Background: Although smoking has been recognized as a risk factor for many respiratory diseases, its effects of influenza-associated morbidity and mortality remain controversial. We conducted a systematic review and meta-analysis to assess the impact of smoking on influenza-associated hospital admissions, intensive care unit (ICU) admissions, and deaths. Methods: We searched the databases of PubMed, CINAHL, EMBASE, and the China National Knowledge Infrastructure for all observational studies published between 1 January 2000 and 30 November 2017 on ever-active/secondhand smoking and influenza-associated hospital admissions, ICU admissions, and deaths. We pooled data using random effect models. Results: The initial search retrieved 7495 articles, of which 20 studies were included for systematic review, and 12 studies (eight case-control studies, two cohort studies, and two cross-sectional studies) with 18612 subjects were included in meta-analysis. The overall quality of selected studies was moderate. Ever-active smokers had higher odds of hospital admissions (odds ratio [OR] = 1.5; 95% confidence interval [CI] = 1.3, 1.7) and ICU admissions (OR 2.2; 95% CI = 1.4, 3.4) after influenza infections, as compared with never smokers. No association was observed between ever-active smoking and influenza-associated deaths. We found a positive association between secondhand smoking and influenza-associated hospital admissions, but only in children below 15 years of age. Conclusions: The literature evidence showed that smoking was consistently associated with higher risk of hospital admissions after influenza infection, but the results for ICU admissions and deaths were less conclusive because of the limited number of studies.
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