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In smokers, a white blood cell called interferon beta is less prevalent in the upper airways, and this could be a prime reason for smokers increased risk of lung infection

The long-term effect of nicotine on the oral mucosa

As between-study heterogeneity can be underestimated or even zero when the number of studies is small 196,197 , we used Fishers information matrix to estimate the uncertainty of the heterogeneity 198 and incorporated that uncertainty into the final results

We analyzed individual-level data using SAS version 9.4 (SAS Institute, Cary, NC, USA) and performed a meta-analysis using STATA version 14 (StataCorp, College Station, TX, USA)