Cigarette smoking and the occurrence of influenza - Systematic review.

OBJECTIVES:The association of current smoking with influenza infection is not widely recognised. The aim of this systematic review was to summarise published evidence and quantify the risk of influenza infection in tobacco smokers compared to non-smokers. METHODS:We systematically searched MEDLINE, EMBASE, CINAHL, LILACS and Web of Science, from inception to 7 November 2017, to identify relevant randomised control trials, cohort and case-control studies. Study quality was assessed using the Newcastle-Ottawa Scale. We included studies defining influenza as a clinical syndrome and those using confirmatory microbiological tests. Pooled odds ratios (ORs) were estimated by using random effects model. RESULTS:The mean quality score across the nine included studies (n = 40,685 participants) was 5.4 of 9 (SD 1.07). Current smokers were over 5 times more likely to develop laboratory-confirmed influenza than non-smokers (pooled OR 5.69 (95% CI 2.79-11.60), 3 studies). For studies reporting the occurrence of an influenza-like illness (ILI), current smokers were 34% more likely to develop ILI than non-smokers (pooled OR 1.34 (95% CI 1.13-1.59), 6 studies). CONCLUSION:Current smokers have an increased risk of developing influenza compared to non-smokers. The association was strongest in studies examining cases with laboratory confirmed influenza.