Epidemiology of acute lower respiratory tract infection hospitalizations in Thai children: A 5‐year national data analysis

Abstract Background Lower respiratory tract infections (LRTIs) are the most common cause for hospitalization in pediatric patients. Pediatric patients with LRTIs are at an increased risk of morbidity and mortality. The national data analysis of epidemiologic variations facilitates awareness and develops solutions to prevent these conditions in the future. Objective This study aims to evaluate the epidemiology, causative pathogens, morbidity, and mortality of LRTIs in pediatric patients of Thailand from 2015 to 2019. Methods This was a retrospective study among pediatric patients aged between 0 and 18 years old admitted in hospitals due to LRTIs in Thailand from January 2015 to December 2019. The data were extracted from National Health Security Office using the International Statistical Classification of Diseases and Related Health Problems, 10th Revision, Thai Modification; ICD‐10‐TM of J09 to J22. Results A total of 1,423,509 children hospitalized due to LRTIs were identified. Most of the patients were of age 1–5 years. Pneumonia was the most common LRTI (876,557 children, 61.58%) in hospitalized children. Respiratory syncytial virus (RSV) is the main etiologic pathogen of bronchiolitis, which presents in approximately 10.86% of all episodes. Influenza viruses were found predominantly in children with pneumonia (15.52%). The mortality rate since 2015–2019 was highest in pneumonia under 1 year old (P < 0.001). Pneumonia in children under 5 years old had the highest mortality rate, which accounted for 11.85 per 100,000 children in 2019. Conclusions LRTIs had a high incidence rate of hospitalization and mortality, especially in children under 5 years old. Influenza virus was the most common pathogen of pneumonia.


| INTRODUCTION
Lower respiratory tract infections (LRTIs) are a group of infections involving the respiratory tract below the level of the larynx. Globally, the incidences of LRTIs among children under 5 years of age have been observed to be 12,197.8 new cases per 100,000 children. 1 LRTIs are the leading cause of hospitalizations in the pediatric population. 2,3 Notably, pneumonia ranks among the top 10 conditions with respect to the cost of hospitalization 3 and has been found to be 0.22 times per child per year in developing countries. 4 The Global Burden of Disease Study 2017 reported 1 that LRTIs caused 808,920 deaths in children younger than 5 years, and no difference in the under 5 LRTIs mortality was observed between the sexes worldwide.
In Thailand, Teeratakulpisarn et al 5  The pathogens that cause LRTIs vary depending on the patient's age. The respiratory syncytial virus (RSV) and influenza viruses are the common causes of LRTIs in children. 6 The Global Burden of Disease Study 2017 contributes 11.5% of LRTI episodes due to the influenza virus in all ages. 7 The seasonal pattern of viruses that cause LRTIs varies depending on the different regions of the world. 8 In countries of the northern hemisphere, the RSV season usually occurs during winters. 9 The influenza season also presents during the cold winter months, related to the increased person-to-person transmission when indoors and exposure to low absolute humidity. 10,11 In the tropical region, the influenza cases were detected in different patterns and lacked seasonality variation. 12 The influenza pattern in tropical Southeast Asia has been less well studied compared with that in industrialized countries. In 2020, Suntronwong et al 13 evaluated the virology study of influenza depending on the seasonal change and discovered two seasonal waves of high influenza infections, during February and August to September, related to the most humid months of the year.
Apart from the seasonal variation of the influenza virus, a study found a correlation between the incidence of pediatric influenza-associated hospitalization in high-poverty and high-crowded areas. 14 LRTIs exert a significant burden on the patients, families, and budgets of the public healthcare systems. WHO

| Statistical analysis
All statistical analyses were performed using the STATA software version 10 (StataCorp LP). The demographic characteristics of the patients were described using the frequency and percentage for categorical data. The continuous data were described as the mean and standard deviation. The admission and mortality rates were calculated based on the universal coverage (UC) scheme. The mortality rate was calculated as per 100,000 population of age groups.
The data of pathogen variation by season each year were presented as monthly trends. The association between the outcome of LRTIs, including the morbidity, mortality, and patient subgroup classifi-   Table 1 presents the patient characteristics in all age groups divided by the diagnosis of LRTIs.   Pneumonia was the most common etiology of LRTI-related mortality (3120/3168, 98.48%), two thirds of the patients were under 5 years of age. The mortality had a statistically significant occurrence in different age groups among pneumonia in children ( Table 2). The etiologic pathogens of LRTIs mortality each year are presented in Figure 6.  identify the risk factors associated with mortality which may represent the cause of a higher mortality rate compared with the previous decade study. 5 We determined that the influenza virus was the most common Two peaks of seasonal distribution of influenza LRTIs were noted during February to March and July to September, which had a similar pattern as the previous studies in Asia and Thailand. 13,24 The peak incidence during July to September may be associated with the rainy season in Thailand, similar to the prior study. 25  The data in this study demonstrated that RSV was the second most common pathogen of LRTIs in the pediatric population in Thailand, especially in children under the age of 5 years. Acute bronchiolitis, which is the disease that occurred in children under 2 years old, was the most common diagnosis of RSV infections, similar to the previous study. 20 The highest incidence of RSV LRTIs was detected in tertiary care hospitals, mainly in Bangkok, the capital city of Thailand.

F I G U R E 4 Distribution of number of admissions in children with respiratory syncytial virus (RSV) lower respiratory tract infections (LRTIs) divided by region of Thailand
It may be because of additional rapid RSV antigen tests than in hospitals in the rural area. RSV LRTIs had a high peak incidence during August to October, similar to the previous study in Thailand. 32 These months were stated as the rainy season of Thailand. Several studies in tropical and developing countries presented the association between rain and RSV infection. 25 Although some studies found that RSV LRTIs had a higher severity of disease than other pathogens, 33

CONSENT FOR PUBLICATION
The author signs for and accepts responsibility for releasing this material on behalf of any and all coauthors.

PEER REVIEW
The peer review history for this article is available at https://publons. com/publon/10.1111/irv.12911.

DATA AVAILABILITY STATEMENT
The datasets generated and/or analyzed during the current study are not publicly available but available from the corresponding author (RU) on request.