Literature search strategy
Meta-analysis was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines[21]. We searched PubMed, EMbase, Web of Science, The Cochrane Library, CBM, CNKI, WanFang Date and VIP from establishment to August 2020. The search strategy included a combination of subjects and words as follows: (‘asthma’) AND (‘albuterol’ OR ‘salbutamol’) AND (‘Receptors, Adrenergic, beta-2’ OR ‘ADRB2 ’ OR ‘adrenergic receptor’ OR ‘Beta 2 Adrenergic Receptors’ OR ‘Receptors, beta-2 Adrenergic’ OR ‘Receptors, beta 2 Adrenergic’ ) AND (‘polymorphism’ OR ‘SNP’ OR ‘variation’ OR ‘variant’ OR ‘mutation’). To find previously unconfirmed studies, we also searched references cited in the relevant articles manually. The search results were limited to human.