data for the medical mask arm with medical mask outcomes in previously published RCTs
Vietnam trial 28/580 (4.83) Ref 1/580 (0.17) Ref 19/580 (3.28) Ref
33/492 (6.70) 1.40 (0.85 to 2.26) 3/492 (0.61) 3.53 (0.37 to 33.89) 13/492 (2.64) 0.80 (0.40 to 1.62)
98/572 (17.13) 3.54 (2.37 to 5.31) 4/572 (0.70) 4.06 (0.45 to 36.18) 19/572 (3.32) 1.01 (0.54 to 1.89)
Bold typeface indicates statistically significant.
CRI, Clinical respiratory illness; ILI, influenza-like illness; RCT, randomised clinical trial; RR, relative risk.
8MacIntyre CR, et al.BMJ Open 2015;5:e006577. doi:10.1136/bmjopen-2014-006577
grant; however they were not involved in study design, data collection or
analysis. The 3M products were not used in this study.
Contributors CRM was the lead investigator, and responsible for the
conception and design of the trial, obtaining the grant funding, overseeing
the whole study, analysing the data and writing of the report. HS
contributed to overseeing the study, staff training, form/database
development and drafting of the manuscript. TCD was responsible for
overseeing the study, database management, recruitment, training and
revision of the manuscript. NTH was responsible for the implementation
of research and revision of the manuscript. PTN was responsible for the
laboratory testing in Vietnam. AAC contributed to the statistical analysis and
drafting of the manuscript. BR was responsible for the statistical analysis
and revision of the manuscript. DED contributed to the laboratory technical
assistance and revision of the manuscript. QW assisted in comparing the
rates of infection from two previous RCTs conducted in China and revision
of the manuscript.
Funding Funding to conduct this study was received from the Australian
Research Council (ARC) (grant number LP0990749).
Competing interests CRM has held an Australian Research Council Linkage
Grant with 3M as the industry partner, for investigator-driven research. 3M
has also contributed masks and respirators for investigator-driven clinical
trials. CRM has received research grants and laboratory testing as in-kind
support from Pfizer, GSK and Bio-CSL for investigator-driven research. HS
had a NHMRC Australian-based Public Health Training Fellowship at the time
of the study (1012631). She has also received funding from vaccine
manufacturers GSK, bio-CSL and Sanofi Pasteur for investigator-driven
research and presentations. AAC used filtration testing of masks for his PhD
thesis conducted by 3M Australia.
Ethics approval National Institute for Hygiene and Epidemiology (NIHE) (approval
number 05 IRB) and the Human Research Ethics Committee of the University of
New South Wales (UNSW), Australia, (HREC approval number 10306).
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement No additional data are available.
Open Access This is an Open Access article distributed in accordance with
the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license,
which permits others to distribute, remix, adapt, build upon this work non-
commercially, and license their derivative works on different terms, provided
the original work is properly cited and the use is non-commercial. See: http://
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