Systematic Review on the Effect of Current Smoking on the Risk of Influenza

Cigarette smoke is thought to increase the risk of respiratory infections through structural changes in the respiratory tract and a decrease in the host immune response. Influenza occurs in epidemics causing severe illness in 3-5 million people worldwide annually (1). We sought to systematically review the literature to establish the effects of current smoking on the risk of influenza in adults. We systematically searched MEDLINE, EMBASE, CINAHL, LILACS and Web of Science to identify published and unpublished randomised control trials, cohort and case-control studies that included adults aged 18 years or above on the association between smoking and influenza. Study quality was assessed using the Newcastle-Ottawa Scale. We included studies defining influenza as a clinical syndrome (an influenza-like illness (ILI)) and those using microbiological methods to confirm the diagnosis. Separate analyses were run on each group. Pooled odds ratios (ORs) with 95% confidence intervals (CIs) were estimated by using random effects model. The mean quality score across the nine included studies was 5.4 (SD 1.07). Current smokers were 34% more likely to develop an influenza-like illness than non-smokers (OR 1.34 (95% CI 1.13 to 1.59), 6 studies). For studies examining microbiologically confirmed influenza, current smokers were over 5 times more likely to develop influenza than non-smokers (OR 5.69 (95% CI 2.79 to 11.60), 3 studies). Smoking is an important risk factor for developing influenza and public health policies to reduce the prevalence of smoking could decrease the economic burden and clinical complications associated with influenza infection. 1. World Health Organisation - Influenza (Seasonal) Fact Sheet January 2018