To investigate the clinical characteristics and laboratory examinations of hospitalized children with influenza, and to provide evidence-based basis for early identification of severe influenza.

Total of 357 children with influenza admitted to Xi’an Children’s Hospital and Xi’an Central Hospital from December 2018 to April 2020 were selected as case group, and divided into mild group (207 cases) and severe group (150 cases) according to disease severity. The clinical data of the case group were analyzed, retrospectively, and the differences of clinical characteristics and laboratory examinations were analyzed, respectively. Meanwhile, 180 healthy children who were randomly selected for physical examination in the same period were selected as control group. Serum 25-(OH)D level was measured by chemiluminescence. The relationship between serum 25-(OH)D level and severe influenza was analyzed.

Among the 357 children with influenza, the male to female ratio was 1.29︰1, and the age ranged from 1 month and 10 days to 12 years old, with 276 cases (77.3%) younger than 5 years old. There were 308 (86.3%) cases of influenza A and 49 (13.7%) cases of influenza B. Patients with underlying diseases (χ² = 5.988, P = 0.014), wheezing (χ² = 5.272, P = 0.022), vomiting (χ² = 6.080, P = 0.014) symptoms in the severe group were more than those in the mild group, with significant differences. The white blood cell count (Z = -2.429, P = 0.015), neutrophil count (Z =-3.106, P = 0.002), C-reactive protein (Z =-4.031, P = 0.001), procalcitonin (Z =-0.970, P = 0.016) in the severe group were significantly higher than those in the mild group, while the lymphocyte count (Z =-3.239, P = 0.001), monocyte count (Z =-2.208, P = 0.027) were lower than those in the mild group, with significant differences. In comparison of serum 25-(OH)D levels, the mild group was significantly lower than that of the control group [(30.47 ± 11.23) ng/ml vs. (44.74 ± 12.57) ng/ml: t =-7.783, P < 0.001], the severe group was lower than that of the control group [(26.33 ± 6.88) ng/ml vs. (44.74 ± 12.57) ng/ml: t = -8.884, P < 0.001], and the severe group was significantly lower than that of the mild group [(26.33 ± 6.88) ng/ml vs. (30.47 ± 11.23) ng/ml: t = 2.311, P = 0.013]. The best cut-off value of serum 25-(OH)D for predicting severe influenza was 22.21 ng/ml which determined by ROC curve, the area under the curve was 0.691, the sensitivity was 68.0%, and the specificity was 65.3%. Multivariate Logistic regression analysis showed that underlying diseases (OR = 2.698, P = 0.028), wheezing (OR = 3.764, P = 0.017), vomiting (OR = 3.455, P = 0.018), serum 25-(OH)D < 22.21 ng/ml (OR = 4.251, P = 0.003) were all risk factors of severe influenza. After oral oseltamivir antiviral and symptomatic treatment, 355 (99.4%) children improved or were cured and discharged, and 2 (0.6%) children in the severe disease group who were not cured were signed out. The mean length of hospitalization was 6.2 days.

Children under 5 years old are susceptible to influenza, and influenza virus infection among children is mainly influenza A. Children with underlying diseases, wheezing and vomiting symptoms during the course of the disease, and lower serum 25-(OH)D are more likely to progress to severe cases, and serum 25-(OH)D cannot be used as a independent predictor of severe influenza.