Epidemiological and clinical characteristics of respiratory syncytial virus infection in hospitalized neonates in Suzhou

Introduction

Respiratory syncytial virus (RSV) is a prominent pathogen linked with acute respiratory infections among the population of infants, immunocompromised individuals, and the elderly around the world (1). After host infection, many times it is seen that RSV can cause clinical conditions such as bronchiolitis, pneumonia, and asthma, along with recurrent patterns of infection being established, along with ease of rapid transmission and course of disease progression (2). Evidence suggests that worldwide, there are approximately 340,000 to 640,000 cases of RSV bronchiolitis and pneumonia annually in children <5 years of age, with approximately 50,000 to 200,000 deaths taking place each year (3,4). This established burden on pediatric health is a major cause of morbidity and mortality in infants. At a minimum, there is no question as to the real-world threat associated with (5).

Neonates have higher levels of susceptibility to illness associated with RSV infection related to the developing infant’s immune system, upper airway (small size), inadequate levels of maternally transferred immunoglobulin, and immunologic immaturity in general. The characteristics of immunologic immaturity expand the potential for risk of infection and clinical issues such as apnea, respiratory failure, and prolonged hospital stay (6). Similar effects are seen, but are typically more pronounced in infants born preterm. Infection tends to be associated with clinical consequences for preterm infant populations. This has been attributed to an inadequate antibody response in neonates, low levels of passively acquired Ig (7). Rates of increased mortality associated with RSV infection are significantly related to being preterm and/or being under three months of age, among hospitalized infants (8). Neonates also have greater risk for associated acute lower respiratory tract infections (ALRTIs) illnesses associated with RSV, with the absence of an effective vaccine (9). It cannot be overstated that with the burden of severe respiratory disease seen across the globe in the infant population as a result of RSV risk, preventive approaches to immunoprophylaxis for RSV have become a priority strategy.

Presently, there are three approved products for the prevention of diseases associated with RSV in infants that include a maternal immunization vaccine, and monoclonal antibodies, more specifically, known generically such as palivizumab and nirsevimab (10). Additionally, there are more than 10 products currently in the preclinical stages of development such as vaccines and monoclonal antibodies (11). Nirsevimab is a long-acting product and is the first biologic product approved by China and registered for the prevention of RSV-related LRTI in infants of a certain age, filling the potential door for a gap in RSV-specific preventive immunization (12).

In China, and specifically in Suzhou, minimal epidemiological studies are focusing solely on RSV infections in hospitalized neonates (particularly over time). Limited literature exists overarching the study of RSV burden on a pediatric population in particular regions, specifically related to seasonal patterns, potential co-infections, and neonatal risk factors alike (13,14).

This study describes an extensive epidemiological study of RSV infections over ten years in hospitalized neonates in East China (Suzhou) with the intention to characterize regional seasonal patterns and unique clinical risk factors for severe disease. The study complements other epidemiological studies of RSV specifically because of the attention to neonates as a unique population that has not been a focus of caregiver studies previously. We present this article in accordance with the STROBE reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-414/rc).

Methods

Subjects

This retrospective study was conducted at the Children’s Hospital of Soochow University and involved neonates admitted with ALRTIs between January 2013 and December 2023. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study protocol was reviewed and approved by the Institutional Review Board of Children’s Hospital of Soochow University (No. 2022IRB056) and informed consent was taken from all the patients’ parents or legal guardians. A total of 7,420 neonates were enrolled based on the following eligibility criteria: full-term birth (gestational age ≥37 weeks); age less than 28 days at admission; and diagnosis of ALRTI according to Zhu Futang Practical Pediatrics (15), 8th edition. The exclusion criteria included neo-nates with severe comorbidities (e.g., congenital heart disease, immunodeficiency, pulmonary hypoplasia, or bronchopulmonary dysplasia); insufficient respiratory pathogen testing; insufficient clinical records; and preterm birth.

Patients were categorized into two groups by disease severity: severe or non-severe. The clinical definition of severe RSV infection was based on diagnostic criteria for severe pneumonia in Guidelines for the Management of Community-Acquired Pneumonia in Children, which stated as any of the following: oxygen saturation <92% on room air, need for supplemental oxygen or mechanical ventilation, or admission to ICU (16).

General clinical data collection and definitions

General clinical data were collected, including age, gender, month and year of onset, duration of hospitalization, clinical symptoms, complications, underlying conditions, and laboratory findings (including radiological results), as well as treatment and outcomes. ALRTI encompasses acute bronchitis, acute bronchiolitis, and acute pneumonia.

Conditions recognized as underlying disorders included all pre-existing structural or functional abnormalities, including congenital heart disease, bronchopulmonary dysplasia, pulmonary malformations, Down syndrome, as well as other neonatal comorbidities that we deemed relevant. An inflammatory biomarker known as high-sensitivity C-reactive protein (hsCRP) was utilized in the analysis and defined as abnormal when hsCRP ≥8 mg/L.

RSV co-infection was defined as RSV with at least one other pathogen, including viral antigens, atypical pathogens, and a positive bacterial culture (the inclusion of bacteria required an acceptable clinical correlation to be considered co-infection, since it is difficult to differentiate bacterial colonization from infection without systemic symptoms, radiology evidence of infection, and elevated inflammatory markers).

Specimen collection and pathogen detection

Respiratory samples were collected by trained health-care workers. For spontaneously breathing neonates, nasal and pharyngeal aspirates were collected, and for neonates on ventilation, sputum was obtained by negative-pressure suction through an endotracheal tube. Pathogen detection methods varied by year.

From 2013 to 2021, direct immunofluorescent assays (DFA) for the detection of influenza A and B, parainfluenza I–III, and adenovirus were used. Human bocavirus, metapneumovirus, Mycoplasma pneumoniae (MP), and Chlamydia pneumoniae (CP) were detected by polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) testing.

From 2022–2023, an upgraded multiplex real-time fluorescence PCR panel was used to detect 13 respiratory pathogens simultaneously. Responsive pathogens included: influenza A virus (H1N1), influenza B virus (H3N2), adenovirus, bocavirus, coronaviruses, human rhinovirus (HRV), metapneumovirus, Chlamydia pneumoniae, and Mycoplasma pneumoniae.

For all respiratory specimens, routine bacterial cultures were conducted. Pathogens were identified by standard microbiological methods, and definitive diagnosis of co-infection only required positive culture and clinical and microbiological evidence of infection.

Statistical analysis

Statistical analyses were performed with SPSS version 26.0. Continuous variables were assessed for normality; normally distributed continuous variables were expressed as mean ± standard deviation and compared using an independent samples t-test. Not normally distributed continuous variables were reported as median (interquartile range between Q1 and Q3) and compared using the Mann-Whitney U test.

Categorical variables were expressed as frequency and percentage and compared by group using Chi-squared or Fisher’s exact test, as appropriate. Multivariate logistic regression analysis was done using the variables which were statistically significant in the univariate analysis to determine the independent risk factors for severe RSV infection. A two-tailed P value of <0.05 was considered statistically significant for all tests.

Results

RSV incidence and co-infection distribution

A total of 875 cases of RSV infection were found, which represented 11.79% (875/7420) of the overall population enrolled in this study. Out of the 875 cases of RSV identified, 229 cases (26.17%) were co-infections of other respiratory pathogens. The commonly encountered bacterial pathogens in these encounters included Staphylococcus aureus (89 cases, 10.17%), Escherichia coli (37 cases, 4.29%), and Klebsiella pneumoniae (24 cases, 2.74%). Viral co-infections were less common and only used parainfluenza virus (7 cases, 0.8%), influenza A (H1N1) (6 cases, 0.69%), and influenza B (H3N2) (5 cases, 0.57%). Moreover, atypical pathogen species were also noted, including Mycoplasma pneumoniae (22 cases, 2.51%) and Chlamydia pneumoniae (12 cases, 1.37%). The results of the data are shown in Table 1.

Demographic characteristics of RSV-positive neonates

Of the 875 neonates with RSV infection, 459 (52.46%) were male and 416 (47.54%) were female, resulting in a male-to-female ratio of 1.10:1. The neonates’ ages ranged from 1 to 28 days, with a median age of 19 days. The occurrence of RSV positivity increased with postnatal age, with an increased number of cases after the first week of life. The age distribution is shown in Figure 1.

Figure 1 Age distribution of ALRTI in newborns with RSV infection. The figure shows the distribution of age (in days) of the 875 neonates with a recent diagnosis of RSV infections and ALRTI at the time of admission. Data were obtained from all neonates admitted to the Children’s Hospital of Soochow University between January 2013 and December 2023. ALRTI, acute lower respiratory tract infections; RSV, respiratory syncytial virus.

Temporal and seasonal distribution

In the years from 2013 to 2023, the annual rate of RSV detection was markedly variable, with the highest rate of 30.79% recorded in 2014 and the lowest rate of detection (7.03%) in 2019. Overall, the annual trend remained relatively stable until 2020, when the rates began to decline, likely due to public health measures associated with coronavirus disease 2019 (COVID-19). This variation in rate was illustrated in Figure 2.

Figure 2 Distribution of RSV-positive detections in newborns from 2013 to 2023. This figure illustrates the annual rates of RSV infection detection among 7420 newborn infants referred to pediatric wards because of ALRTI in the city of Suzhou, China, from 2013–2023. The bars demonstrate the percentage of RSV-positive cases to ALRTI admissions each year. It shows that there is a seasonal pattern and a significant increase in sustainability demonstrated by off-season peaks in 2023. ALRTI, acute lower respiratory tract infections; RSV, respiratory syncytial virus.

The RSV infections also showed strong seasonality. Of 875 RSV-positive neonates, there were 82 RSV cases in spring (March–May), 33 in summer (June–August), 177 in autumn (September–November), and 583 in winter (December–February). The greatest detection of RSV was in winter (24.83%), followed by autumn (8.75%), spring (5.77%), and summer (2.02%) (Table 2). The seasonal activity of RSV from October to March in these years heartily differed in 2023, when multiple peaks were detected in April, May, July, August, and again in November–December, as opposed to the typical winter peak.

Clinical features of severe vs. non-severe RSV infections

Among the RSV-infected neonates, we classified 191 (21.83%) as severe. The groups had similar symptoms, including cough and sputum production [871]; nasal congestion or rhinorrhea [839]; and respiratory distress [312]. On physical examination, moist rales [700 cases] and wheezing [164] were found.

Severe cases differed from non-severe cases on several clinical features. The neonates had a higher incidence of fever, respiratory distress, cyanosis, and a higher probability of abnormal auscultation findings (moist rales). In addition, the severity group received more overall oxygen therapy, more IVIg therapy, and a higher hospital length of stay than the non-severity group (Table 3).

Multivariate analysis of risk factors for severe RSV infection

To determine factors independently associated with severe RSV infection, a multivariable logistic regression analysis was performed. As shown in Table 4, preterm birth [odds ratio (OR) =2.325, 95% confidence interval (95% CI): 1.112–4.859], hsCRP ≥8 mg/L (OR =2.580, 95% CI: 1.527–4.359), mixed infection (OR =1.493, 95% CI: 1.038–2.146), and underlying disease (OR =3.679, 95% CI: 2.247–6.023) significantly increased risk of severe RSV infection (Table 4). These factors continued to be statistically significant after adjusting for potential confounders, emphasizing their role in disease severity.

Discussion

In this study, RSV was the most commonly identified viral pathogen in hospitalized neonates with ALRTI in Suzhou, with a positivity rate of 11.79%. The age distribution indicated that RSV infection became more prevalent as neonates aged past one week, likely due to more environmental exposure in addition to the decreasing levels of maternally derived transplacental antibodies (12). This is supported by the literature from other regions in China and overseas, demonstrating more susceptibility to RSV infections with advancing age in neonates, such as maternal antibodies decline and immune systems mature (15,17).

The annual detection rates of RSV in our cohort had considerable variation, with an increase particularly in 2014, and a downward trend during the years 2019–2022. These trends are consistent with reports from the United States and Australia, indicating that RSV circulation was considerably limited during the COVID-19 pandemic due to public health interventions including lockdowns, masks, and school closures (13,18). However, a significant finding in our study was the off-season RSV peak detected in 2023, which had multiple surges in April, May, July, August, and the last surge in November–December. This represented a dramatic off-season departure from normal RSV seasonal patterns, which in Suzhou, like most subtropical regions, is generally performed from October to March, peaking in the winter months (19). Further post-pandemic shifts in RSV seasonality have been similarly reported in France, Japan and Canada. Easing restrictions resulted in a delayed but certainly amplified RSV epidemic (20). This shift may result from a population-level immunity gap engendered by a decline in viral exposure caused by pandemic-related mitigations. It could also stem from decreased maternal antibodies due to the decline in viral circulation.

It may be time to change the monoclonal antibody prophylaxis (e.g., palivizumab or nirsevimab) policy from simply being seasonal (i.e., we start prophylaxis every fall) to utilizing the information on local RSV viral circulation to optimize prophylaxis as opposed to having a predetermined seasonal window. Enhanced year-round monitoring is paramount in identifying potential post-RSV surges (bounce-back RSV waves), especially in Suzhou, where winter restrictions facilitated reduced RSV seasonality.

The coinfection rate from our data was 26.16%, coincident with studies conducted in Italy and South Korea stating this was 20–30% of hospitalized neonates with RSV co-infection with a bacterial or viral pathogen (21-24). Our findings indicated that neonates with co-infections had an increased probability of requiring oxygen therapy and had a negative severity of illness trajectory. Co-pathogens were also common in the antibiotic-resistant S. aureus. This finding was similar to Rich et al., which stated the frequency of isolation of S. aureus, some of which were methicillin resistant (25). These findings support a requirement for empirical antibiotic coverage in neonates admitted with severe RSV infection, where laboratory or imaging elucidated a likely secondary bacterial involvement.

Clinically, during RVSV infection in neonates and those who presented with severe disease had more frequent fever, were more cyanotic, had more respiratory distress than those who did not have RSV or had mild-moderate RSV disease. Our logistic regression model identified prematurity, hsCRP ≥8 mg/L, coinfection and existence of underlying disease as independent risk factors for severe illness associated with RSV. The association remained significant after adjustment and is highly consistent with findings from multiple studies worldwide. First, prematurity (OR =2.325), as a key risk factor, supports the repeatedly emphasized mechanism in the previous studies that “immature immune systems and underdeveloped lungs are the core reasons why preterm infants are susceptible to severe infection” (26-28). Second, the strong association with hs-CRP ≥8 mg/L (OR =2.580) underscores the role of hyperactivated inflammatory responses in disease progression, aligning with the hypothesis that bacterial exposure exacerbates viral pathogenesis (29). Third, the risk associated with co-infection (OR =1.493) supports multiple reports of “worsened clinical symptoms due to viral co-infection” (30), particularly consistent with findings from a study in Thailand that co-infection increases the risk of mortality in lower respiratory tract infections (LRTIs) (31). Finally, the markedly high risk conferred by underlying diseases (OR =3.679) highlights the vulnerability of special populations, corroborating reports that children with cystic fibrosis are more prone to severe RSV infection (32). These findings provide objective indicators for the early clinical identification of high-risk neonates and establish an evidence-based foundation for targeted preventive strategies, such as immunoprophylaxis in high-risk groups (33,34).

We believe our findings in the SUS of RSV have important implications for clinical care. Our recommendations include targeted surveillance on neonates with risk factors and early intervention, particularly for preterm neonates and those with inflammatory markers. These neonates must be prioritized for the passive immunization strategy of nirsevimab and palivizumab, where indicated, and will mitigate complications. Active maternal immunization during pregnancy would further improve protection for neonates, particularly in regions with a poor maternal rate of breastfeeding (35-37). Overall, our work reiterates the benefit of integrated seasonal risk surveillance with individual neonate risk profiling per five-six cohort levels as they relate to RSV prevention and response.

This study has some limitations. First, since it was conducted in a unique tertiary center, it may restrict its general availability of findings to other populations in China; additionally, the retrospective nature of the study may create a bias due to record-level information or contrary variables that we may not have recorded in the clinical records. Second, we could not perform RSV subtyping, preventing us from recognizing any ease in recognizing strain-specific impacts on the severity of illness. Finally, we did not account for socioeconomic and demographic information; for example, parental information may include education or anything related to their level of income—this may affect their hospitalization risk and ease of care. In future studies, we will conduct multicenter prospective investigations to address the limited representativeness of single-center samples, employ standardized protocols for RSV genotyping to assess the impact of different strains on disease severity, and systematically collect socioeconomic and demographic information (such as parental education level and income) to improve the evaluation of hospitalization risks and disparities in care.

Conclusions

To summarize, RSV remains the foremost contributor to the morbidity of ALRTIs in hospitalized neonates in the Suzhou area, occurring seasonally in the autumn and winter seasons, with a significant shift in unexpected off-season surges in 2023. Prematurity, elevated hsCRP (≥8 mg/L), co-infection with other pathogens, and comorbidities remained significantly associated risk factors for severe RSV.

These findings support the requirement for systematic surveillance and focused preventive measures in the future, especially in high-risk neonates. Preventive measures such as passive immunization (i.e., nirsevimab & palivizumab) and maternal vaccination in preterm and chronic NICU infants during RSV season should be prioritized. Health education resources for caregivers and infection control in the NICU are important as well.

Validation of these findings and longer-term outcomes, including the ongoing implementation of new RSV immunization programming, in a prospective multicenter study would be ideal.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-414/rc

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-414/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-414/prf

Funding: This study was funded by the 2022 Medical research project of Jiangsu Provincial Health Commission (No. M2022058).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-414/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study protocol was reviewed and approved by the Institutional Review Board of Children’s Hospital of Soochow University (No. 2022IRB056) and informed consent was taken from all the patients’ parents or legal guardians.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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