Note: The risk of bias by domain corresponds to the highest risk of bias among outcomes by domain.
The overall risk of bias corresponds to the overall highest risk of bias assessed among outcomes.
| Bias | Author's judgement | Support for judgement |
| Randomization |
Some concerns |
Quote: “Eligible participants were randomly assigned 1:1 to the mask or control group using a computer algorithm and were stratified by the 5 regions of Denmark”.
Comment: Allocation sequence random. No information on allocation concealment. Imbalances in baseline characteristics appear to be compatible with chance. Comorbidities were not reported. |
| Deviations from intervention |
Some concerns |
Quote: “unblinded, randomized controlled trial”.
Comment: Unblinded study (participants and personnel/carers). Deviations from intended intervention arising because of the study context: "based on the lowest adherence reported in the mask group during follow-up, 46% of participants wore the mask as recommended, 47% predominantly as recommended, and 7% not as recommended". There was no information on whether control participants crossed over and wore masks. No co-interventions of interest (vaccines) available at time of study. Hence, no information (or not enough information) on whether deviations arose because of the trial context. Participants were analyzed according to their randomized groups for the outcome. Of note, 638 vs 524 participants were excluded from the analysis post-randomization, of which 35 vs 33 (positive antibody results at baseline, which is exclusion of ineligible participants pot-randomization, hence acceptable) were for reasons other than missing data. Finally, 69 vs 65 were due to study kit distribution error and this method was considered inappropriate to estimate the effect of assignment to intervention for this outcome. There was probably no substantial impact of failure to analyze participants according to their randomized groups. Risk assessed to be some concerns for the outcomes: Symptomatic or asymptomatic confirmed COVID-19. Confirmed COVID-19. |
| Missing outcome data |
Some concerns |
Comment: 6024 participants randomized; 4862 participants analyzed.
Data not available for all or nearly all participants randomized. No evidence that the result is not biased. Reasons for missing data: Did not finalize participation (534 vs 426); Had study kit distribution error (69 vs 65); Had positive results on antibody test at baseline (35 vs 33). Missingness could depend on the true value of the outcome. Not likely that missingness depended on the true value of the outcome (equal proportion of missingness between arms and same reasons for missingness) Risk assessed to be some concerns for the outcome: Symptomatic or asymptomatic confirmed COVID-19. Confirmed COVID-19. |
| Measurement of the outcome |
Some concerns |
Quote: "unblinded, randomized controlled trial”.
Comment: Method of measuring the outcome probably appropriate. Measurement or ascertainment of outcome probably does not differ between groups. Unblinded study (outcome assessor) CONFIRMED COVID-19 PCR-confirmed status is an observer-reported outcome not involving judgement. Risk assessed to be low for outcome: Confirmed COVID-19. SYMPTOMATIC OR ASYMPTOMATIC CONFIRMED COVID-19 This outcome (as it depends on PCR-confirmed or probable or suspected COVID) could be influenced by knowledge of intervention assignment but was not considered likely to in the context of a pandemic. Risk assessed to be some concerns for the outcomes: Symptomatic or asymptomatic confirmed COVID-19. |
| Selection of the reported results |
Low |
Comment: The prospective protocol (dated April 7th, 2020) and trial registry (dated April 5th, 2020) were available.
Outcomes pre-specified. Results were not selected from multiple outcome measurements or analyses of the data. Trial analyzed as pre-specified. Risk assessed to be low for the outcomes: Symptomatic or asymptomatic confirmed COVID-19. Confirmed COVID-19. |
| Overall risk of bias |
Some concerns |