Table 3.
Summary of findings.
| Wearing a mask compared to no mask in a community setting | ||||||
|---|---|---|---|---|---|---|
| Patient or population: community | ||||||
| Intervention: mask wearing | ||||||
| Comparison: no mask wearing | ||||||
| Outcomes | N°of participants (studies) Follow up |
Certainty of the evidence (GRADE) |
Relative effect (95% CI) |
Anticipated absolute effects* | ||
| Risk with no mask wearing | Risk difference with mask wearing | |||||
| Mortality rate | The general consensus points toward a reduction of deaths when the population mask coverage is near-universal, regardless of mask efficacy. | |||||
| Respiratory infection | In randomized controlled trials | 4017 (3 RCTs) |
⊕◯○○ VERY LOW a,b,c |
RR 0.97 (0.72 to 1.31) |
112 per 1,000 |
3 fewer cases per 1,000 (31 fewer to 35 more) |
| In cross sectional studies | 16,413 (four observational studies) |
⊕○○○ VERY LOW b,d |
RR 0.90 (0.74 to 1.10) |
172 per 1.000 |
17 fewer per 1.000 (45 fewer to 17 more) |
|
| In case-control studies | 1,501 (fourobservational studies) |
⊕○○○ VERY LOW b,d,e |
RR 0.59 (0.34 to 1.03) |
405 per 1.000 |
166 fewer per 1.000 (267 fewer to 12 more) |
|
| In prospective studies | 960 (two observational studies) |
⊕○○○ VERY LOW b,f,g,h |
RR 0.55 (0.11 to 2.75) |
584 per 1.000 |
263 fewer per 1.000 (520 fewer to 1,022 more) |
|
| Basic reproduction number (R0) of viral respiratory infection |
In the worst-case scenario with a mask efficacy at 30% and a population coverage at 20%, the R0 reduced from the initial value of 2.0 to just 1.9 whereas in the best-case scenario when the mask efficacy is at 95%, the R0 can fall to 0.99 from an initial value of 16.90, even though no population neither coverage nor time horizon are reported | |||||
| Filtering capacity of masks | Seven experimental laboratory studies | ⊕○○○ VERY LOWi |
High degree of variation of filtration efficiency depending on the materials used. All types of masks might reduce aerosol exposure. However, personal respirators were more efficient than surgical masks, which were more efficient than home-made masks. | |||
| Viral load reduction | One experimental laboratory study (37 volunteers) |
⊕○○○ VERY LOW i |
RR 0.25 (0.09 to 0.67) |
432 per 1.000 |
324 fewer per 1.000 (394 fewer to 143 fewer) |
|
| *The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: Confidence interval; OR: Odds ratio | ||||||
|
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different Low certainty: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect Very low certainty: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect | ||||||
a
High risk from multiple bias.
b
Not COVID-19 population.
c
The line of “no difference” included important benefit and harms.
d
I2>75%.
e
Ascertainment of exposure.
f
Wide confidence intervals comprising important benefit and harm.
g
Ascertainment of exposure and assessment of the outcome.
h
I2>90%.
i
Experimental study, high variability in type of masks equipment.