"The high risk of bias in the trials, variation in outcome measurement, and relatively low adherence with the interventions during the studies hampers drawing firm conclusions."
Means they aren't sure themselves, if their results are usable at all. For the individual, one unmasked exposure undoes the entire effort. We're not talking about toxicity.
This seems to be clear to the authors aswell. The strongest language in their conclusions reads like this:
"There is uncertainty about the effects of face masks. The low to moderate certainty of evidence means our confidence in the effect estimate is limited, and that the true effect may be different from the observed estimate of the effect."
Going from that to "Masks don't work" is quite the leap.
"We included 12 trials (10 cluster‐RCTs) comparing medical/surgical masks versus no masks to prevent the spread of viral respiratory illness (two trials with healthcare workers and 10 in the community). Wearing masks in the community probably makes little or no difference to the outcome of influenza‐like illness (ILI)/COVID‐19 like illness compared to not wearing masks (risk ratio (RR) 0.95, 95% confidence interval (CI) 0.84 to 1.09; 9 trials, 276,917 participants; moderate‐certainty evidence. Wearing masks in the community probably makes little or no difference to the outcome of laboratory‐confirmed influenza/SARS‐CoV‐2 compared to not wearing masks (RR 1.01, 95% CI 0.72 to 1.42; 6 trials, 13,919 participants; moderate‐certainty evidence)."
Means they aren't sure themselves, if their results are usable at all. For the individual, one unmasked exposure undoes the entire effort. We're not talking about toxicity.
This seems to be clear to the authors aswell. The strongest language in their conclusions reads like this:
"There is uncertainty about the effects of face masks. The low to moderate certainty of evidence means our confidence in the effect estimate is limited, and that the true effect may be different from the observed estimate of the effect."
Going from that to "Masks don't work" is quite the leap.