key: cord-0880054-fpqgs6q0
authors: Haller, S.; Guesewell, S.; Egger, T.; Scanferla, G.; Thoma, R.; Leal-Neto, O. B.; Flury, D.; Brucher, A.; Lemmenmeier, E.; Moeller, J. C.; Rieder, P.; Ruetti, M.; Stocker, R.; Vuichard-Gysin, D.; Wiggli, B.; Besold, U.; Kuster, S. P.; McGeer, A.; Risch, L.; Schlegel, M.; Friedl, A.; Vernazza, P.; Kahlert, C.; Kohler, P.
title: Use of respirator vs. surgical masks in healthcare personnel and its impact on SARS-CoV-2 acquisition: a prospective multicentre cohort study
date: 2021-06-01
journal: nan
DOI: 10.1101/2021.05.30.21258080
sha: 26bf1d084cd7f5c3b66bacbec5be9e0b3ae71775
doc_id: 880054
cord_uid: fpqgs6q0

Background There is insufficient evidence regarding the role of respirators in the prevention of SARS-CoV-2 infection. We analysed the impact of filtering facepiece class 2 (FFP2) vs. surgical masks on the risk of SARS-CoV-2 acquisition among Swiss healthcare workers (HCW). Methods Our prospective multicentre cohort enrolled patient-facing HCWs from June to August 2020. Participants were asked about COVID-19 risk exposures/behaviours, including preferred mask type when caring for COVID-19 patients outside of aerosol-generating procedures (AGP). For those performing AGPs, we asked whether they used FFP2 irrespective of the patients COVID-19 status (i.e. universal use). The impact of FFP2 on i) self-reported SARS-CoV-2-positive nasopharyngeal PCR/rapid antigen tests captured during weekly surveys, and ii) SARS-CoV-2 seroconversion between baseline and January/February 2021 was assessed. Results We enrolled 3259 participants from nine healthcare institutions, whereof 716 (22%) preferentially used FFP2 respirators. Among these, 81/716 (11%) reported a SARS-CoV-2-positive swab, compared to 352/2543 (14%) surgical mask users (median follow-up 242 days); seroconversion was documented in 85/656 (13%) FFP2 and 426/2255 (19%) surgical mask users. Adjusted for baseline characteristics, COVID-19 exposure, and risk behaviour, FFP2 use was non-significantly associated with a decreased risk for SARS-CoV-2-positive swab (adjusted hazard ratio [aHR] 0.8, 95% CI 0.6-1.0, p=0.052) and seroconversion (adjusted odds ratio [aOR] 0.7, 95% CI 0.5-1.0, p=0.053); household exposure was the strongest risk factor (aHR for positive swab 10.1, p<0.001; aOR for seroconversion 5.0, p<0.001). In subgroup analysis, FFP2 use was clearly protective among those with frequent (>20 patients) COVID-19 exposure (aHR 0.7, p<0.001; aOR 0.6, p=0.035). Universal FFP2 use during AGPs showed no protective effect (aHR 1.1, p=0.7; aOR 0.9, p=0.53). Conclusion Respirators compared to surgical masks may convey additional protection from SARS-CoV-2 for HCW with frequent exposure to COVID-19 patients.

The most common mode of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS- transmission is believed to be via respiratory droplets and -probably less important -via fomites. 1, 2 The role of microdroplets or aerosols, which might travel beyond one to two meters from an infected person, as a potential mode of SARS-CoV-2 transmission has been heavily debated. [3] [4] [5] [6] Reports from healthcare and non-healthcare settings suggest that SARS-CoV-2 may indeed be transmitted via aerosols, particularly in poorly ventilated indoor environments, even outside of so-called aerosol-generating procedures (AGPs). 7, 8 Healthcare workers (HCW) remain at the front line, with a high risk of exposure to and infection with SARS-CoV-2. 9 For HCW involved in AGPs, international guidelines unanimously recommend the use of so-called respirators, which include filtering facepiece class 2 (FFP2), N95, or KN95, and which have the ability to filter microparticles. In contrast to respirators, surgical (also termed medical) masks are designed to provide barrier protection and only block larger respiratory droplets. As a consequence of the conflicting opinions about aerosol transmission, guidelines differ regarding recommendations for the use of respirators outside of AGPs. Whereas the Centers of Disease Control and Prevention (CDC) and the European Centre for Disease Prevention and Control (ECDC) recommend respirators if available, the World Health Organization (WHO) and the Swiss National Centre for Infection Prevention (Swissnoso) recommend surgical masks. [10] [11] [12] [13] The Infectious Diseases Society of America's recommendation is to use either a surgical mask or a respirator. 14 Prospective head-to-head comparisons evaluating the protective effect of these mask types against SARS-CoV-2 acquisition are sparse. In a meta-analysis including mostly studies on non-SARS-CoV-2 coronaviruses, use of respirators was associated with a stronger protective effect, although no study directly compared respirators to surgical masks. 15 A meta-analysis comparing the clinical effectiveness of respirators to surgical masks for other respiratory viruses, including coronaviruses, found no significant difference concerning infection risk in HCW. 16 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint For SARS-CoV-2, we identified three studies not covered in the above-mentioned meta-analysis.

An online survey among HCW from different countries showed a protective effect of respirators compared to surgical masks for those performing AGPs on Coronavirus Disease 2019 (COVID- 19) patients. 17 A cross-sectional study from the US (in preprint) found respirator use to be associated with decreased seropositivity rate, although no multivariate analysis was perforned. 18 In a prospective single-centre HCW cohort from Western Switzerland, respirators were protective regarding SARS-CoV-2 seroconversion, although use of other PPE was not documented and residual confounding was suspected. 19 To summarize, there is currently insufficient evidence to determine if the use of FFP2 respirators reduces the risk of SARS-CoV-2 infections. In this analysis of prospective cohort data from Swiss HCWs, we sought to assess the effectiveness of FFP2 compared to surgical masks regarding SARS-CoV-2 protection for HCW involved in patient care.

We performed a prospective observational multicentre cohort consisting of employees (aged 16 years or older) from different healthcare institutions in Northern and Eastern Switzerland (from four different cantons). All hospital employees irrespective of involvement in patient care were eligible to participate; however, for the current analysis we excluded those without patient contact. Employees registered online and provided electronic consent. Enrolment took place from June 22 nd to August 15 th 2020, between the end of the first COVID-19 wave in Switzerland and the surge of the second wave; 20 data were analysed up to March 9 th 2021, when the second wave had abated (Figure 1) . The study was approved by the ethics committee of Eastern Switzerland (#2020-00502).

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint During the study period, a national policy required Swiss residents (including HCW) to wear at least a surgical mask at work. The Swiss National Centre for Infection Prevention (Swissnoso) suggests the use of a respirator mask only while performing AGPs on confirmed or suspected COVID-19 patients. 13 However, this was considered minimum standard and institutions were free to recommend respirators outside of AGPs. Also, in most institutions HCW could make the personal choice of wearing a respirator or surgical mask at work. To characterize institutionlevel mask recommendation, we conducted a survey of representatives from participating institutions asking about local policies for FFP2 use and estimated compliance with those policies (among a choice of "good", "more FFP2 used than recommended", "less FFP2 used than recommended)".

The study timeline is shown in Figure 1 . Upon inclusion, participants answered a baseline questionnaire asking about anthropometric data, pregnancy and comorbidities, job description (including full-time equivalent [FTE] percentage, profession, involvement in AGPs, working in intensive care, exposure to COVID-19 patients, use of PPE, or visit to staff restaurant), and nonwork related risk behaviour such as wearing a mask outside of work, leisure and shopping activities, but also opinions on adequacy of regulatory measures. During follow-up, participants received weekly text messages and emails with an electronic link to a questionnaire where they indicated results of nasopharyngeal swabs (polymerase chain reaction or rapid antigen tests) for SARS-CoV-2. Additionally, participants answered whether they had been exposed to confirmed COVID-19 patients, co-workers, household contacts, or other COVID-19 cases during the previous week. In January 2021, (i.e. before follow-up serology was performed), a follow-up questionnaire asked about use of mask type (FFP2 vs. surgical mask) outside of AGPs. For those involved in care of COVID-19 patients, this question explicitely asked about mask use during COVID-19 patient contact. AGPs were defined as bronchoscopies, in-/extubation, gastroscopy, . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint transesophageal echocardiography, reanimation, non-invasive ventilation, and suction of tracheal secretions. Participants had the choice among "Use of surgical mask only"; "Mostly use of surgical mask"; "Equal use of both mask types"; "Mostly use of FFP2"; "Use of FFP2 only". For the purpose of this analysis, the two latter categories were classified as "Mostly FFP2", whereas the first three categories were merged into the group "Not mostly FFP2" (for better comprehensibility termed "Mostly surgical masks"). For HCW involved in AGPs, we also asked whether they always used FFP2 during AGPs, irrespective of the patient's COVID-19 status (i.e. termed "universal FFP2 use"). Furthermore, use of other PPE including gowns, gloves and goggles while caring for COVID-19 patients was asked, as well as the number of COVID-19 patients and of positive co-workers HCW had been knowingly exposed since March 2020 (for description and categorisation of variables, see Table S1 ). 

For analysis of nasopharyngeal swabs, we performed Cox regression with time intervals between consecutive weekly questionnaires as response. Intervals were censored as long as no positive swab (event) was reported, and those following the first event were excluded. The model . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint included COVID-19 exposures reported in any of the three weekly questionnaires submitted before the end of each time interval as well as the cumulative number of negative swabs up to an interval's end (to account for different testing behaviour) as time-dependent co-variables, and answers from the baseline and follow-up questionnaire (including mask type) as timeindependent co-variables. These variables were a priori chosen from the baseline questionnaire, 20 based on their expected potential to confound the association between mask use and risk for SARS-CoV-2 acquisition. Cantons and institutions were included as cluster terms.

For seroconversion, we used logistic regression including the same time-independent covariables as for nasopharyngeal swabs as fixed effects and cluster terms as random effects.

Instead of the time-dependent covariables, we included overall household exposure (summarizing weekly reports into "any" vs. "none") as well as the total number of COVID-19 patient and co-worker exposures. Variance inflation factors (VIF) were calculated to check that no co-variable had a VIF>5. R statistical software Version 4.0.2 was used for all statistical analyses.

We performed three sensitivity analyses to assess the influence of possible confounders on the estimated effect of mask type on the two outcomes: (1) excluding HCW with COVID-19 cases within their household (which might reduce the importance of professional exposure and protection), (2) including cantons and institution as fixed effects (to account for regional incidence and institutional factors), and (3) excluding HCW tested positively before December 1 st 2020 (because mask use may have changed through time and was asked only in January 2021).

Because mask type was likely to be most important for HCW with frequent COVID-19 exposure, we performed a subgroup analysis according to frequency of COVID-19 patient contact (no known contact vs. 1-20 patients vs. >20 patients since March 2020). Also, we . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint repeated the analysis for those performing AGPs, using a model including whether they always used FFP2 during AGPs.

We included participants from seven acute care institutions (with 14 different sites), one rehabiliation clinic, and three psychiatry clinics (analysed as one institution). Policies on respiratory protection according to institution are summarized in Table 1 . Most institutions followed the Swissnoso recommendations for use of FFP2 respirators. Actual FFP2 use varied considerably between institutions and ranged from 3% to 52% of participants. All acute care institutions with less strict local guidelines (i.e. no FFP2 mask required during contact with COVID-19 patients) reported that FFP2 masks were more frequently used than recommended ( Table 1) .

Among the 3'259 participants, 614 (19%) were male and median age was 39 years (interquartile range [IQR] 30-49 years). Most were nurses (n=1'724, 53%), followed by physicians (n=671, 21%). Preferential use of FFP2 while caring for COVID-19 patients was reported by 716 (22%).

HCW who preferred using FFP2 were more likely to be male (OR 1·5, p<0·001), to be ≤ 50 years old (OR 1·4, p=0·001), to support stronger public restrictions regarding the pandemic (OR 1.7, p<0·001), to be involved in AGPs (OR 4·2, p<0·001), to work in intensive care (OR 8·4, p<0·001), to be exposed to >20 COVID-19 patients (OR 2.8, p<0·001), to use gowns (OR 8·7, p<0.001), gloves (OR 2·5, p<0.001) and goggles (OR 8·0, p<0.001) while caring for COVID-19 patients, and to undergo testing for SARS-CoV-2 (OR 1·4, p<0·001) ( Table 2) .

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021.

Median follow-up was 242 days, both for respirator and for surgical mask users (Wilcoxon test, p=0·49). The number of self-reported positive SARS-CoV-2 tests was 81/716 (11%) for FFP2 users compared to 352/2543 (14%) in users of surgical masks (hazard ratio [HR] 0·8; 95% confidence interval [CI] 0·6-1·0; p=0·06 log-rank test). In the Cox regression model, the factor most strongly associated with a positive SARS-CoV-2 test was exposure to a positive household contact (adjusted HR [aHR] 10·1, 95% CI 7·5-13·5, p<0·001). Use of FFP2 while caring for COVID-19 patients was associated with a decreased risk of acquiring SARS-CoV-2 (aHR 0·8, 95% CI 0·7-1·0, P=0·052) (Figure 2, Table S2 ). In sensitivity analyses, restriction to data collected after December 1 st (aHR 0·7, p=0.03) showed similar results; removing participants with a positive household member (aHR 0·8, p=0.31) and treating institutions/cantons as fixed effect (aHR 0·9, p=0.43) resulted in non-significant associations ( Table S2) .

We included 2'916 HCW with negative baseline serology, who had a second serology performed in January/February 2021. Seroprevalence was 12·9% (85/658) for FFP2 users compared to 18·9% (429/2258) for users of surgical masks (OR 0·6, 95% CI 0·5-0·8, p<0·001). In multivariable analysis, the strongest risk factor for seroconversion was again having a positive household member with an adjusted odds ratio (aOR) of 5·0 (95% CI 3·9-6·5, p<0·001). FFP2 use was non-significantly associated with decreased risk for seroconversion (0·7, 95% CI 0·5-1·0, p=0.053) (Figure 3, Table S3 ). In sensitivity analyses, removal of variable on household exposure (aOR 0·7, p=0.046) and including cantons/institutions as fixed effects (aOR 0·8, p=0.088) did not significantly change the point estimates for FFP2 use nor the significance levels (Table S3) .

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint HCW with frequent (>20) exposure to COVID-19 patients were less likely to report a positive SARS-CoV-2 swab when mostly wearing FFP2, with an unadjusted HR of 0·6 (p<0·001) compared to 0·8 (p=0·18) for those exposed to 1-20 patients (Figure 4) . Results of multivariable analyses were similar to those in univariable analysis, both for positive swabs (aHR 0·7, p<0·001, vs. aHR 1·1, p=0·77) and seroconversion (aOR 0·6, p=0·036, vs. aOR 0·8, p=0·32) ( Table S4 ). The number of positive swabs for HCW without COVID-19 patient contact was too small to perform multivariable analyses (2 events among 40 FFP2 users, 29 events among 480 surgical mask users).

For HCW performing AGPs, universal use of FFP2 during AGPs (irrespective of the patients COVID-19 status) showed no effect (aHR 1·1, p=0·66 and aOR 0·9, p=0·54, respectively) ( Table   S5 ).

In this prospective multicentre HCW cohort, FFP2 use outside of AGPs was marginally associated with a decreased risk for SARS-CoV-2 infection when compared to surgical masks.

The effect was however pronounced for those with frequent COVID-19 patient exposure. Using FFP2 irrespective of the patient's COVID-19 status did not provide additional protection for HCWs involved in AGPs. The large sample size, the dual approach for outcome assessment, and consideration of a variety of potential confounder variables (including personal risk factors, use of other PPE, and general risk perception) are among the strengths of this study. This is, to our knowledge, the first prospective multicentre study comparing the effect of respirators and surgical masks regarding protection from SARS-CoV-2. In Switzerland, national guidelines do not recommend the use of respirators for HCW caring for COVID-19 patients outside of AGPs; at the same time institutional guidelines are heterogeneous. This unique setting allowed us to study this important research question. The overall association between FFP2 use . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint and risk for SARS-CoV-2 infection was just marginally non-significant. This and the inconsistencies seen in our sensitivity analyses are probably a reflection of the heterogeneous study population, which consisted by two thirds of HCWs with only sporadic (or even no known) COVID-19 exposure.

However, for HCW with more frequent exposure, we found a clearly significant protective effect associated with FFP2 use. Several reports suggest that aerosol transmission is indeed a nonnegligible mode of SARS-CoV-2 transmission and that respirator masks may provide additional protection compared to surgical masks. [17] [18] [19] 21, 22 On the other hand, single case reports have suggested that surgical masks are equivalent to respirators in protecting HCW from SARS-CoV-2 infection. 23, 24 These supposedly contradictory findings can be reconciled when considering a particular feature of SARS-CoV-2, namely its high overdispersion. 25 Overdispersion describes the highly variable transmissibility of infected individuals; in other words, only a minority of infected individuals actually transmit the virus to others, often within so-called superspreading events. As a consequence, the probability of being exposed but not infected -as seen in the case reports above -is relatively high (irrespective of mask type). Supporting this hypothesis, a simulation study by Chen et al. describes how the SARS-CoV-2 viral load of infectious individuals can vary largely and how this influences infection probability and the effectiveness of the different mask types. 26 Notably, we did not observe any protective effect of FFP2 for HCW performing AGPs in the absence of any COVID-19 suspicion in the patient. We extrapolate from these findings that universal FFP2 use in the hospital setting, where the average risk exposure is usually lower than during AGPs, does not provide additional protection compared to surgical masks. We acknowledge however that in settings with a high proportion of undiagnosed, asymptomatic or presymptomatic patients, an additional benefit through universal FFP2 use cannot be excluded.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 17, 19 Given the fact that many HCWs in our study did not consistently wear either FFP2 or surgical masks, we assume that the protective effect of FFP2 might be even higher in reality. However, the clinical significance of the protective effect mediated by FFP2 use can be questioned, given the dominating impact of extra-occupational risk exposures on the COVID-19 risk of HCWs, as seen in other studies. 27 Also, the disadvantages of respirators -the discomfort of wearing such masks over long periods of time, the fact that their protective effect can be diminished without prior HCW training and fit testing (which was indeed not done in most of our institutions), as well as their cost -have to be considered when assessing the net benefit of FFP2 over surgical masks. 28, 29 To adjust for potential confounding, we included the use of gloves, gowns and goggles in our multivariable analysis. None of these measures were associated with any clear additional protective effect. Further studies, ideally randomized controlled trials, are needed to evaluate the role of these components of PPE in SARS-CoV-2 protection. Other associations with SARS-CoV-2 infection found in our study, such as the "protective" effect of active smoking or the increased risk associated with working as a nurse, have been discussed earlier 20 .

Our study has limitations. First, residual confounding is possible. Yet, we have included multiple co-variables accounting for risk exposures and risk behaviours. Also, the fact that use of other PPE or universal respirator use among HCW performing AGPs (representing HCW with particularly risk-averse behaviour) were not associated with reduced seroconversion rate, supports our argument of a valid multivariable model with low risk of residual confounding.

Second, the question about respirator use was asked in a questionnaire in January 2021, at a time when most SARS-CoV-2 positive participants had already had their infection. A positive test result could have led to a change in preferred mask type (in either direction). However, . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint restricting the analysis to the time period close to the follow-up questionnaire showed similar or even stronger assocations compared to the full model. Third, we did not specifically ask about duration of contact to individual COVID-19 patients, although type of profession, work percentage, or involvement in AGP can be regarded as proxy for this potentially important variable. Fourth, although we included multiple institutions, settings, and geographical regions in our study, the generalizability of the results can be questioned due to the fact that participation in the study was non-mandatory. However, distribution of key variables (e.g. age, sex, profession)

were similar between the total HCW population (from the largest participating institution) and the cohort population. 20 To conclude, FFP2 use outside of AGPs is associated with a reduced risk of SARS-CoV-2 acquisition for HCW with high COVID-19 exposure, while those with only sporadic or no known contact do not seem to benefit. The effect size should be interpreted in the context of the global COVID-19 risk for HCW, which is driven by exposure to positive household contacts. No were involved in data collection; TE and LR were responsible for laboratory analyses; SH, SG, TE, CRK and PK were responsible for data analysis; SH, GS and RT performed the literature search; SH and SG were responsible for creation of figures; SH, SG, TE, MS, CRK and PK interpreted the data; CRK and PK supervised the study.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint Table 2 . Factors associated with preferential use of FFP2 vs. surgical masks among 3'259 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted June 1, 2021. ; https://doi.org/10.1101/2021.05.30.21258080 doi: medRxiv preprint

Pathophysiology, Transmission, Diagnosis, and Treatment of Coronavirus Disease 2019 (COVID-19): A Review

Low risk of SARS-CoV-2 transmission by fomites in real-life conditions

Ten scientific reasons in support of airborne transmission of SARS-CoV-2

It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19)

Risk of SARS-CoV-2 transmission by aerosols, the rational use of masks, and protection of healthcare workers from COVID-19

Use of medical face masks versus particulate respirators as a component of personal protective equipment for health care workers in the context of the COVID-19 pandemic

COVID-19 Outbreak Associated with Air Conditioning in Restaurant

Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals

Risk of COVID-19 in health-care workers in Denmark: an observational cohort study

Updated Healthcare Infection Prevention and Control Recommendations in Response to COVID-19 Vaccination

European Centre for Disease Prevention and Control ECDC. Infection prevention and control and preparedness for COVID-19 in healthcare settings, Sixth update -9

Mask use in the context of COVID-19, Interim guidance

Swiss National Centre for Infection Prevention. Interims Vorsorgemassnahmen in Spitälern für einen hospitalisierten Patienten mit begründetem Verdacht oder mit einer bestätigten COVID-19 Infektion, Version 8.7, Swissnoso

Infectious Diseases Society of America Guidelines on Infection Prevention for Healthcare Personnel Caring for Patients With Suspected or Known Coronavirus Disease

Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis

Medical masks vs N95 respirators for preventing COVID 19 in healthcare workers: A systematic review and meta analysis of randomized trials

Assessing coronavirus disease 2019 (COVID-19) transmission to healthcare personnel: The global ACT-HCP case-control study

Seroepidemiology among Employees of New York City Health and Hospitals during the First Wave of the SARS-CoV-2 Epidemic. Occupational and Environmental Health

Severe acute respiratory coronavirus virus 2 (SARS-CoV-2) seroconversion and occupational exposure of employees at a Swiss university hospital: A large longitudinal cohort study

Non-occupational and occupational factors associated with specific SARS-CoV-2 antibodies among hospital workers -A multicentre crosssectional study

Long-distance airborne dispersal of SARS-CoV-2 in COVID-19 wards

Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 during Border Quarantine and Air Travel

COVID-19 and the Risk to Health Care Workers: A Case Report

Aerosolized SARS-CoV-2 transmission risk: Surgical or N95 masks?

Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China

Face masks effectively limit the probability of SARS-CoV-2 transmission

healthcare workers.

Frequency of the factor (n and % with it) in relation to FFP2 use during contact with COVID- 19