BACKGROUND-AIM:
In recent years, the use of electronic cigarettes (e-cigarettes) has increased among adolescents and young adults. Although many studies suggest an increased likelihood of respiratory symptoms among e-cigarette users, the evidence remains inconsistent. The objectives of this study were to identify and describe studies evaluating the prevalence of respiratory symptoms among e-cigarette users under 25 years old and to estimate the overall risk of developing these symptoms according to patterns of use.

METHODS:
A systematic review was conducted following the PRISMA 2020 guidelines. A bibliographic search was performed in PubMed (MEDLINE), Embase, CINAHL and Web of Science. In order to identify studies that compared e-cigarette users (exclusive, dual, or general, meaning a mix of exclusive and dual users) with non-users which reported prevalence or effect measures for respiratory symptoms among people aged under 25 years. Titles, abstracts, and full texts were screened independently in pairs. Data extraction was performed using an ad hoc table. When possible, pooled estimates were obtained through random effects meta-analysis. Heterogeneity, publication bias, subgroup and sensitivity analyses were performed. Study quality was assessed using an adapted version of the Newcastle-Ottawa scale.

RESULTS:
Twenty-seven studies were included. Most of them were cross-sectional studies (n=19) and were conducted in the United States (n=18). The respiratory symptoms most frequently assessed were cough (n=13), wheezing (n=8) and shortness of breath (n=8), with substantial variability in prevalence across comparing groups. In general, the prevalence of respiratory symptoms was higher among e-cigarette users than among non-users. The meta-analysis showed significant associations with any respiratory symptom (OR=1.50; 95%CI: 1.27–1.77), wheezing (OR=1.54; 95%CI: 1.29–1.84), cough (OR=1.39; 95%CI: 1.19–1.62), shortness of breath (OR=1.66; 95%CI: 1.47–1.89), bronchitis symptoms (OR=1.37; 95%CI: 1.10–1.69), chest pain (OR=1.58; 95%CI: 1.43–1.75), and phlegm (OR=1.93; 95%CI: 1.76–2.12). Substantial heterogeneity was observed in half of the models. Subgroup analyses showed that dual users were at higher risk. No association was found between exclusive e-cigarette use and any of the assessed respiratory symptoms. Most of the included studies were of moderate quality.

CONCLUSIONS:
Our findings showed an association between e-cigarette use and the presence of respiratory symptoms among young people. However, this association did not persist when the analysis was restricted to exclusive e-cigarette users. These results highlight the need for additional longitudinal studies to better understand the long-term effects of e-cigarette use, as well as the importance of strengthening prevention efforts, regulatory measures and health education targeting young people.