Respiratory outcomes in Korean COPD patients using e-cigarettes: methodological and interpretive limitations

Dear Editor,

We read with interest the analysis from the Korean National Health Insurance Sharing Service by Kim et al. [1] examining clinical outcomes in COPD patients who smoke, stratified by subsequent changes in nicotine consumption. The topic is important and the large, real-world dataset is a major strength.

However, a central methodological issue substantially limits interpretability because the manuscript’s “switched to e-cigarettes” category appears to combine patients who fully substituted cigarettes with those who used e-cigarettes while continuing to smoke (dual use).

This distinction is critical. Dual users often have heavier smoking histories, greater baseline morbidity, and most importantly may not meaningfully reduce cigarette consumption. Table 1 in the article by Kim et al. confirms that the “switched to e-cigarette” group (which included dual use) were not smoking much less than the exclusive smoking group (12.8 cigs/day vs. 15.7; not statistically significant). Pooling dual users with complete switchers can dilute potential benefits of full substitution and may also introduce confounding by baseline risk, potentially attributing worse prognosis to e-cigarette use when it reflects underlying smoking severity and health status.

Notably, the full text reports a co-primary analysis comparing exclusive e-cigarette users with dual users at follow-up. This comparison is obviously the most clinically meaningful, yet it is not presented prominently; instead, the headline presentation focuses on the pooled “switching” category. Supporting the risk of exposure misclassification, the manuscript’s baseline/behavioral data indicate that the pooled e-cigarette group was not smoking substantially less than continuing smokers, consistent with a large proportion being dual users rather than true switchers.

In addition, the exclusive-switching subgroup in the present study appears small, yielding wide confidence intervals and limited power. In this context, conclusions should not hinge solely on p-values, particularly when effect sizes may still be clinically relevant and directionally consistent with benefit.

Finally, the manuscript appears to mischaracterize the literature it cites. The authors write: “Although recent studies involving several dozen patients with COPD who switched to e-cigarettes have reported improvements… the small sample size and absence of a control group limit the generalizability of these findings [14, 15].” While these studies are indeed modest in size and observational, both were conducted in respiratory outpatient clinic cohorts and included comparator groups of COPD smokers who continued smoking, allowing longitudinal between-group comparisons [2, 3]. Describing them as lacking control groups is therefore inaccurate and risks misleading readers about the existing evidence base.

Author’s contributions

DC FC and LS wrote the main manuscript text.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.