From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7992121/pdf/406...
The World Health Organisation (WHO) initially declined to endorse wearing of ‘medical masks’ as a non-pharmaceutical intervention to prevent or reduce the spread of the SARS-Cov-2 virus and strongly opposed using other forms of masks. In the second version of the WHO’s advice in March 2020 (WHO, 2020a) it stated that:
A medical mask is not required for people who are not sick as there is no evidence of its usefulness in protecting them.
Cloth (e.g. cotton or gauze) masks are not recommended under any circum- stances.
However, by December 2020, the WHO recommended wearing of non-medical masks of various kind in certain community settings—subject to recommendations on how this is done. It did so, “Despite the limited evidence of protective efficacy of mask wearing in community settings” (WHO, 2020b). Various public health experts and epidemiologists have gone further, asserting that community use of non-medical masks is crucial for slowing transmission and attributing better outcomes in some countries to early adoption of widespread mask usage. Some countries that were initially reticent to even recommend mask use have made a comparable about-turn, making mask wearing mandatory and failure to do so subject to criminal penalty.
Under the currently-dominant hierarchy in evidence-based medicine, credible evidence on mask usage must come from randomized control trials; it was the absence of significant positive effects from RCTs prior to the pandemic that informed the WHO’s initial stance.