Table 1.

Characteristics of eligible studies.

	Study	Year	Country	Virus	Mask type	Type of Study	Population	Main findings & comments
1	Yin et al.	2004	China	SARSa	Paper mask, cotton mask	Case-control study	Healthcare workers	Wearing a mask is effective for medical personnel in preventing SARS hospital infections.
2	Wu et al.	2004	China	SARSa	Mask	Case-control study	Population	The mask use lowered the risk for disease supports the community's use of this strategy
3	Ma et al.	2004	China	SARSa	Mask	Case-control study	Healthcare workers	Wearing masks is of great significance to prevent respiratory infections. There are many types of masks used clinically.
4	Loeb et al.	2004	Canada	SARS	Medical Mask, N95	Case-control study	Healthcare workers	Consistently wearing a mask (either surgical or particulate respirator type N95) while caring for a SARS patient was protective for the nurses.
5	Teleman et al.	2004	Singapore	SARSa	N95	Case-control study	Healthcare workers	Both hand washing and wearing of N95 masks remained strongly protective but gowns and gloves did not affect.
6	Nishiura et al.	2005	Vietnam	SARS	Surgical mask	Case-control study	Employees and relative	Masks and gowns appeared to prevent SARS transmission.
7	Wilder-Smith et al.	2005	Singapore	SARS	N95	Case-control study	Healthcare workers	Asymptomatic SARS was associated with lower SARS antibody titers and higher use of masks when compared to pneumonic SARS.
8	MacIntyre et al.	2011	China	Respiratory virus	Medical Mask, N95 Fit tested, N95 non-fit tested	Cluster randomized trial	Healthcare workers	There was no significant difference in outcomes between the N95 arms with and without fit testing.
9	Barasheed et al.	2014	Australia	Respiratory virus	Mask	Cluster randomized trial	Pilgrims	The laboratory results did not show any difference between the ‘mask’ group and ‘control’ group.
10	Sung et al.	2016	USA	Respiratory virus	Mask	Cohort study	HSCT patients	The requirement that all individuals in direct contact with HSCT patients wear surgical masks will reduce RVI.
11	Zhang et al.	2017	China	Respiratory virus	Masks	Case-control study	Healthcare workers	Choosing the right disposable respirator also plays an important role in controlling hospital viral infections.
12	Cowling et al.	2008	China (Hong Kong)	Influenza virus	Mask	Cluster randomized trial	Household	The laboratory-based or clinical secondary attack ratios did not significantly differ across the mask group and control group. Adherence to interventions was variable.
13	Cowling et al.	2009	China (Hong Kong)	Influenza virus	Mask	Cluster randomized trial	Household	Hand hygiene and facemasks seemed to prevent household transmission of influenza virus when implemented within 36 h of index patient symptom onset.
14	Suess et al.	2012	Germany	Influenza virus	Mask	Cluster randomized trial	Household	The secondary infection in the mask groups was significantly lower compared to the control group.
15	Aiello et al.	2012	USA	Influenza virus	Mask	Cluster randomized trial	Student	Face masks and hand hygiene combined may reduce the rate of ILI and confirmed influenza in community settings.
16	Cheng et al.	2010	China (Hong Kong)	H1N1	Surgical mask	Case-control study	Healthcare workers	Not wearing a surgical mask during contact with the index case were found to be significant risk factors for nosocomial acquisition of S-OIV.
17	Jaeger et al.	2011	USA	H1N1	Mask or N95	Cohort study	Healthcare workers	The use of a mask or N95 respirator was associated with remaining seronegative.
18	Chokephaibulkit et al.	2012	Thailand	H1N1	Mask	Case-control study	Healthcare workers	During the H1N1 outbreak in 2009, the wearing of masks by medical personnel was not related to the infection. There was a weak association in the nurse subgroup.
19	Zhang et al.	2012	China	H1N1	Mask	Case-control study	Healthcare workers	The results suggest that the protective effect of wearing a mask is not significant.
20	Zhang et al.	2013	China (Hong Kong)	H1N1	Mask	Case-control study	Population	Wearing masks is a protective factor against H1N1 infection when taking a plane.
21	Wang et al.	2020	China	SARS-CoV-2	N95	Case-control study	Healthcare workers	The 2019-nCoV infection rate for medical staff was significantly increased in the no-mask group compared with the N95 respirator group (adjusted odds ratio (OR): 464.82, [95% CI: 97.73-infinite]).

^aPatients met local clinical diagnostic criteria during an acute large-scale infectious disease.