Comparison 18. Sensitivity analysis: comparison 01: subgroup analysis by study quality.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
18.1 Influenza‐like illness	25	9211	Risk Ratio (M‐H, Random, 95% CI)	0.75 [0.65, 0.87]
18.1.1 Quality A	8	4502	Risk Ratio (M‐H, Random, 95% CI)	0.78 [0.65, 0.94]
18.1.2 Quality B	13	3854	Risk Ratio (M‐H, Random, 95% CI)	0.82 [0.65, 1.03]
18.1.3 Quality C	3	389	Risk Ratio (M‐H, Random, 95% CI)	0.66 [0.43, 1.00]
18.1.4 Quality D	1	466	Risk Ratio (M‐H, Random, 95% CI)	0.44 [0.35, 0.57]