People should wear masks; masks may help.

But all the good quality evidence we have so far struggles to find a benefit to masks. This is especially true for DIY cloth masks that are being mandated for the public, but it's also true of high quality N95 masks. We only see a benefit when we drop the quality of evidence down, but this means we have less confidence in the result.

https://www.publish.csiro.au/hc/pdf/HC15950

> RCT evidence was limited and showed no effect but accumulated evidence from retrospective case controls and cohorts all showed these strategies decreased transmission. N95 respirator mask OR 0.17 (CI 0.07–0.43), Gloves OR 0.32 (CI 0.23–0.45), Gowns OR 0.33(CI 0.24–0.45), All OR 0.09 (CI 0.02–0.35)

Note that this is for an N95 mask, not worse masks like surgical loop masks or cloth face coverings.

There's some research looking at post-operative wound infection rates. It can't find a benefit to surgeons wearing masks in clean surgery.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7138271/

> Main results

> We included three trials, involving a total of 2106 participants. There was no statistically significant difference in infection rates between the masked and unmasked group in any of the trials. We identified no new trials for this latest update.

> Authors' conclusions

> From the limited results it is unclear whether the wearing of surgical face masks by members of the surgical team has any impact on surgical wound infection rates for patients undergoing clean surgery.

https://pubmed.ncbi.nlm.nih.gov/20524498/

> Conclusion: From the limited randomized trials it is still not clear that whether wearing surgical face masks harms or benefit the patients undergoing elective surgery.

Here's the most recent, very high quality, paper about masks and covid-19. This wasn't available when Public Health organisations were making their recommendations.

https://www.thelancet.com/journals/lancet/article/PIIS0140-6...

> Although direct evidence is limited, the optimum use of face masks, in particular N95 or similar respirators in health-care settings and 12–16-layer cotton or surgical masks in the community, could depend on contextual factors; action is needed at all levels to address the paucity of better evidence. Eye protection might provide additional benefits. Globally collaborative and well conducted studies, including randomised trials, of different personal protective strategies are needed regardless of the challenges, but this systematic appraisal of currently best available evidence could be considered to inform interim guidance.

This para is saying we just don't have the evidence yet. It's saying people should wear facemasks because masks may help, but we don't know, and maybe masks cause harm.

> Interpretation

> The findings of this systematic review and meta-analysis support physical distancing of 1 m or more and provide quantitative estimates for models and contact tracing to inform policy. Optimum use of face masks, respirators, and eye protection in public and health-care settings should be informed by these findings and contextual factors. Robust randomised trials are needed to better inform the evidence for these interventions, but this systematic appraisal of currently best available evidence might inform interim guidance.

Look at the difference in the description of distancing (supported) and masks (optimum use may help).

> Face mask use could result in a large reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD −14·3%, −15·9 to −10·7; low certainty), with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar (eg, reusable 12–16-layer cotton masks; pinteraction=0·090; posterior probability >95%, low certainty).

Tucked away in this para is "low certainty". This comes from GRADE. Low certainty means that masks may be much more effective, or much less effective, than they estimate.

https://bestpractice.bmj.com/info/toolkit/learn-ebm/what-is-...

> Low The true effect might be markedly different from the estimated effect