Discussion
None of the studies we reviewed established a conclusive relationship between mask/respirator use and protection against influenza infection. Some useful clues, however, could be gleaned. Subanalyses performed for one of the larger randomised controlled studies in a household setting found evidence of reduced rates of influenza-like illness if household contacts consistently wore the mask or respirator. The authors of a randomised trial of mask plus alcohol-based sanitiser and mask-only group amongst U.S. university students living in residence halls noted that their study may have been better positioned to identify a protective effect because participants initiated the interventions at the beginning of the influenza season. Cowling's finding that there was a significant reduction in the secondary attack ratio if the hand hygiene and mask plus hand hygiene interventions were begun within 36 hours of the index case lends support to this hypothesis.
Anticipating the paucity of studies that focused solely on influenza, we included the effect of masks/respirators on respiratory viruses other than influenza. Such studies have often been used to support infection control guidance for influenza. However, the difficulties in interpreting the observational studies of SARS suggest that they are of limited use for guiding policy on influenza. Firstly, SARS is an unusual acute viral respiratory infection with a very different epidemiology to almost all other respiratory viral infections. It is fundamentally different from human influenza: it rarely infects children, has a long incubation period, transmits little early on, mostly transmits in healthcare settings, is not prone to extensive global spread and has only appeared once. Secondly, the studies were poorly designed, had many weaknesses and so were very difficult to interpret. Issues of concern include the use of a non-specific definition for exposure to a SARS patient (e.g. coming within one metre of a patient), inconsistency in providing information about the comparability of cases and controls and collection of data after a lengthy period following the outbreak. Several lacked microbiological confirmation of cases or controls and it would seem likely that a number of the SARS cases were not cases at all. Because all the cases knew they were cases, recall bias was highly likely. The single case–control study that tried to address some of these limitations did not find that inconsistent use of masks or respirators was associated with SARS infection.
It is important to note three considerations when assessing the practical implications of the review's findings. Firstly, development of evidence-based guidance about mask/respirator use is inextricably linked to what is known about how influenza is spread and specific risk factors that can affect transmissibility (e.g. host factors, pathogen factors, environmental factors and particle size). However, this is an area equally fraught with uncertainty; there are limited and conflicting evidence regarding the relative importance and frequency of direct contact, indirect contact, droplet and aerosol modes of transmission. Historically, transmission has been thought to occur principally through respiratory droplets and masks have been used as a barrier against droplets emitted by coughing and sneezing. In the last decade, there has been increasing interest in a possible role for aerosol transmission of influenza and the advisability of filtering respirators to block such transmission. For example, studies have found that infected patients can produce aerosol particles containing influenza virus and that hospital airflow patterns can influence influenza transmission via aerosols.
Secondly, although the focus of this review has been on masks and respirators, limiting transmission of influenza in both healthcare and community settings requires a multifaceted approach, of which masks and respirators are but one component. In the healthcare setting, this 'hierarchy of controls' includes administrative controls help to reduce the introduction and spread of infection (e.g. policies to restrict entrance of ill visitors and workers, vaccination of healthcare workers); environmental/engineering controls (e.g. adequate ventilation); and lastly, use of personal protective equipment and hand hygiene. In the community setting, a similarly structured approach is advised. However, during both the planning for an eventual pandemic and the subsequent public health response to the H1N1 pandemic, concern over policy and guidance related to mask/respirator use has at times seemed to overshadow other important controls. It is somewhat paradoxical that whilst continued effort and resources are needed to assess the independent effect of masks and respirators on influenza transmission, their use would always be recommended in combination with other control measures.
Thirdly the practical implications of policy, guidance and recommendations on mask/respirator use and other infection control measures must be considered. The only two studies that compared mask and respirators to protect healthcare workers from influenza infection essentially reached different conclusions illustrating the difficulties facing policymakers. Further, a simulation study found that strict adherence to guidance about personal protective equipment (which included masks and respirators) compromised normal ward functioning in a UK hospital setting.
This review had a prescribed narrow focus that permitted us to examine a relatively small number of studies. We considered employing quantitative techniques, but on analysis found the studies comprised a range of study designs, pathogens, participants, interventions and opportunities for bias and confounding would render any meta-analysis findings open to criticism. A review that included interventions other than mask/respirator use, experimental laboratory and/animal–human studies on mask/respirator efficacy, cost-effectiveness studies and the occurrence of adverse events would present a more comprehensive picture.
Several systematic reviews of interventions to limit the transmission of respiratory viral infections and/or specifically influenza have been undertaken. Most have considered a range of interventions; one focused specifically on respiratory protection. Within the boundaries established by our inclusion criteria, our search strategy captured essentially the same studies on masks and respirators that others have identified. Jefferson et al derived pooled estimates of the effectiveness of wearing an N95 respirator (91%) and wearing a mask (68%) for any respiratory viral infection; however, these estimates were derived from the analyses of six SARS studies whose methodology was problematic. We carefully noted how well exposures in various studies were detailed and if cases and controls were laboratory-confirmed to avoid misclassification bias. We did not feel that such a heterogeneous group of studies could be combined even for SARS.
In conclusion, there is a limited evidence base to support the use of masks and/or respirators in healthcare or community settings. Mask use is best undertaken as part of a package of personal protection, especially including hand hygiene in both home and healthcare settings. Early initiation and correct and consistent wearing of masks/respirators may improve their effectiveness. However, this remains a major challenge – both in the context of a formal study and in everyday practice.
Continued research on the effectiveness masks/respirators use and other closely associated considerations remains an urgent priority with emphasis being on carefully designed observational studies and trials best conducted outside a crisis situation. However, examination of the literature has highlighted that well-designed studies in this field are challenging. Studies need to be adequately powered to assess potentially small differences between interventions and the independent effect of mask/respirator wearing when a second intervention (e.g. hand hygiene) is employed; an appropriate control group must be identified (e.g. no use of masks/respirators). Most of the studies we examined were too small to reliably detect what would be anticipated to be moderate effects. Perhaps, one solution is to fund large multi-centre trials with similar protocols in different sites for multiple years to achieve sufficient power. Protocols should include the collection of detailed exposure data, objective monitoring of compliance and assessment of potential confounders. It may be difficult to design studies employing a control group that does not use any protective equipment (including masks/respirators), particularly in healthcare settings, as such precautions are routinely recommended. Finally, there is a striking paucity of published studies with microbiologically proven influenza infection as an outcome; inclusion of laboratory outcomes is essential in any future study of masks/respirators on transmission of influenza.