BACKGROUND:
Although smoking has been recognized as risk factor for many respiratory diseases, its effects on 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 01 January 2000 and 30 November 2017 on ever-active / second-hand smoking and influenza-associated hospital admissions, ICU admissions and deaths. We pooled data using random effect models.
RESULTS:
The initial search retrieved 7,495 articles, of which twenty studies were included for systematic review, and twelve studies (eight case-control studies, two cohort studies and two cross-sectional studies) with 18,612 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 to never smokers. No association was observed between ever-active smoking and influenza-associated deaths. We found a positive association between second-hand smoking and influenza-associated hospital admissions, but only in children under 15 years.
CONCLUSION:
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 due to the limited number of studies.