Characteristics of the study population
A total of 30,994 children with CAP requiring hospitalization were enrolled in the present study. The characteristics of the study population are presented in Table 1. The mean age of the participants was 41.9 months (SD, 40.8 months) and 54.6% of the total study population were boys. The number of children hospitalized due to CAP was highest in fall (September to November), followed by winter (December to February), spring (March to May), and summer (June to August).
Annual and seasonal patterns of CAP caused by respiratory viruses and MP
During the study period, the number of children hospitalized due to CAP was highest in 2011, followed by 2015 (Figure 1). The number of children hospitalized with CAP due to various respiratory virus infections was highest in 2015, with similar seasonal and annual patterns of total CAP. Among the various respiratory viruses, RSV was the most commonly identified virus, especially from November to December, with a gradually increasing trend until each peak (Figure 2). The number of children hospitalized due to FLU-associated CAP peaked from January to March in each year, with the largest number of affected patients in 2015. The number of children hospitalized with HMPV-associated CAP peaked in April, with its largest number of affected patients in 2014 (Supplementary Figure 1).
The two epidemics of MP pneumonia (2011 and 2015) corresponded with the two peaks of CAP. The incidence of MP pneumonia requiring hospitalization in children peaked from October to November in each year (Figure 1). The most common clinical phenotype of MP pneumonia was MSMP, followed by MLEP and MRMP (Figure 3A). During the study period, the monthly rate of MSMP/total MP pneumonia was 63.6-100.0% (mean ± SD, 81.3 ± 7.7), that of MRMP/total MP pneumonia was 0.0-11.0% (4.2 ± 4.3), and that of MLEP/total MP pneumonia was 0.0-30.3% (14.5 ± 14.2) (Figure 3B). The rates of clinical MLEP and MRMP pneumonia showed increasing trends (seasonal Mann-Kendall trend tests, P = 0.0644 and P = 0.0066, respectively), whereas the rates of clinical MSMP pneumonia showed a significant reduction during the study period (seasonal Mann-Kendall trend test, P = 0.0028).
Causative pathogens of CAP according to age
The number of children hospitalized with CAP decreased with age (Table 2). In children younger than 2 years of age, RSV (34.0%) was the most commonly detected respiratory pathogen, followed by PIV (19.7%), and HRV (18.8%). In children 2-4 years of age, RSV (14.9%) was the most commonly identified respiratory virus, followed by HRV (14.8%), and PIV (13.6%). In children 5-17 years of age, HRV (10.6%) was the most commonly detected virus, followed by FLU (9.2%). In children older than 2 years of age, MP (45.3%) was the most common cause of CAP, whereas 9.44% of children younger than 2 years of age exhibited MP infections. The rate of clinical MRMP/total MP pneumonia was highest in children 5-9 years of age (244/3475, 9.8%), followed by adolescents 10-17 years of age (47/905, 6.9%), children 2-4 years of age (154/3526, 4.4%) and those younger than 2 years of age (25/1277, 2.0%).
Clinical characteristics and laboratory findings according to etiologies of CAP
The clinical features and laboratory findings, according to the etiologies, in children admitted due to CAP are described in Table 3. The median length of hospital stay due to CAP was 181.39 hours. The duration of hospitalization due to CAP was longest in those with AdV pneumonia, followed by RSV pneumonia. Systemic corticosteroids were most commonly administered to those with MP pneumonia (1645/7455, 22.1%), followed by RSV pneumonia (622/4521, 13.8%). The number of children with CAP who required oxygen supplementation was greatest in children with RSV pneumonia (905/4521, 20.0%); ventilator care was most commonly applied in children with AdV pneumonia (13/680, 1.9%), followed by those with RSV pneumonia (119/4521, 2.6%).
The whole blood counts were highest in AdV, followed by RSV, FLU, HMPV, and MP. Blood neutrophil percentages were highest in MP, followed by FLU, AdV, HMPV, and RSV. C-reactive protein levels were highest in MP, followed by AdV, HMPV, FLU, and RSV.