The number and percentages of missing values.

Background
The aim of this study was to explore the association between serum copper-zinc (Cu-Zn) ratio and the risk of respiratory tract infection in children and adolescents.
Methods
This cross-sectional study collected the data of 1695 participants who aged 6–17 years with follow-up data on respiratory tract infection in 2011–2012, 2013–2014 and 2015–2016 cycles from the National Health and Nutrition Examination Survey (NHANES) database. Univariate logistic regression analysis was applied to explore the covariates. Each covariate was adjusted in multivariate logistic regression analysis to explore the correlation between serum Cu-Zn ratio and respiratory tract infection. Subgroup analysis was performed to stratify the data according to age, gender and BMI. Restricted cubic spline (RCS) curve was plotted to identify the association between serum Cu-Zn ratio and respiratory tract infection.
Results
The results of RCS curve depicted that the risk of respiratory tract infection was increased as the elevation of the serum Cu-Zn ratio. After adjusting for confounders, risk of respiratory tract infection in children and adolescents was elevated with the increase of serum copper-zinc ratio (OR = 1.38, 95%CI: 1.19–1.60). Compared with people with serum copper-zinc ratio <1.25, subjects who had serum copper-zinc ratio >1.52 was associated with increased risk of respiratory tract infection in children and adolescents (OR = 1.88, 95%CI: 1.19–2.98). Subgroup analysis demonstrated that the risk of respiratory tract infection was elevated as the increase of serum copper-zinc ratio in participants <12 years (OR = 1.65, 95%CI: 1.28–2.12), ≥12 years (OR = 1.27, 95%CI: 1.03–1.57), males (OR = 1.63, 95%CI: 1.29–2.06), females (OR = 1.26, 95%CI: 1.01–1.57), underweight and normal (OR = 1.35, 95%CI: 1.11–1.65), and overweight and obese participants (OR = 1.44, 95%CI: 1.15–1.80).
Conclusion
Higher serum Cu-Zn ratio was associated with increased risk of respiratory tract infection in children and adolescents, which suggests the importance of Zn supplement and the balance of serum Cu-Zn ratio in children and adolescents.