Universal use of face masks for success against COVID-19: evidence and implications for prevention policies

To the Editor:

A debate is taking place on the use of face masks (both cloth and surgical) as a prevention tool in the community vis à vis the recent World Health Organization (WHO) recommendations. To shed light on this important topic, we reviewed relevant literature, focusing on the keywords “infection control”, “prevention”, “masks”, “respirators”, “viral infections” and “COVID-19”, without time restrictions to identify a minimum set of references from an electronic database (PUBMED), existing guidelines, viral diseases, airborne diseases and grey literature.

The core findings of the references identified are summarised in table 1.

According to the WHO report published in February 2020, the proportion of truly asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection was relatively small and was not a major driver of virus circulation, infection transmission and new disease development [15]. However, in blanket testing of an isolated village of ∼3000 people in northern Italy, 50–75% of people with positive pharyngeal molecular tests were totally asymptomatic [1]. This finding was echoed by a more recent daily surveillance report from China, where all people arriving from overseas were rigorously tested: among 166 persons with newly identified infections, 78% were asymptomatic [2]. Although the infective dose associated with transmission is not known, the viral load in the respiratory tract in an asymptomatic patient has been reported to be similar to patients with symptoms [3], and transmission of SARS-CoV-2 infection from an asymptomatic contact has also been described [4].

Until recently, it was thought that the virus could only be transmitted by droplets that are coughed or sneezed out or by contaminated fomites, with differences according to the initial load and surface characteristics [5]. Airborne transmission of SARS-CoV-2 was considered possible only when care procedures generating aerosols (e.g. intubation, bronchoscopy and positive-pressure ventilation) were performed [6]. However, other studies seemed to indicate the opposite, i.e. that the virus is present in exhaled air produced by talking and breathing [6]. Moreover, a potential role for aerosols in virus diffusion was evident in a complex laboratory study. Aerosols containing a viral load quite similar to that observed in human respiratory samples were created to generate an aerosolised environment. SARS-CoV-2 was detected ≤3 h after the start of the study [7]. Although these findings were not considered fully convincing by some authors [5], they deserve attention and further studies are required to establish whether and when airborne transmission of SARS-CoV-2 truly occurs and how it can be reduced.

It is well known that surgical masks can prevent the inhalation of large droplets and sprays but have limited ability to filter submicron-sized airborne particles [8, 9]. As SARS-CoV-2 is also embedded in aerosols <5 μm in diameter, it cannot be determined whether they are always effective. However, mask wearing by patients with pulmonary tuberculosis (an airborne infectious disease) has been shown to reduce infectivity to guinea pigs by 56% [9, 10]. The surgical mask has also been shown to intercept other human coronaviruses during coughing [11]. A meta-analysis of randomised controlled trials has also shown that surgical masks and N95 respirators were similarly effective in preventing influenza-like illness and laboratory-confirmed influenza among healthcare workers [12]. Similar results were obtained in a case–control study comparing the protective effect of surgical masks and N95 respirators against SARS among healthcare workers in five Hong Kong hospitals [13].

Controlling a respiratory infection at source using a face mask is a well-established strategy. For example, symptomatic patients with cough or sneezing are generally advised to put on a face mask, and this applies equally to patients with pulmonary tuberculosis (airborne transmission) and influenza (predominantly droplet-transmitted). With the large number of asymptomatic patients unaware of their own infection [1, 2], the comparable viral load in their upper respiratory tract [3], droplet and aerosol dispersion even during talking and breathing [6], and prolonged viral viability outside our body [7], we strongly advocate universal use of face masks as a means of source control in public places during the COVID-19 pandemic. Extreme forms of social distancing are not sustainable, and complete lockdown of cities or even whole countries is devastating to the economy. Universal masking in public complements social distancing and hand hygiene in containing or slowing down the otherwise exponential growth of the pandemic. Universal masking protects against cross-transmission through unavoidable person-to-person contact during the lockdown and reduces the risk for resurgence during relaxation of social distancing measures.

A high degree of compliance will maximise the impact of universal masking in public. The global shortage of surgical masks and N95 respirators is a serious concern. In line with the recent recommendation by the US Centers for Disease Control and Prevention (CDC) for healthy people to wear a cloth face cover in public [14], we strongly support the use of cloth masks as a simple, economic and sustainable alternative to surgical masks as a means of source control for general community use, so that disposable surgical masks and N95 respirators can be reserved for use in healthcare facilities. Such an intervention is likely to be life saving in many resource-limited settings.

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