Effectiveness of Face Masks in Preventing COVID-19 Transmission in Real-World Settings: A Systematic Literature Review

doi:10.21203/rs.3.rs-2534269/v1

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Effectiveness of Face Masks in Preventing COVID-19 Transmission in Real-World Settings: A Systematic Literature Review

https://doi.org/10.21203/rs.3.rs-2534269/v1

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Background. Since its emergence, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused an unprecedented global pandemic. Evidence regarding the effectiveness of mask wearing is important to support guidelines for public health practice and implementation of infection-control strategies. Previous systematically reviews within controlled settings suggest wearing face masks are ‘efficacious’ at preventing COVID-19. However, a comprehensive review of the ‘effectiveness’ of face masks in preventing COVID-19 transmission in real-world settings is needed in order to generalize findings. The present study reviewed literature on the effectiveness of wearing face masks in preventing COVID-19 transmission in real-world settings.

Methods. Following PRISMA guidelines, three databases and gray literature sources were searched up until January 2022. Inclusion criteria for studies were: 1) written in or translated into English; 2) assess the SARS-CoV-2 transmission; 3) assess the behavior of mask wearing and 4) study conducted in a non-laboratory-controlled setting (e.g., worksites, schools etc.). All 2,240 search results were reviewed for inclusion. Two reviewers extracted study characteristics. Three reviewers independently assessed quality of evidence and risk of bias using JBI critical appraisal tools.

Results. This review identified 48 unique studies that met the criteria. Most studies investigated the effect of face masks in a healthcare setting (n=21, 43%). The majority (n=44; 91.6%) of studies showed that wearing face masks led to a reduction in transmission or risk of transmission of COVID-19, and the protective effect of masks was increased when combined with other preventive measures. The findings support that face masks may be up to 79% effective in preventing transmission in non-controlled real-life settings.

Conclusions. The results of this systematic review strongly support public health recommendations for wearing a mask to reduce the risk of COVID-19 infection in real-world settings. Effective communication efforts are needed to inform the general public of the benefits of wearing face masks to prevent COVID-19.

SARS-CoV-2

masks

real-world settings

Since COVID-19 was declared a pandemic by the World Health Organization (WHO) in March of 2020,¹the U.S. and many other countries have faced several waves of infections and deaths attributed to different variants, which have put great strain on hospitals and the health system.² COVID-19 was the third leading cause of death in the U.S. in 2020, the number one cause of death for people aged 45-84 years, and in the top four leading causes of death for all other age groups.³ To date, over 6.1 million people globally have died due to COVID-19.⁴

Public health recommendations for prevention of COVID-19 include vaccination, wearing a face mask, social distancing (i.e., staying six feet away from others), avoiding crowds and poorly ventilated spaces, washing hands often, and cleaning and disinfecting high touch surfaces.⁵ Nonpharmaceutical interventions (NPIs), which are actions that persons and communities can take to help slow the spread of respiratory virus infections, often are the most readily available interventions to help slow transmission of infectious viruses in communities.⁶ Wearing personal protective equipment (PPE), such as face masks, have long been used in previous pandemics, epidemics and outbreaks as an important strategy to protect against the spread of infectious disease.⁷ Despite the ample evidence for the effectiveness of PPE to prevent transmission of respiratory diseases (e.g., seasonal flu, tuberculosis etc.), the U.S. and nations around the world continue to struggle to contain COVID-19 and avoid preventable suffering and deaths. The emergences of newer and more infectious variants such as the Delta and the Omicron have renewed questions about the role of masks to prevent the spread of COVID-19.⁸ These variants contain mutations that can affect the virus’s transmissibility and virulence, potentially impacting the effectiveness of vaccines, therefore increasing the need for public health officials to enact NPIs for help in containing viral spread.⁹ Additionally, Americans’ usage of face masks to curb the spread of COVID-19 has been, and continues to be, a controversial and politicized issue.¹⁰ Misinformation surrounding this topic has become a threat to the health of the public. Policy makers and public health officials urgently need high quality evidence on mask effectiveness and the use of masks by the general population to craft public and institutional guidance to contain SARS-CoV-2.¹¹ Previous literature reviews have shown that masks and facial coverings are efficacious at reducing risk of transmission in controlled laboratory settings.^{11, 12} However, evidence is needed for the ‘effectiveness’ of mask wearing in non-controlled settings that are generalizable to the real world.

Purpose

The efficacy of face masks has been studied in controlled environments, but there are gaps in research regarding how face mask effectiveness translates to the real world. Public health recommendations rely heavily on human behavior, therefore a review exploring the behavior of mask use in various ‘real world’ community settings will be beneficial to advancing our understanding of their level of protection. A comprehensive systematic review is needed to gather the evidence to inform mask wearing during the present pandemic and in the future. There are no literature reviews, to the author’s knowledge, that assess the behavior of mask wearing on the outcome of SARS-CoV-2 disease transmission in multiple real-world settings. While there have been studies on the efficiency of face masks against airborne transmission of infectious SARS-CoV-2 droplets/aerosols,¹³ a systematic review will be helpful to summarize what is known about mask effectiveness against COVID-19 in real world settings (i.e., outside laboratory and controlled settings).

This study followed the structure and recommendations for reporting a systematic literature review as outlined by the PRISMA guidelines.¹⁴ Inclusion criteria were: 1) studies written in or translated into English; 2) studies that assess SARS-CoV-2; 3) studies that assess the behavior of mask wearing; 4) studies that assess the outcome of SARS-CoV-2 disease transmission, and 5) studies conducted in non-laboratory or controlled settings such as health care settings, worksites, schools etc. Electronic databases were searched for relevant literature. Databases include SCOPUS, PubMed, and CINAHL. Gray literature such as conference abstracts, county data, and government reports were also identified through the following databases and methods: 1) ProQuest, 2) MedRxiv, 3) BioRxiv, 4) backward referencing by examining the references cited in articles that met inclusion criteria; 5) Google Scholar searches which were limited to first 100 results (i.e., first 10 pages). Studies were excluded if: 1) they did not parse out data specific to the effect of mask wearing and subsequent transmission; 2) they relied solely on statistical models to estimate the effects of mask wearing on transmission (i.e., no primary data were collected on behavior). The timespan for the literature search included all years available in the databases up to January of 2022.

Eligibility Criteria and Article Screening Process

Key terms were used to capture the relevant literature on mask wearing and disease transmission. The string of key terms include the following: ("sars cov 2"[MeSH Terms] OR "sars cov 2"[All Fields] OR "covid"[All Fields] OR "covid 19"[MeSH Terms] OR "covid 19"[All Fields]) AND ("masks"[MeSH Terms] OR "masks"[All Fields] OR "mask"[All Fields]) AND ("transmissibility"[All Fields] OR "transmissible"[All Fields] OR "transmissibilities"[All Fields] OR "transmissibility"[All Fields] OR "transmissible"[All Fields] OR "transmissibles"[All Fields] OR "transmission"[MeSH Subheading] OR "transmission"[All Fields] OR "transmissions"[All Fields]).

Database searches and gray literature searches were conducted independent of each other. After identifying articles from each database, duplicates were identified and removed before screening for inclusion criteria. Articles were then screened and evaluated first by title and abstract for meeting inclusion and exclusion criteria (phase 1) by a primary and secondary reviewer. Discrepancies between reviewers were resolved after a discussion following the initial screening. Articles that met criteria in phase 1 were further reviewed via the full text (phase 2). Final eligibility of articles was determined after the full-text review. The included articles were also explored for works cited by using the backward referencing technique to search for additional literature that met the inclusion criteria. For all articles that meet the inclusion criteria, relevant data and study characteristics were extracted (Table 1). These included: author, year, country, study design, outcome measures, study sample, main finding, adherence, duration, and study limitations. Results of the article screening process are presented in the PRISMA flow-diagram (Figure 1).

Risk of Bias Assessment

Two independent reviewers assessed the quality of included studies using Joanna Briggs Institute (JBI) critical appraisal tools.¹⁵ A third reviewer was consulted when discrepancies were identified. JBI critical appraisal checklists are considered suitable and the most preferred for scoring non-experimental study designs which make up the majority of studies included in this review.¹⁶Checklists for each study design varied but all evaluated criteria such as desciption of clinical condition, validity of measurement of disease outcome, appropriateness of statistical approaches, and mitigation of confounders. Each criteria was rated as red = “no”, green = “yes”, or yellow = “unclear.” Risk of bias was ranked as high when the study was rated with ≤ 49% of “yes” scores, moderate when the study was rated between 50 – 79% of “yes” scores, and low when the study was rated with ≥80% of “yes” scores.

A total of 2,240 articles and gray literature were identified, and 48 met the final criteria for inclusion. These studies all assessed SARS-CoV-2, the behavior of mask wearing, and the outcome of disease transmission in various settings. The majority of studies were from the United States (n=22)^17-38 while the rest were conducted in China (n=9),^{37, 39-46} Germany (n=4),^47-50 Japan (n=2),^{51, 52} Denmark (n=1),⁵³ Spain (n=1),⁵⁴ South Korea (n=1),⁵⁵ Pakistan (n=1),⁵⁶ Switzerland (n=1),⁵⁷ Finland (n=1),⁵⁸ Israel (n=1),⁵⁹ Greece (n=1),⁶⁰ Ethiopia (n=1),⁶¹ Thailand (n=1),⁶² and India (n=1).⁶³ Studies were conducted in healthcare settings (n=21), community and neighborhood settings (n=12), and in schools and universities (n=5; Table 1). Most studies were cross-sectional (n=22) followed by case-control (n=9), case reports (n=7), cohorts (n=6), and other study designs (n=3). COVID-19 infection was the outcome of interest and was primarily identified through lab-verified test results such as polymerase chain reaction (PCR) nasal swab tests (n=42).

Type of Mask

This review identified studies that evaluated various types of masks including cloth masks, respirators, disposable surgical masks, double-layer gaiters, N95 and KN95 masks. The majority of included studies did not address the specific type of masks used by participants (n=23). Studies that looked exclusively at surgical masks included Meylan et al.⁵⁷and Wang et al.³⁷ Both studies found that adherence to a mask policy within a hospital setting was associated with lower SARS-CoV-2 positivity.^{37, 57} Gras-Valentí et al.⁵⁴ also evaluated the impact of the continuous use of a surgical mask among HCWs. The accumulated incidence of COVID-19 among HCWs during the preintervention period until the implementation of the continuous use of a surgical mask was 22.3 for every 1000 HCWs, and the risk during the intervention period was 8.2 for every 1000 HCWs.⁵⁴

Li et al.²⁷ looked exclusively at cloth masks and the daily cumulative laboratory confirmed cases in New York City and COVID-19 transmissibility. They found that a cloth face covering was effective at reducing transmission capacity by 60% at the individual level for those susceptible to COVID-19.²⁷ Ranjan et al.⁶³ assessed N95 masks exclusively. They found that within a tertiary care setting, adhering to N95 mask fit checks were significantly associated (OR = 0.35, CI: 0.18 – 0.66, p<0.001) with the prevention of transmission of SARS-CoV-2 infection.⁶³ Thompson et al.³⁴ assessed implementation of universal masking with both surgical and cloth face masks. Among 250 potentially exposed patients and staff, 14 confirmed cases of COVID-19 were identified. All transmissions occurred among patients and staff with direct interactions or close contact without use of surgical, cloth, or N95 respirators.³⁴ Hendrix et al.²⁴ assessed the role of source control (i.e., masks) in preventing COVID-19 transmission in a hair salon. Types of face covering used by clients varied; 49 (47.1%) wore cloth face coverings, 48 (46.1%) wore surgical masks, five (4.8%) wore N95 respirators, and two (1.9%) did not know what kind of face covering they wore. When asked what the stylists wore, 64 (61.5%) reported that their stylist wore a cloth face covering (39; 37.5%) or surgical mask (25; 24.0%); 40 (38.5%) clients did not know or remember the type of face covering worn by stylists. They found that among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19 while both the stylists and the clients wore face masks, no symptomatic secondary cases were reported. Among 67 clients who tested for SARS-CoV-2, all test results were negative, suggesting that wearing a face mask was associated with reduced transmission independent of the type of mask.²⁴

Mask Use and Transmission of SARS-CoV-2

Of a total of 48 included studies, 44 (91.6%) showed that face masks played a role in reducing or preventing transmission of SARS-CoV-2. Each analysis demonstrated that, following directives from organizational and political leadership for universal masking policies, new COVID-19 cases fell significantly. Four studies showed that masks had no clear effect on reducing the wearers’ risk of infection.^{26, 31, 38, 53} Xie et al.³⁸ noted that mask mandates may not directly increase the adoption of mask wearing behavior in the public. Klompas et al.²⁶ assessed 3 case studies which all resulted in COVID-19 infection despite one or both parties wearing medical masks. Sasser et al.³¹ found no significant associations between COVID-19 incidence and face mask use during high school sports games (p-values > .05). Bundegaard et al.⁵³ assessed whether recommending surgical mask use outside the home reduces wearers' risk for SARS-CoV-2 infection in a setting where masks were uncommon. The recommendation to wear a surgical mask when outside the home did not reduce incident SARS-CoV-2 infection compared with no mask recommendation. In a per protocol analysis that excluded participants in the mask group who reported nonadherence (7%), SARS-CoV-2 infection occurred in 40 participants (1.8%) in the mask group and 53 (2.1%) in the control group (between-group difference, −0.4 percentage point [CI, −1.2 to 0.5 percentage point]; P = 0.40) (OR, 0.84 [CI, 0.55 to 1.26]; p = 0.40).⁵³

Healthcare Settings

Many studies took place in healthcare settings (n=21; 43.7%). Most of these articles demonstrated a positive association (44 of 48; 91.6%) of a mask wearing intervention on minimizing or controlling transmission of COVID-19 within a healthcare setting (Table 1). Wendt et al. (2020) investigated potential transmissions of a symptomatic SARS-CoV-2–positive physician in a tertiary-care hospital who worked for 15 cumulative hours without wearing a face mask. All high-risk contacts committed to wearing a face mask during work. All 254 potential contacts of the symptomatic positive index physician, 67 patients, and 187 nurses and doctors were assessed. They found that, after day 5, all tested persons were negative for COVID-19. Wang at el.⁵⁰ assessed the association of hospital masking policies with the SARS-CoV-2 infection rate among HCWs and found that universal masking within the hospital was associated with a significantly lower rate of SARS-CoV-2 positivity among HCWs. During the universal masking intervention period, the positivity rate decreased linearly from 14.65% to 11.46%, with a weighted mean decline of 0.49% per day and a net slope change of 1.65% (95% CI, 1.13%-2.15%; p< .001) decline per day compared with the preintervention period.⁵⁰ Seidelman et al.³² also found that universal masking within the healthcare setting resulted in a significant decrease in the cumulative incidence rate of healthcare-acquired SARS-CoV-2 infections among HCWs (log-rank test = 4.38, p= 0.03).³² Lastly, Ambrosch et al.⁴⁷ noted a large decline of nosocomial SARS-CoV-2 infections by almost 80% after the introduction of a strict mask requirement for all employees during the entire shift and the instruction to wear masks on patients.

Community and Neighborhood Settings

Studies were also conducted in various community and neighborhood settings (n=12; 25%). A prospective cohort study by Riley et al.³⁰ which assessed residents of Johnson County, Iowa who tested positive for COVID-19 found that mask use was significantly associated with lower secondary attack rates (SARs) (odds ratio [OR] 0.7, 95% CI 0.57–0.84), and that mask use by both the case-patient and the close contact reduced the SAR by half, from 25.6% to 12.5%. Shaweno et al.⁶¹ conducted SARS-CoV-2 serosurveys, a survey designed to measure the seroprevalence of SARS-CoV-2 antibodies among individuals, in overcrowded neighborhoods in Dire Dawa, Ethiopia. They found that individuals who reported wearing a mask reduced had lower risk of seroprevalence. They observed 4.5 times higher prevalence of SARS-CoV-2 antibodies among individuals who did not wear facemasks, 10.1% (95% CI 4.1, 16) prevalence compared to 2.3% (95% CI 1.0, 4.0) prevalence among individuals who reported wearing a face mask when leaving home. Similarly, they found that not using face masks while leaving home showed significant association with SARS-CoV-2 seroprevalence.⁶¹ Comparably, Cheng et al.⁴⁰ assessed the effect of community-wide mask usage to control COVID-19 in Hong Kong Special Administrative Region (HKSAR), a densely populated cosmopolitan city in China. The compliance of face mask usage by HKSAR in the general public was 96.6% (range: 95.7% to 97.2%), which was measured by direct observation. This study observed 11 COVID-19 clusters in recreational ‘mask-off’ settings compared to only 3 in workplace ‘mask-on’ settings (p = 0.036, Chi square test of goodness-of-fit).⁴⁰

Schools and University Settings

A small number of studies took place in schools and universities (n=5; 10.4%). One study was conducted in a university setting and four were conducted in K-12 schools. Schools and universities have been an area of concern throughout the pandemic due to the inherent difficulty in maintaining physical distance while still providing instruction. Four out of five studies in school-based settings showed that mask policy and/or requirements is an important strategy in decreasing COVID-19 case rates and keeping students, staff, faculty, and visitors safe. Jehn et al.²⁵ found that the odds of a school-associated COVID-19 outbreak in schools with no mask requirement were 3.7 times higher than those in schools with an early mask requirement. Gettings at al.²² noted that in Georgia K-5 schools, COVID-19 incidence was 37% lower in schools that required teachers and staff members to use masks and 39% lower in schools that improved ventilation, compared with schools that did not use these prevention strategies. Budzyn et al.²⁰ found that counties throughout the U.S. with mask requirements for K-12 schools (16.32 cases per 1000,000 children) had a significantly lower number of daily cases of pediatric COVI-19 (18.53 per 100,000 children; β = −1.31; 95% CI = −1.51 to −1.11; p<0.001) than countries with no mask requirements (34.85 per 100,000 per day).

Models

Four of the included studies used a combination of real-world data plus a variation of a compartmental model to analyze the effectiveness of masking interventions on case growth and death rates (n=4). Li et al.²⁷ used the effective reproduction number (Rt) to evaluate the effectiveness of multiple interventions. It was found that after social distancing, the Rt value was reduced by 68%, while after the mask recommendation, it was further reduced to about 59.8%. Zhang et al.⁴⁶ simulated the spread of COVID-19 in Hong Kong, using actual demographic and geographic data, as well as case reports issued by the Centre for Health Protection of Hong Kong, to analyze the efficiency of several main NPIs. Using an improved agent-based SEIR model, they found that wearing a mask in public places can reduce the infection risk via the close contact route (i.e., short-range airborne transmission). A key finding was that if all people could wear masks in all public indoor environments, the total infection risk could be reduced by 32.5%.⁴⁶

Other Community Settings

Other settings which were studied included households (n=2),^{28, 37} airplanes (n=2),⁵² a hair salon (n=1),²⁴ and an indoor sports facility (n=1).⁴⁹ A study by Wang et al.³⁷ which examined face masks within households found that face mask use by the primary case and family contacts before the primary case developed symptoms was 79% effective in reducing transmission (OR=0.21, 95% CI 0.06 to 0.79). Similarly, in another study of household transmission, it was found that transmission was significantly lower in households in which the index case wore a face mask compared with those did not [17% (7%–37%) vs. 48% (31%–66%), P = 0.02].²⁸ Toyokawa et al.⁵² investigated the association between the use of face masks and COVID-19 among passengers and flight attendants exposed to a COVID-19 passenger in a domestic flight. Risk factors for infection included not using a face mask (adjusted odds ratio [aOR]: 7.29, 95% CI 1.86‐28.6) and partial face mask use (aOR: 3.0, 95% CI: 0.83-10.8). Nonuse of face masks was identified as an independent risk factor for contracting COVID-19 on the airplane.

Risk of Bias Assessment

Table 2 – Table 8 shows the risk of bias assessment across all studies included. Seven JBI checklists based on study design were used to assess bias. Risk of bias was classified as low in 1 included study, moderate in 10 studies, and low in 36 studies. In case reports (n=5), there were biases in the description of adverse events (n=3; Table 2). Both the quasi-experimental and RCT studies included in this review were classified as moderate bias due to unclear allocation procedures, blinding issues, and inadequate description of follow up (Table 4 and Table 8). Among the 6 cohort studies included, the majority (83%) were rated as having unclear descriptions of follow up and lacking reasons for loss to follow up (Table 5). Almost all case-control studies (89%) were rated as low risk of bias (Table 6). While the majority of cross sectional studies (95%) were classified as low risk of bias, almost all studies were unreliable in their measurement of SARS-CoV-2 infection as many patients self-reported their COVID-19 status (Table 7).

This systematic literature review identified and described 48 studies that focused on the behavior of mask wearing and the outcome of SARS-CoV-2 disease transmission in various community and real-life settings. Various types of masks were reported, and heterogeneity was seen in study designs and settings. A large majority of the studies (44 of 48; 91.6%) indicated that mask wearing reduced the risk of transmission of COVID-19. Overall, the evidence overwhelmingly supports that using face masks is effective at slowing or preventing the transmission of SARS-CoV-2 infection in a variety of real-world settings. Broader implementation of face covering policies and adherence to mask wearing can and does mitigate the spread of infection in the general population.²⁴ The results of this systematic literature review are important for public health practice given that the data are more generalizable to real world settings. Additionally, mask mandates and recommendations do help limit community transmission of COVID-19 and subsequent cases and death rates.²³ The results of this review align with the CDC’s recommended guidance on wearing masks to protect the individual and the broader community against the spread of COVID-19.⁶⁴

The evidence of this review is also in line with previous systematic review and meta-analysis which also showed that wearing a face mask reduces the risk of COVID-19 infection.⁶⁵ One meta-analysis found that wearing a mask is very effective in preventing the spread of COVID-19 (OR = 0.38, 95% CI: 0.21-0.69).⁶⁵ Inclusion and exclusion criteria were very similar; however, Li et al.⁶⁵ primarily included studies that assessed mask wearing effectiveness in healthcare workers and incorporated a study design with a comparative group (i.e., case-control studies). The current systematic review included studies from community settings regardless of comparator condition to assess the impact of mask mandates and policies.

Four of the included studies did not find a significant benefit for wearing face masks. The lack of a significant benefit may have been due to relatively low rates of COVID-19 circulating in Wisconsin during September 2020.³¹ Likewise, Xie et al.³⁸ did not find that a mask mandate reduced SARS-CoV-2 transmission. Their study found that mobility restricting policies (i.e., stay-at-home orders) may have had a larger effect on reducing transmission than masks. In these studies, data on self-reported mask wearing were not available which may have limited the ability to evaluate the effect of masking compared to other mitigations measures. Bundegaard et al.⁵³ did not test the role of masks in source control of SARS-CoV-2 infection, rather it only tested the wearers’ risk. These methodological limitations may partially explain the non-significant findings for the effectiveness of mask wearing on the prevention of SARS-CoV-2 transmission.

Limitations

This study followed a rigorous and systematic approach using PRISMA guidelines to identify relevant studies and three independent reviewers followed a strict protocol for assessing inclusion and abstracting study data. The results are largely consistent and provide compelling support for an association between mask wearing and a decreased risk of COVID-19 infection. However, a meta-analysis was not performed to directly attribute the effect of mask wearing to transmission of SARS-CoV-2. Considering the heterogeneity of settings, types of masks, study designs, and statistical methods in the identified articles, it was challenging to identify data from all studies that could allow for meaningful comparisons. Furthermore, several studies used multiple strategies to prevent infection including social distancing, ventilation, and policy-level mandates. This was to be expected since public health recommendations for the prevention of SARS-CoV-2 include the use of several strategies in combination. Other factors may have influenced disease transmission such as vaccination status, individual adherence to mask recommendations, social distancing behavior, and hygiene practices. However, articles for this review were excluded if they did not parse out data specific to the effect of mask wearing and subsequent transmission.

Another limitation was that mask adherence was not assessed in many of the included studies. Only a small number of these studies (n=8) reported measuring adherence to mask wearing (n=4) through self-report surveys (n=4) and via observations and publicly available sources (n=4). Additionally, many included studies (n=23) did not specify the type of masks used. Studies that did report mask type were heterogeneous in the types of masks used. The efficacy of masks varies considerably across different types, and real-world effectiveness is likely to show similar variation. This is an important note as adherence is essential for policy effectiveness.²¹

Furthermore, only one randomized controlled study design was identified. This is not surprising given ethical concerns that would be raised in conducting such studies in the context of a new and deadly respiratory disease. In addition, the large heterogeneity of study designs and data from the studies would preclude from conducting a meaningful meta-analysis. Lastly, due to the newness of COVID-19 research, the initial gray literature search may have excluded articles that may have met inclusion criteria. There was likely some research that has yet to be translated into English and was thus not included in this study.

Evidence from this review strongly supports the role of masking policies and mask wearing in prevention of the spread of SARS-CoV-2 in various real-world settings. Additionally, there is evidence that various types of masks can reduce the transmission of COVID-19. There is a need for consistent and clear public health messaging that reaches a wide audience and that communication addresses health literacy. Also, there is a need to invest in providing an environment in which people can easily engage in risk-reducing behaviors (such as mask wearing). This can include providing high quality masks to those who otherwise do not have access to them or would not actively seek to obtain one on their own, and by providing a positive and supportive social environment for mask wearing.

Ethics approval and consent to participate: not applicable

Consent for publication: not applicable

Availability of data and materials: All data from the current study are available from the corresponding author on reasonable request.

Competing interests: The authors declare that they have no competing interests

Funding: Dr. Crespo received support from Family Health Centers of San Diego’s Scientist in Residence Program sponsored by The Conrad Prebys Foundation.

Authors’ contributions: NC conceptualized the current study and made substantial contributions to data acquisition, writing, and revision of the work. JF led acquisition of data and interpretation. PD assisted with acquisition of data, risk of bias assessment, and revision of paper drafts. JG, CR, and JE all interpreted study findings, and substantively revised initial and final drafts of the paper. All authors read and approved the final manuscript.

Acknowledgements: Not applicable.

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Effectiveness of Face Masks in Preventing COVID-19 Transmission in Real-World Settings: A Systematic Literature Review

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