Table 3.

GRADE assessment of the included references—factors that can reduce quality: domain 1 limitations in study design or execution; domain 2 inconsistency of results; domain 3 indirectness of evidence; domain 4 imprecision; domain 5 publication bias; factors that can increase the quality: domain 6 large magnitude of effect; domain 7 residual confounding; domain 8 dose–response gradient. Assessment $\downarrow$–assessed down 1 or 2 levels; $\uparrow$–assessed up 1 or 2 levels; - –no effect.

	factors that can reduce quality				factors increase the quality
study	domain 1	domain 2	domain 3	domain 4	domain 5	domain 6	domain 7	domain 8
ventilation	$\downarrow 2$	$\downarrow 2$	$\downarrow 1$	$\downarrow 2$	—	—	—	—
air cleaning
devices	$\downarrow 2$	$\downarrow 2$	$\downarrow 1$	$\downarrow 2$	—	—	—	—
occupancy	$\downarrow 2$	$\downarrow 2$	$\downarrow 1$	$\downarrow 2$	—	—	—	—
disinfection	$\downarrow 2$	$\downarrow 1$	$\downarrow 2$	$\downarrow 2$	—	—	—	—