Background: The purpose of this study was to examine the prevalence and effects of current smoking on adverse outcomes among hospitalized COVID-19 patients. Methods: We performed a systematic review of the literature (PubMed) for studies published until April 25. Studies were included into the analysis if they satisfied all of the following criteria:
- To present hospitalized patients with COVID-19.
- To classify patients into less and more severe disease, irrespective of the severity definition (defined as “adverse ourtcome”).
- To present data on the smoking status, separately for each severity classification. We identified 18 (from a total of 1398) relevant studies. Pooled current smoking prevalence was compared with the gender-adjusted, population-based expected prevalence by calculating Prevalence Odds Ratio (POR).
The association between current, compared to non-current and former, smoking and adverse outcome was examined by calculating Odds Ratio (OR). All analyses were performed using random-effects meta-analysis. Results: Among 6515 patients, 440 of whom were current smokers, the pooled prevalence of current smoking was 6.8% (95%CI: 4.8-9.1%) and the POR was 0.21 (95%CI: 0.16-0.26, P < 0.001). In Chinese studies only, the POR was 0.22 (95%CI: 0.17-0.27, P < 0.001). Current smokers were more likely to have an adverse outcome compared to non-current smokers (OR: 1.53, 95%CI: 1.06-2.20, P = 0.022). However, they were less likely to have an adverse outcome compared to former smokers (OR: 0.42, 95%CI: 0.27-0.74, P = 0.003). Conclusion: An unexpectedly low prevalence of current smoking was observed among hospitalized patients with COVID-19. Hospitalized current smokers had higher odds compared to non-current smokers but lower odds compared to former smokers for an adverse outcome. The possibility that nicotine may have a protective effect in COVID-19 which may be masked by smoking-related toxicity and by the abrupt cessation of nicotine intake when smokers are hospitalized should be explored.