Table 4.

Challenges in interpreting findings from ecological mask studies

Diagnostic capabilities

Accounting for changes in SARS-CoV-2 infection diagnostic capacity and types of diagnostic tools used is critical for interpreting changes in incidence associated with policy implementation [53]. Testing capacity was only assessed by one study included in this review [17]. More affluent regions may have a greater ability to detect COVID-19 cases than less affluent regions, including among asymptomatic individuals, and the practice of face mask wearing, including availability and quality, may also be related to area-level affluence (meaning that both exposure and outcome are influenced by affluence).

Concurrent preventive interventions

Policies mandating community mask wearing are accompanied by policies promoting other nonpharmaceutical interventions known to prevent SARS-CoV-2 transmission – notably physical distancing, schools and workplace closures, and hand hygiene. Seventeen studies considered one or more nonpharmaceutical interventions in addition to mask wearing, with statistical adjustments made in nine of these studies[9,17,20,23,25,29,31,33,37]; the remainder did not consider the influence of other nonpharmaceutical interventions. It is not possible to reliably estimate the benefit of face-mask policies over and above the benefits that are brought about by the other measures. Furthermore, the contribution of vaccines will need to be accounted for as vaccination programmes reach scale.

Mask policy vs mask wearing

Mask quality varies by type of mask fabric [54,55], and when and how they are worn [56]. Only 2 studies reported mask type [19,21] and two considered adherence to mask policies [22,29]. None of the studies evaluated individual-level adherence to and appropriate wearing of masks. The ability to categorize mask policies is relatively crude (i.e. requiring people to wear masks in the community could encompass a lot of variability in terms of where and when and in whom masks are required)

Differences in virus transmission

None of these studies accounted for the potential contribution of superspreading events [57] or the emergence of new SARS-CoV-2 variants of concern that have higher transmissibility [58] on incidence trends. While most studies were completed prior to the emergence of variants of concern, and it is uncertain that the contribution of such variants on incidence trends could be assessed through ecological analyses.

Timing from policy to outcome

There is a time lag between policy implementation and possible impact on incidence, followed by hospitalization, and then mortality. Interpreting studies on the effect of mask and other policies requires appropriate consideration of timing. There is also variability in the requirements of mask policies, including differences in exemptions for certain age groups, places of worship, and other specific settings [59].