Factors associated with severe pneumonia among hospitalised children under five years of age: a cross-sectional study at Hai Duong Children's Hospital, Hai Duong, Vietnam

¹Department of Paediatrics, Hai Duong Medical Technical University, Hai Duong, Vietnam, ²Department of Pediatrics, Hanoi Medical University, Hanoi, Vietnam

^&Corresponding author
Thuy Thi Dieu Nguyen, Department of Pediatrics, Hanoi Medical University, Hanoi, Vietnam

Introduction    

Pneumonia remains a leading cause of morbidity and mortality among children under five years of age, particularly in low- and middle-income countries (LMICs). In these settings, pneumonia diagnosis and severity classification often rely on syndromic clinical definitions because access to advanced diagnostic tools is limited. The World Health Organization (WHO) Integrated Management of Childhood Illness (IMCI) guidelines define pneumonia based on cough or difficulty breathing accompanied by fast breathing and/or chest indrawing, while additional danger signs such as inability to drink, convulsions, lethargy, or central cyanosis indicate severe disease [1,2]. According to the WHO classification, cases are categorised as non-severe pneumonia and severe pneumonia, the latter defined by the presence of any general danger sign [1,3]. This classification framework has been widely used in epidemiological studies and large multicenter investigations, including the Pneumonia Etiology Research for Child Health (PERCH) study, to standardise case definitions and facilitate comparisons across settings [1,4].

Despite improvements in prevention and case management, the burden of severe childhood pneumonia remains substantial in LMICs. Globally, approximately 120 million episodes of childhood pneumonia occur each year, of which around 14 million progress to severe disease requiring hospitalisation [5,6]. Africa and South-East Asia account for nearly 69% of global severe pneumonia cases and deaths [1,6]. Severe pneumonia is associated with significantly higher mortality, particularly among children presenting with hypoxaemia or other danger signs, highlighting the importance of early identification and management of high-risk cases [7-9].

Several host-related, environmental, and health system factors have been associated with increased severity of pediatric pneumonia. Younger age, particularly during infancy, is consistently linked to higher risk due to immature immune function [9-11]. Nutritional deficits such as underweight, stunting, and severe malnutrition also increase vulnerability to severe disease and mortality [9,12,13]. Perinatal conditions including preterm birth and low birth weight, chronic underlying diseases, incomplete vaccination, and socioeconomic disadvantages such as low caregiver education have likewise been identified as important determinants of pneumonia severity and poor outcomes in children [9-18].

However, evidence on factors independently associated with severe pediatric pneumonia in Vietnam remains limited, particularly from hospital-based studies using standardized WHO definitions. Local epidemiological evidence is needed to better understand the determinants of severe disease in the Vietnamese context and to support targeted prevention and early management strategies. This study aimed to determine the rate of severe pneumonia and identify factors independently associated with severe pneumonia among hospitalised children aged 2-59 months at Hai Duong Children's Hospital, Vietnam.

Methods     

Study design and setting: a hospital-based cross-sectional study was conducted to identify factors associated with pneumonia severity among children aged 2-59 months hospitalised with community-acquired pneumonia. The study was carried out at Hai Duong Children's Hospital, a provincial tertiary pediatric hospital providing specialised respiratory and intensive care services for children in Hai Duong province and neighbouring northern regions of Vietnam. Data collection was conducted from November 2024 to April 2025 at the Department of Respiratory Medicine and the Pediatric Intensive Care Unit.

Study population: the target population included children aged 2-59 months admitted with community-acquired pneumonia during the study period. Eligible participants were children diagnosed with community-acquired pneumonia according to the Vietnamese Ministry of Health 2014 guideline and receiving inpatient treatment at Hai Duong Children's Hospital. Patients were excluded if essential clinical or laboratory information required for analysis was incomplete. A consecutive convenience sampling strategy was applied, whereby all eligible patients admitted during the study period were invited to participate. A total of 570 children met the inclusion criteria and were included in the final analysis.

Data collection: data were collected prospectively at hospital admission using a standardised case-record form specifically developed for the study. Information was obtained through structured caregiver interviews and review of hospital medical records. Collected variables included demographic characteristics, perinatal history, vaccination status, caregiver education, clinical history, nutritional status, and laboratory findings. Anthropometric measurements and clinical assessments were performed according to routine hospital procedures. Each participant was assigned a unique study identification code, and completed forms were reviewed for completeness and internal consistency before entry into the study database.

Definitions: the primary outcome variable was pneumonia severity, classified according to the Vietnamese Ministry of Health 2014 criteria and dichotomised into non-severe pneumonia and severe pneumonia for analysis. Independent variables included demographic and socioeconomic factors (age group, sex, place of residence, caregiver educational level), perinatal history (preterm birth), clinical background (presence of underlying diseases), vaccination status (receipt of combined 5-in-1 or 6-in-1 vaccine, pneumococcal vaccine, influenza vaccine, and measles vaccine), nutritional status (underweight defined as weight-for-age < −2 SD according to WHO 2006 growth standards), duration of illness before hospital admission, and microbiological profile (single infection versus viral-bacterial co-infection).

Statistical analysis: data were entered, cleaned, and analysed using IBM SPSS Statistics version 26.0 (IBM Corp., Armonk, NY, USA). Categorical variables were summarised as frequencies and percentages, while continuous variables were summarised as means and standard deviations or medians and interquartile ranges depending on distribution. Differences between severe and non-severe pneumonia groups were assessed using the chi-square test or Fisher's exact test for categorical variables. Factors associated with pneumonia severity were examined using logistic regression analysis to estimate crude odds ratios (OR) and adjusted odds ratios (aOR) with 95% confidence intervals (CI). Variables considered clinically relevant or statistically significant in univariable analysis were included in the multivariable logistic regression model. Statistical significance was determined using two-sided tests with P < .05.

Ethical considerations: the study protocol was approved by the Institutional Review Board of Hanoi Medical University (Approval No: 1891/QD-DHYHN) and authorised by Hai Duong Children's Hospital. Written informed consent was obtained from parents or legal guardians before enrollment. All diagnostic and treatment procedures followed routine clinical practice and posed no additional risk to participants. Participant confidentiality was strictly maintained by using anonymized study identification codes. Participation was voluntary, and caregivers retained the right to withdraw at any time without affecting the child's clinical care.

Results     

Demographic characteristics: among the 570 hospitalised children with pneumonia, 313/570 (54.9%) were aged 12-<60 months and 257/570 (45.1%) were aged 2-<12 months, while male patients accounted for 319/570 (56.0%). Most children resided in rural areas (460/570, 80.7%). Preterm birth was reported in 41/570 children (7.2%), and underlying diseases were present in 76/570 (13.3%), most commonly respiratory disorders. Vaccination coverage for the combined 5-in-1 or 6-in-1 vaccine was high (533/570, 93.5%), whereas pneumococcal (364/570, 63.9%), measles (358/570, 62.8%), and influenza vaccination (177/570, 31.1%) were less common. Underweight status was observed in 36/570 children (6.3%). Most primary caregivers had at least a high school education (514/570, 90.2%). Overall, 167/570 children (29.3%) were classified as having severe pneumonia. Detailed baseline characteristics are presented in Table 1.

Factors associated with severe pneumonia: in the multivariable logistic regression analysis (Table 2), several factors were independently associated with severe pneumonia. Preterm birth was associated with higher odds of severe pneumonia (aOR: 2.32, 95% CI 1.08-4.95; P=0.030). Children with underlying diseases also had increased odds of severe pneumonia (aOR: 2.83, 95% CI 1.62-4.94; P<0.001). Lack of routine vaccination with the combined 5-in-1 or 6-in-1 vaccine was associated with severe pneumonia (aOR: 2.68, 95% CI 1.20-5.95; P=0.016), as was lack of pneumococcal vaccination (aOR: 1.78, 95% CI 1.12-2.84; P=0.015). In addition, children whose primary caregivers had an education level below high school had higher odds of severe pneumonia (aOR: 2.45, 95% CI 1.34-4.49; P=.004). Other variables, including younger age, underweight status, and lack of influenza or measles vaccination, were not significantly associated with severe pneumonia after adjustment.

Discussion     

This study aimed to determine the rate of severe pneumonia and identify factors independently associated with severe pneumonia among hospitalised children aged 2-59 months at Hai Duong Children's Hospital, Vietnam. In this hospital-based cohort, severe pneumonia accounted for 29.3% of admissions. Multivariable analysis showed that preterm birth, the presence of underlying diseases, lack of routine childhood vaccination (5-in-1 or 6-in-1 vaccine), lack of pneumococcal vaccination, and caregiver education below high school level were independently associated with severe pneumonia. In contrast, age group, underweight status, and lack of influenza or measles vaccination were not independently associated with severe pneumonia after adjustment.

The proportion of severe pneumonia observed in this study (29.3%) is within the range reported in other hospital-based studies, although substantial variation exists across settings. In Bangladesh, a multicenter study reported that 60.1% of hospitalised children aged 2-59 months had severe pneumonia and an additional 19.9% had very severe pneumonia, meaning that approximately 80% of admitted cases were classified as severe according to WHO criteria [19]. In Kenya, severe pneumonia accounted for approximately 21% of admissions but was associated with markedly higher mortality compared with non-severe pneumonia (14.2% vs. 2.7%) [5]. Community-based cohort studies report lower progression rates; for example, only 8.7% of pneumonia episodes progressed to severe disease in a large urban cohort in Dhaka [12], while 23% of pneumonia episodes were classified as severe in a South African birth cohort [10]. The rate observed in our study therefore appears consistent with hospital-based cohorts that include a mix of early and advanced clinical presentations.

Preterm birth remained independently associated with severe pneumonia in this study. This finding is consistent with evidence showing that prematurity confers long-term biological vulnerability to respiratory infections. A South African birth cohort demonstrated that preterm birth independently predicted serious pneumonia outcomes such as ICU admission or death [11]. Prematurity is also strongly associated with severe viral lower respiratory tract infections; preterm infants account for approximately 25% of RSV-associated ALRI hospitalisations and have substantially higher hospitalisation rates compared with term infants [14]. These findings support the role of prematurity as a key biological risk factor for severe pneumonia in early childhood.

Underlying diseases were also independently associated with severe pneumonia (aOR: 2.83, 95% CI 1.62-4.94; P<.001). Previous studies have shown that comorbid conditions can substantially worsen clinical outcomes in children with pneumonia. In Kenyan hospital cohorts, neurological disorders and other chronic conditions significantly increased the risk of readmission and mortality among children with severe pneumonia [20]. A meta-analysis conducted in low- and middle-income countries reported that chronic underlying diseases increased ALRI mortality nearly fivefold (OR 4.76, 95% CI 3.27-6.93) [9]. These findings are consistent with the biological mechanisms through which comorbid conditions reduce cardiopulmonary reserve and increase vulnerability to complications.

Preventive and social determinants also played an important role in our study. Children who had not received routine vaccination with the combined 5-in-1 or 6-in-1 vaccine had higher odds of severe pneumonia (aOR: 2.68, 95% CI 1.20-5.95; P=.016), and lack of pneumococcal vaccination was similarly associated with increased risk (aOR: 1.78, 95% CI 1.12-2.84; P=0.015). Evidence from other settings also highlights the protective role of vaccination against severe pneumonia outcomes. For example, in Kenyan hospitalised children, complete vaccination was associated with reduced in-hospital mortality among severe pneumonia cases (aHR 0.74, 95% CI 0.60-0.91) [20-23], while in an Indian community study, partial immunisation more than doubled the odds of pneumonia compared with full immunisation (OR 2.41, 95% CI 1.06-5.49) [17]. In addition, caregiver education below high school level remained independently associated with severe pneumonia (aOR: 2.45, 95% CI 1.34-4.49; P=.004). Socioeconomic disadvantages, including low maternal education, have consistently been associated with increased ALRI mortality and severity in children [9,18]. These findings suggest that both biological vulnerability and social determinants contribute to severe pneumonia risk in this population.

From a public health perspective, these findings highlight several potential intervention points. Improving routine immunisation coverage, particularly for combination and pneumococcal vaccines, may reduce the risk of severe pneumonia among hospitalised children. In addition, children born preterm or with underlying diseases may benefit from targeted preventive care and early clinical monitoring when respiratory symptoms occur. Caregiver education may also play a role in promoting timely healthcare seeking and preventive practices, which could reduce the likelihood that pneumonia progresses to severe disease.

This study has several strengths and limitations. A key strength is the prospective collection of clinical and epidemiological data using standardised national diagnostic criteria, which improves data completeness and comparability with other studies. However, several limitations should be acknowledged. The cross-sectional design limits the ability to establish causal relationships between risk factors and disease severity. The study was conducted at a single tertiary pediatric hospital, which may limit the generalizability of the findings to other healthcare settings. Some variables, including vaccination history and duration of illness before admission, relied on caregiver report and may therefore be subject to recall bias. In addition, microbiological confirmation was not available for all cases, limiting pathogen-specific analysis.

Conclusion     

In this hospital-based study, severe pneumonia accounted for nearly one-third of pneumonia admissions among children under five years of age. Preterm birth, underlying diseases, lack of routine childhood vaccination, lack of pneumococcal vaccination, and lower caregiver educational level were independently associated with severe pneumonia. These findings suggest that strengthening vaccination coverage, improving caregiver awareness, and providing targeted preventive care for high-risk children may help reduce the occurrence of severe pneumonia in similar hospital settings.

Competing interests     

The authors declare no competing interests.

Authors' contributions     

Conception and study design: Hien Thi Thu Nguyen, Thuy Thi Dieu Nguyen. Data collection: Hien Thi Thu Nguyen, Anh Thi Hoang Vu, Thuy Thi Thu Bui. Data analysis and interpretation: Tuyen Van Chu, Thuy Thi Dieu Nguyen. Manuscript drafting: Hien Thi Thu Nguyen. Manuscript revision: Thuy Thi Dieu Nguyen, Anh Thi Hoang Vu, Thuy Thi Thu Bui, Tuyen Van Chu, Ha Dang Thi. Guarantor of the study: Thuy Thi Dieu Nguyen. All authors have read and agreed to the final manuscript.

Acknowledgments     

The authors sincerely thank the leadership, physicians, nurses, and medical staff of Hai Duong Children's Hospital for their support in patient recruitment and data collection. We are also grateful to the participating children and their caregivers for their cooperation and contribution to this study.

Tables     

Table 1: baseline characteristics of pediatric pneumonia patients (n = 570)

Table 2: univariate and multivariable logistic regression analysis of factors independently associated with severe pneumonia

References