Hostname: page-component-74656f9884-7drmj Total loading time: 0 Render date: 2026-11-05T08:15:24.917Z Has data issue: false hasContentIssue false
  • English
  • Français

Impact of national RSV immunization programme on the epidemiology of bronchiolitis in Greece

Published online by Cambridge University Press:  30 October 2026

Argyro Konstantopoulou Open the ORCID record for Argyro Konstantopoulou [Opens in a new window] ,
Kyriaki Velali ,
Panagiota Markopoulou ,
Aliki Anastasiadou ,
Theodoros Nikolopoulos ,
Panagiota Panagiotou ,
Maria-Eleni Bompou ,
Anna Messaritaki ,
Athanasios Michos Open the ORCID record for Athanasios Michos [Opens in a new window]  and
Christina Kanaka-Gantenbein
...Show all authors
Argyro Konstantopoulou
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece First Department of Pediatrics, Aghia Sophia Children’s Hospital, Greece
Kyriaki Velali
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Panagiota Markopoulou
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece 1st Neonatal Intensive Care Unit, Aghia Sophia Children’s Hospital, Greece
Aliki Anastasiadou
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Theodoros Nikolopoulos
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Panagiota Panagiotou
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Maria-Eleni Bompou
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Anna Messaritaki
Affiliation:
First Department of Pediatrics, Aghia Sophia Children’s Hospital, Greece
Athanasios Michos
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Christina Kanaka-Gantenbein
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
Tania Siahanidou*
Affiliation:
First Department of Pediatrics, Medical School, National and Kapodistrian University of Athens , Athens, Greece
*
Corresponding author: Tania Siahanidou; Email: siahan@med.uoa.gr
Rights & Permissions [Opens in a new window]

Abstract

Universal RSV immunization strategies for infants, including monoclonal antibody administration and maternal vaccination during pregnancy, were implemented in Greece for the first time during the 2025–2026 season. We conducted a prospective observational study of infants aged <12 months hospitalized with bronchiolitis at a quaternary paediatric hospital in Greece between November 2025 and April 2026. Bronchiolitis incidence was compared with that of the 2023–2024 and 2024–2025 seasons. Severity of bronchiolitis was assessed using the modified Tal score. Across the three seasons, 854 of 15246 admissions were due to bronchiolitis. The incidence decreased significantly in 2025–2026 to 42 per 1000 admissions, compared to 64 and 59 in 2024–2025 and 2023–2024, respectively. The 2025–2026 epidemic curve of bronchiolitis was flatter and more prolonged, with a delayed peak in March. Of 186 infants with bronchiolitis, 19.9% had received RSV immunization. Disease severity and oxygen requirements did not differ significantly between immunized and unimmunized infants, although unimmunized infants were older and had a trend towards higher complication rates. In conclusion, national RSV immunization was associated with reduced bronchiolitis hospitalizations and complications. However, the smaller-than-expected reduction in disease rates underscores the need to improve awareness, coverage, and timely immunization.

Keywords

bronchiolitisRSV bronchiolitisRSV immunization

Information

Type
Original Paper
Information
Epidemiology & Infection , Volume 154 , 2026 , e143
Check for updates
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press

Introduction

Acute bronchiolitis is the most common lower respiratory tract infection in infants and young children up to 2 years of age [Reference Mahant1]. Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis. Approximately 2 million hospitalizations and 20000 in-hospital deaths due to RSV bronchiolitis are estimated to occur annually worldwide among infants aged <12 months [Reference Li2]. It has also been shown that 1 in 5 children under the age of 2 has an outpatient medical visit for RSV infection, and 1 in every 56 healthy term-born infants is hospitalized due to RSV in high-income settings [Reference Wildenbeest3, Reference Lively4].

The epidemiological characteristics of RSV bronchiolitis have changed in recent years following the introduction of universal passive immunization strategies against RSV in infants [Reference Coma5–Reference Andina Martínez7]. These strategies include administration of a long-acting monoclonal antibody (Nirsevimab) to infants or maternal vaccination during pregnancy (RSVpreF vaccine – Abrysvo). The implementation of immunization strategies has reduced bronchiolitis-related hospitalizations by 43–75% and admissions to paediatric intensive care units (PICUs) by more than 60% among infants [Reference Villani6–Reference Manzoni10]. Specifically, among infants aged <6 months, hospitalizations and PICU admissions due to RSV decreased by 74–89% and up to 90%, respectively [Reference Coma5, Reference Drysdale11]. Moreover, RSV immunization appears to reduce the need for oxygen supplementation for RSV bronchiolitis in infants, with a seven-fold lower relative risk compared with non-immunized infants [Reference Villani6].

In Greece, bronchiolitis follows the typical epidemiological pattern of the Northern Hemisphere, with a season lasting from November to April and having a peak in January [Reference Koloi12]. A previous study showed that hospitalizations due to RSV infection account for 28 children per 1000 paediatric admissions in this country [Reference Berikopoulou13]. During the 2025–2026 season, RSV immunization was introduced for the first time into the Greek National Immunization Programme, a development expected to substantially affect the epidemiology not only of RSV but also of other causes of bronchiolitis [14]. According to the National Immunization Programme, there is an option between maternal vaccination during pregnancy, at 32–36 weeks of gestation, for pregnant women expected to deliver during the RSV season (November 1 to March 31), and passive immunization of newborns born during the RSV season with the monoclonal antibody Nirsevimab [14]. Administration of the monoclonal antibody is also indicated at the start of the RSV season for preterm infants born at <35 weeks of gestation and aged <6 months, as well as other groups of infants at high risk for severe RSV infection, including infants with bronchopulmonary dysplasia, severe immunosuppression, cystic fibrosis or other severe lung diseases, hemodynamically significant congenital heart disease, trisomy 21, or severe neurological disorders affecting the respiratory function [14]. As this was the first year of implementing passive RSV immunization in infants in Greece, to our knowledge, there are no published data yet on its impact on the disease’s epidemiology in this country.

The present study aims to describe the epidemiological characteristics of neonates and infants hospitalized with bronchiolitis during the 2025–2026 season and to investigate possible epidemiological changes in the disease compared with previous years in Greece.

Methods

Study design

We conducted a prospective observational study of infants admitted for hospitalization due to bronchiolitis at the ‘Aghia Sophia’ Children’s Hospital, the largest paediatric hospital in Greece, with 750 beds and serving more than 40% of the population of Attica. The study period extended from 1 November 2025 to 15 April 2026.

To assess possible changes in the epidemiology of bronchiolitis compared with previous years, infants hospitalized with bronchiolitis (RSV-positive or RSV-negative) during the two preceding seasons, from 1 November 2023 to 15 April 2024 and from 1 November 2024 to 15 April 2025, were also recorded. These data were retrieved from the admission registries of each department and from the hospital’s electronic medical record system. Furthermore, data regarding the overall number of hospitalizations for any cause in the above paediatric departments during each of the three RSV periods were collected from the hospital’s statistical department.

Study population

The study population consisted of infants aged <12 months who were hospitalized with bronchiolitis during the study period. Infants with underlying diseases (such as congenital heart disease, bronchopulmonary dysplasia, neurological disorders, etc.), for whom RSV immunization is indicated, were also included. The diagnosis of bronchiolitis was based on the NICE 2021 criteria [15]. Data were collected from the patients’ medical records.

Data collection

Demographic and epidemiological data of the infants were recorded. RSV immunization status, type of immunization (monoclonal antibody or maternal vaccination during pregnancy), and timing of immunization were documented. The severity of bronchiolitis was assessed using the Modified Tal Score (MTS). To calculate the score and classify disease severity as mild, moderate, or severe, the following parameters were assessed at the time of maximum clinical deterioration: respiratory rate, transcutaneous oxygen saturation, chest auscultation findings, and signs of respiratory distress [Reference Martín-Latorre16]. Duration of hospitalization, duration of oxygen supplementation, need for PICU admission, and complications were also recorded. RSV positivity was assessed either by antigen testing for RSV or Polymerase Chain Reaction (PCR) testing of a nasopharyngeal sample collected with a sterile nylon nasopharyngeal swab. The PCR assay was designed to detect both respiratory viruses and bacterial pathogens of the upper respiratory tract.

Statistical analysis

For data analysis, infants were divided into non-immunized and RSV-immunized groups. A subgroup analysis was also performed among unimmunized infants who had an indication for immunization and those who did not. Infants who had received the RSV monoclonal antibody less than 7 days before admission were considered non-immunized. In addition, infants whose mothers had been vaccinated against RSV less than 14 days before delivery and who did not receive a monoclonal antibody after birth were also classified in the non-immunized group. The in-hospital incidence per 1000 hospitalizations for both overall bronchiolitis and RSV-positive bronchiolitis was calculated for each period by dividing the number of these cases by the total hospital admissions during that time.

Categorical variables are presented as absolute and relative frequencies (N, %). Normality of quantitative variables was assessed using the Shapiro–Wilk test. As the data were not normally distributed, results are expressed as median and interquartile range (median, IQR). Categorical variables were compared using the Chi-square test (Pearson’s χ2). For comparison of continuous non-parametric variables between two independent groups, the Mann–Whitney U test was used, while the Kruskal–Wallis test was applied for comparisons among three groups. Statistical significance was set at p < 0.05. Statistical analysis was performed using IBM SPSS Statistics software.

Ethics statement

The study protocol was approved by the Scientific Committee of the hospital. Patient anonymity was ensured.

Results

Demographic and clinical characteristics of infants with bronchiolitis, 2025–2026

Between November 2025 and April 2026, a total of 4417 children were admitted to the hospital. Of these, 186 were treated for bronchiolitis, with boys accounting for 108 (58%) of cases. The median age was 3.8 (1.8–6.3) months, and 28 (15%) patients were neonates. Among hospitalized infants with bronchiolitis, 26 (14%) had an underlying condition, including congenital heart disease (n = 3), bronchopulmonary dysplasia (n = 2), prematurity (n = 18), neurological disease (n = 2), and immunodeficiency (n = 1). The remaining 160 (86%) infants were term-born and previously healthy.

Among infants with bronchiolitis, 37 (19.9%) were immunized against RSV; 28 infants through monoclonal antibody administration and 9 through maternal vaccination during pregnancy. The median age at administration of the monoclonal antibody among immunized infants was 28 (10.0–47.5) days. Out of the total infants, 126 (67.7%) tested positive for RSV. Other viruses isolated included adenovirus (n = 1), bocavirus (n = 1), SARS-CoV-2 (n = 1), influenza A virus (n = 1), and rhinovirus (n = 1). No pathogen was isolated in 56 infants (30.1%). Among RSV-positive infants, 24 (19%) were immunized against RSV.

The median severity score of bronchiolitis, based on the MTS, was 5 (3–7), corresponding to mild–moderate bronchiolitis, with the median day of clinical deterioration being day 3 (3–4) of illness. Oxygen supplementation was required in 97 (51.9%) infants, with a median duration of 3 (2–4) days. Among the infants who required oxygen supplementation, only 5 (5.1%) – 4 neonates and a 3-month-old infant, and among them only one RSV-immunized infant – were supported with high-flow nasal cannula (HFNC); no child required intubation/invasive mechanical ventilation. The median length of hospital stay was 5 (3–6) days. Complications occurred in 43 (23%) infants, including pneumonia in 27 (14.5%), acute otitis media (AOM) in 13 (6.9%), and other complications in 3 (1.6%) infants. No infant required transfer to the PICU or died.

A comparison was performed between infants immunized against RSV and unimmunized infants (Table 1). No statistically significant difference was found between the two groups in terms of bronchiolitis severity based on MTS, duration of oxygen supplementation, or length of hospital stay. The median age at onset of bronchiolitis was higher among unimmunized (4.4 (2.1–6.7) months) compared with immunized infants (2.1 (0.9–3.8) months) (p < 0.001). In addition, the complication rate was higher among unimmunized than immunized infants (25.5% vs. 13.5%, p = 0.08) (Figure 1).

Table 1.

Comparison between RSV-immunized and unimmunized infants with bronchiolitis

A data table comparing clinical outcomes between R S V-immunized and unimmunized infants with bronchiolitis. See long description.
Table 1. Long description

The table consists of four columns and six data rows. The columns are: Variable, Immunized infants n equals 37, Unimmunized infants n equals 149, and p-value.

* Age in months: Immunized infants have a median of 2.1 with a range of 0.9 to 3.86. Unimmunized infants have a median of 4.4 with a range of 2.16 to 6.7. The p-value is less than 0.001.

* R S V, n percent: Immunized infants are 24 (64.8 percent). Unimmunized infants are 102 (68.4 percent). The p-value is 0.68.

* M T S (I Q R): Immunized infants have a score of 4 (2 to 6). Unimmunized infants have a score of 5 (3 to 7). The p-value is 0.45.

* L O S in days (I Q R): Immunized infants have a median of 5 (3 to 7). Unimmunized infants have a median of 4 (3 to 6). The p-value is 0.19.

* Oxygen requirements in days (I Q R): Immunized infants have a median of 2 (0 to 3). Unimmunized infants have a median of 1 (0 to 3). The p-value is 0.46.

* Complications, n percent: Immunized infants are 5 (13.5 percent). Unimmunized infants are 38 (25.5 percent). The p-value is 0.08.

Footnotes indicate M T S stands for modified Tal score and L O S stands for length of hospital stay. Comparisons were made using Mann-Whitney U and chi square tests.

MTS: modified Tal score; LOS: length of hospital stay. Comparisons were made using Mann–Whitney U and chi square (χ2) tests.

Figure 1.

Rates of overall and specific complications (pneumonia, ΑΟΜ, other complications) in RSV-immunized and unimmunized infants with bronchiolitis. AOM: acute otitis media.

A bar chart shows lower complication rates for R S V immunized infants compared to unimmunized infants across all categories. Total complications are 13.50% versus 25.50%, pneumonia 10.80% versus 15.40%, and A O M 2.70% versus 8.10%.

An indication for RSV immunization according to the current National Immunization Programme in Greece was present in 50 of 149 non-immunized infants with bronchiolitis (33.5%). The remaining 99 of 149 infants (66.5%) did not have an indication for immunization according to the national programme but were aged <6 months at the beginning of the RSV season. No statistically significant difference was found between infants without an indication for immunization and those with an indication in terms of RSV positivity (N = 71 (71.7%) vs. 31 (62%), respectively; p = 0.15), bronchiolitis severity (MTS 5 (3–7) vs. 5 (3–7), respectively; p = 0.8), need for oxygen supplementation, (N = 48 (48.5%) vs. 28 (56%), respectively; p = 0.2), or frequency of complications, (N = 13 (26%) vs. 25 (25%), respectively; p = 0.4).

Epidemiological changes in bronchiolitis: Comparison of the 2025–2026 season with the previous epidemic seasons (2023–2024 and 2024–2025)

A total of 15246 hospital admissions were analysed across the three study periods, of which 854 (5.6%) were due to bronchiolitis. Table 2 summarizes the comparative data throughout the three epidemic periods.

Table 2.

Comparative analysis of infant bronchiolitis hospitalizations in the 2023–2024, 2024–2025, and 2025–2026 seasons

A data table comparing infant bronchiolitis hospitalizations across three seasons from 2023 to 2026, showing incidence rates, R S V prevalence, age, and length of stay. See long description.
Table 2. Long description

The table consists of five columns: Category, 2023–2024, 2024–2025, 2025–2026, and p-value.

• Number of infants with bronchiolitis, n: 314 in 2023–2024; 354 in 2024–2025; 186 in 2025–2026.

• Incidence per 1,000 admissions: 59 in 2023–2024; 64 in 2024–2025; 42 in 2025–2026. The p-value is less than 0.001.

• R S V, n (percentage): 170 (54.1 percent) in 2023–2024; 173 (48.8 percent) in 2024–2025; 126 (67.7 percent) in 2025–2026. The p-value is less than 0.001.

• Age in months (I Q R): 3.1 (1.4–5.6) in 2023–2024; 3.4 (1.7–6) in 2024–2025; 3.8 (1.8–6.3) in 2025–2026. The p-value is 0.165.

• L O S in days (I Q R): 4.5 (3–7) in 2023–2024; 4 (2–5) in 2024–2025; 5 (3–6) in 2025–2026. The p-value is less than 0.001.

Footnote: L O S indicates length of hospital stay. Comparisons were performed using Kruskal–Wallis and chi-square tests.

LOS: length of hospital stay. Comparisons were performed using Kruskal–Wallis and chi-square (χ2) tests.

During the 2023–2024 period, a total of 5316 admissions were recorded; of these, 314 cases were diagnosed with bronchiolitis, yielding an incidence of 59 per 1000 admissions. For the 2024–2025 period, there were 354 bronchiolitis cases out of 5513 total admissions, corresponding to an incidence at 64 per 1000 admissions. Conversely, a decline was recorded during the 2025–2026 period; out of 4417 total admissions, 186 were attributed to bronchiolitis and the incidence dropped significantly to 42 per 1000 admissions. Consequently, there was a significant reduction in the relative risk of admission for bronchiolitis (RR = 0.68, 95% CI: 0.58–0.80). Statistical analysis confirmed that the variation in the incidence of bronchiolitis admissions across the three periods was highly statistically significant (χ2 = 24.093, p < 0.001). During the 2025–2026 season, the absolute number of infants admitted with bronchiolitis dropped by 47.5% compared to 2024–2025, and by 40.7% compared to 2023–2024.

Focusing specifically on RSV, 469 RSV-positive cases were recorded throughout the study, accounting for 3.1% of all hospitalizations. The absolute number of RSV-related hospitalizations decreased notably during the 2025–2026 season, representing a 27% reduction compared with 2024–2025 and a 26% reduction compared with 2023–2024. The incidence of RSV-related hospitalizations per 1000 admissions did not differ significantly among the three epidemic periods (32 in 2023–2024, 31 in 2024–2025, and 29 in 2025–2026) (p = 0.5) although a significant reduction was observed during the RSV peak months (December-February) in 2025-2026 (RR=0.71, 95% CI: 0.56-0.91).

Regarding the neonatal population of the study, no statistically significant reduction in admissions for bronchiolitis (99, 107, and 105 per 1000 admissions in 2023–2024, 2024–2025, and 2025–2026, respectively; p = 0.9) or RSV bronchiolitis (63.5, 55.4, and 75.2 per 1000 admissions in 2023–2024, 2024–2025, and 2025–2026, respectively; p = 0.6) was observed.

Analysing the seasonal patterns of bronchiolitis, the epidemic peak incidence shifted from January during the 2023–2024 season (92.9 per 1000 admissions) to February in 2024–2025 (108.2 per 1000 admissions), whereas during the 2025–2026 period, the epidemiological curve became visibly flatter and more prolonged. The incidence of bronchiolitis remained substantially lower in 2025–2026 than in the previous peak season throughout the winter months, ultimately reaching a delayed and much lower maximum in March (56.3 per 1000 admissions). March thus emerged as the new peak for overall bronchiolitis (Figure 2), while February and March represented the peak months for RSV-specific bronchiolitis (Figure 3).

Figure 2.

Monthly incidence of bronchiolitis hospitalizations (per 1000 admissions) across the 2023–2024, 2024–2025, and 2025–2026 seasons.

A line graph with an integrated data table showing bronchiolitis hospitalization rates across three seasons from November to April. See long description.
Figure 2. Long description

The y-axis is labeled Bronchiolitis per 1,000 admissions with a scale from 0.0 to 120.0. The x-axis lists months from November to April. A data table at the bottom provides exact values for three seasons.

* 2023–2024 (red line): Starts at 19.7 in November, peaks early in January at 92.9, then declines steadily to 28.7 by April.

* 2024–2025 (purple line): Starts at 27.9 in November, rises sharply to a season-high peak of 108.2 in February, then drops to 48.9 by April.

* 2025–2026 (blue line): Shows the lowest overall incidence, starting at 12.9 in November, reaching a plateau between January (55.2) and March (56.3), before falling to 21.0 in April.

Vertical grid lines align the data points for each month across the three seasons.

Figure 3.

Monthly incidence of RSV bronchiolitis hospitalizations (per 1000 admissions) across the 2023–2024, 2024–2025, and 2025–2026 seasons.

A line graph with an integrated data table comparing R S V bronchiolitis hospitalization rates across three consecutive seasons from November to April. See long description.
Figure 3. Long description

The Y-axis represents R S V bronchiolitis per 1,000 hospitalizations, ranging from 0.0 to 70.0. The X-axis lists months from November to April. Three colored lines represent different seasons.

* The 2023-2024 season (red line) shows an early, sharp peak in January at 58.4, followed by a steady decline to 5.1 in April.

* The 2024-2025 season (purple line) shows a later, higher peak in February at 64.7, starting from 5.4 in November and ending at 12.8 in April.

* The 2025-2026 season (blue line) shows a broader, lower peak reaching 44.3 in February and maintaining a higher rate through March (40.0) before dropping to 13.1 in April.

A data table at the bottom provides exact values for each month. In November, rates are low (1.1 to 5.4). By January, all seasons show significant increases (35.7 to 58.4). The 2024-2025 season reaches the highest single point of the entire dataset in February.

Discussion

This was a prospective observational study including a total of 186 infants hospitalized due to bronchiolitis during the period from November 2025 to April 2026, following the first-year implementation of the National Infant RSV Immunization Programme in Greece. To our knowledge, this is the first study in Greece focusing on the epidemiology of bronchiolitis and RSV-bronchiolitis before and after the implementation of immunization measures.

According to the results of this study, a significant reduction in bronchiolitis cases and rates per 1000 admissions was observed during the 2025–2026 season compared with the 2024–2025 and 2023–2024 seasons. The absolute number of hospitalizations due to RSV bronchiolitis also declined in 2025–2026; however, there was no corresponding reduction in the rate of hospitalizations due to RSV bronchiolitis per 1000 all-cause admissions. The lack of a significant reduction in the incidence of RSV bronchiolitis may be attributed to several factors. First, as this was the first year of implementation of the RSV prevention programme in Greece, hesitancy in the general population towards ‘new vaccines’ may have represented an obstacle to achieving particularly high immunization rates [Reference Manzoni10, Reference Moline17]. Preliminary data from our hospital showed that fewer than 20% of neonates were passively immunized against RSV through maternal vaccination during pregnancy, whereas among infants born to unvaccinated mothers, less than 40% received immunization with Nirsevimab [Reference Velali18]. Moreover, it should be noted that approximately 50% of infant hospitalizations due to RSV concern infants born outside the RSV season [Reference Sarna19–Reference Drajac22]. Under the current RSV prevention programme in Greece, which does not include healthy infants <6 months at the start of the RSV season (catch-up group), no more than 50% of infants not belonging to a high-risk group are eligible for immunization [14]. Second, immunization with the monoclonal antibody Nirsevimab was delayed. In our study population, infants were immunized with Nirsevimab at a median age of 28 days, which is considerably older than the recommended age for immunization, namely before discharge from the maternity hospital or as soon as possible thereafter and preferably within the first 7 days of life [14]. Third, prescription data from Greece indicate that prescriptions for Nirsevimab decreased after the first months of the RSV season (personal communication with Greek authorities). This may have contributed to the substantially flattened and prolonged epidemiological curve of RSV bronchiolitis during the 2025–2026 period, with many new cases appearing in March compared with previous years. Finally, it cannot be excluded that reduced sensitivity of the antigen test used to detect RSV bronchiolitis during previous years in our hospital may have contributed to the absence of a significant decrease in the frequency of RSV bronchiolitis during the 2025–2026 period.

The reduction in bronchiolitis rates in our total study population is consistent with the findings of previous studies, highlighting the important contribution of RSV immunization to reducing the overall burden of viral lower respiratory tract infections during infancy [Reference Andina Martínez7, Reference Manzoni10]. In contrast, the number of admissions due to bronchiolitis among neonates did not decrease during the 2025–2026 season, possibly because of the low uptake of maternal RSV vaccination during pregnancy, as well as the fact that the median age at monoclonal antibody administration corresponded to the end of the neonatal period. Additionally, the effectiveness of transplacental antibody transfer to the neonate following maternal vaccination can be influenced by the timing of vaccination relative to delivery, prematurity, maternal immunocompromised states, and other maternal or placental factors [Reference Ciobanu23, Reference Atwell24]. This finding underscores the need to raise awareness and provide adequate information to both healthcare professionals and parents regarding the value of maternal RSV vaccination and the timely administration of monoclonal antibodies to all eligible neonates in order to increase immunization coverage. The broader impact of RSV – including healthcare costs, the financial burden on families, and reduced quality of life among affected children and their caregivers – provides an additional rationale for strengthening preventive strategies and expanding RSV immunization [Reference Mao25–Reference Bozzola27]. The absence of a reduction in neonatal hospitalization rates, combined with the increased vulnerability of this age group, may also have contributed to the longer length of hospital stay observed among infants during the 2025–2026 season compared with previous years.

Among infants hospitalized with bronchiolitis during the 2025–2026 season, almost 20% had been immunized against RSV. When immunized and unimmunized infants with bronchiolitis were compared, immunized infants were, as expected, younger in chronological age; however, they also tended to have a lower complication rate, a finding that has not previously reported in relevant studies. Interestingly, no significant differences were observed between the two groups regarding disease severity, oxygen requirement, or length of hospitalization. Importantly, one in three unimmunized infants (33.5%) had missed the opportunity for immunization despite being eligible according to the Greek National Immunization Programme. The remaining unimmunized infants (66.5%) did not meet the current criteria for immunization according to the national programme but were younger than 6 months of age at the beginning of the RSV season. Therefore, increasing immunization coverage, but more importantly extending the immunization programme to include healthy infants aged <6 months at the beginning of the RSV season, similarly to policies adopted in other countries, appears necessary in order to maximize the benefits of infant RSV immunization [Reference Brault28–Reference Kiefer31]. Supporting this perspective, RSV positivity, bronchiolitis severity according to the MTS, need for oxygen supplementation, and frequency of complications did not differ significantly between infants eligible for RSV immunization according to the Greek National Immunization Programme and those without an indication for immunization but aged <6 months at the beginning of the RSV season. Without implementation of an expanded RSV immunization programme covering both infants born during the RSV reason and those born outside the season but younger than 6 months of age, RSV is likely to remain a substantial cause of healthcare resource utilization in the coming years [Reference Ares-Gómez32, Reference Martinón-Torres33].

The strengths of the present study include its prospective design, the recruitment of patients from the largest paediatric hospital in the country, the objective assessment of bronchiolitis severity using the MTS, and the fact that it is the first study to examine changes in bronchiolitis epidemiology following the implementation of infant RSV immunization measures in Greece. The study also has certain limitations. It was a single-centre study conducted in a public hospital, and the number of studied infants was rather small. The incidence of bronchiolitis was evaluated among hospitalized infants, but changes in the overall epidemiology of the disease in the community were not investigated. Finally, comparisons of clinical characteristics between immunized and non-immunized infants may be subject to bias, as immunized infants who ultimately became ill and required hospitalization may, by definition, represent more severe breakthrough cases.

Conclusions

This is the first study examining changes in the incidence of bronchiolitis among hospitalized infants following the first year of implementation of RSV immunization strategies in Greece. A significant reduction was observed in both the total number and incidence of hospitalizations due to bronchiolitis, as well as in the total number of RSV-related hospitalizations, compared with previous years. Moreover, a borderline reduction in post-RSV complications, including pneumonia and AOM, was observed among immunized infants. Therefore, it is important to inform and raise awareness among the medical community and the general population regarding the benefits of universal infant RSV immunization, as well as to conduct further studies investigating potential changes in the infectious causes of bronchiolitis or the development of RSV resistance to immunization products.

Abbreviations

AOM

acute otitis media

CI

confidence interval

HFNC

high-flow nasal canula

ICU

intensive care unit

IQR

interquartile range

MTS

modified Tal score

RR

relative risk

RSV

respiratory syncytial virus

Data availability statement

Data are available from Tania Siahanidou () upon request.

Author contributions

Data curation: A.K., K.V., P.M., T.N., A.M.; Formal analysis: A.K., K.V., P.M.; Investigation: A.K., K.V., P.M., A.A., T.N., P.P., M-E.B., A.M., A.M., C.K.; Methodology: A.K., K.V., P.M., A.A., T.N., P.P., M-E.B., A.M., C.K., T.S.; Software: A.K., K.V., P.M.; Writing - original draft: A.K., K.V.; Writing - review & editing: P.M., A.A., T.N., P.P., M-E.B., A.M., A.M., C.K., T.S.; Conceptualization: P.P., A.M., C.K., T.S.; Resources: P.P., M-E.B., A.M., C.K., T.S.; Supervision: C.K., T.S.; Validation: T.S.

Funding statement

This research received no specific grant from any funding agency, commercial or not-for-profit sectors. The publication of the article in OA mode was financially supported by HEAL-LINK.

Competing interests

The authors declare none.

References

Mahant, S, et al. (2022) Rates in bronchiolitis hospitalization, intensive care unit use, mortality, and costs from 2004 to 2018. JAMA Pediatrics 176(3), 270–279. https://doi.org/10.1001/jamapediatrics.2021.5177.CrossRefGoogle ScholarPubMed
Li, Y, et al. (2022) Global, regional, and national disease burden estimates of acute lower respiratory infections due to respiratory syncytial virus in children younger than 5 years in 2019: A systematic analysis. Lancet 399(10340), 2047–2064. https://doi.org/10.1016/S0140-6736(22)00478-0.CrossRefGoogle ScholarPubMed
Wildenbeest, JG, et al. (2023) The burden of respiratory syncytial virus in healthy term-born infants in Europe: A prospective birth cohort study. The Lancet Respiratory Medicine 11(4), 341–353. https://doi.org/10.1016/S2213-2600(22)00414-3.CrossRefGoogle ScholarPubMed
Lively, JY, et al. (2019) Respiratory syncytial virus-associated outpatient visits among children younger than 24 months. Journal of the Pediatric Infectious Diseases Society 8(3), 284–286. https://doi.org/10.1093/jpids/piz011.CrossRefGoogle ScholarPubMed
Coma, E, et al. (2024) Effectiveness of nirsevimab immunoprophylaxis against respiratory syncytial virus-related outcomes in hospital and primary care settings: A retrospective cohort study in infants in Catalonia (Spain). Archives of Disease in Childhood 109(9), 736–741. https://doi.org/10.1136/archdischild-2024-327153.CrossRefGoogle ScholarPubMed
Villani, A, et al. (2025) RSV bronchiolitis: A disease only for those who do not receive prophylaxis. European Journal of Pediatrics 184(7), 437. https://doi.org/10.1007/s00431-025-06275-6.CrossRefGoogle Scholar
Andina Martínez, D, et al. (2024) Nirsevimab and acute bronchiolitis episodes in pediatric emergency departments. Pediatrics 154(4), e2024066584. https://doi.org/10.1542/peds.2024-066584.CrossRefGoogle ScholarPubMed
Blyth, CC, et al. (2026) Respiratory syncytial virus epidemiology and effectiveness of infant nirsevimab: 2024 results from the Australian sentinel hospital network (FluCAN-PAEDS). Eurosurveillance 31(2), 2500275. https://doi.org/10.2807/1560-7917.ES.2026.31.2.2500275.CrossRefGoogle ScholarPubMed
Bennett, JC, et al. (2026) RSV prevention products and severe RSV-associated disease among infants. JAMA Network Open 9(4), e265695. https://doi.org/10.1001/jamanetworkopen.2026.5695.CrossRefGoogle ScholarPubMed
Manzoni, P, et al. (2026) Did nirsevimab shift pediatric hospitalizations due to lower respiratory tract infections? A nationwide Italian study (2024-2025). Viruses 18(3), 274. https://doi.org/10.3390/v18030274.CrossRefGoogle ScholarPubMed
Drysdale, SB, et al. (2023) Nirsevimab for prevention of hospitalizations due to RSV in infants. The New England Journal of Medicine 389(26), 2425–2435. https://doi.org/10.1056/NEJMoa2309189.CrossRefGoogle ScholarPubMed
Koloi, A, et al. (2025) Impact of COVID-19 pandemic on bronchiolitis epidemiology in Greece. Medicina (Kaunas, Lithuania) 61(10), 1746. https://doi.org/10.3390/medicina61101746.CrossRefGoogle ScholarPubMed
Berikopoulou, MM, et al. (2024) Epidemiology of respiratory syncytial virus in hospitalized children before, during, and after the COVID-19 lockdown restriction measures in Greece. Epidemiology and Infection 152, e94. https://doi.org/10.1017/S0950268824000724.CrossRefGoogle ScholarPubMed
Ministry of Health (2025) National Immunization Program for Children and Adolescents 2025 - Schedule and Recommendations. https://www.moh.gov.gr/articles/health/dieythynsh-dhmosias-ygieinhs/emboliasmoi/ethniko-programma-emboliasmwn-epe-paidiwn-kai-efhbwn/13267-laquo-ethniko-programma-emboliasmwn-paidiwn-kai-efhbwn-2025-xronodiagramma-kai-systaseis-raquo (accessed 25 May 2026).Google Scholar
NICE (2015) Overview | Bronchiolitis in Children: Diagnosis and Management | Guidance. https://www.nice.org.uk/guidance/ng9 (accessed 25 May 2026).Google Scholar
Martín-Latorre, MDM, et al. (2023) Comparison of easy-to-use bronchiolitis scores in the post-COVID-19 era—An observational study. Children (Basel) 10(12), 1834. https://doi.org/10.3390/children10121834.Google ScholarPubMed
Moline, HL, et al. (2024) Early estimate of nirsevimab effectiveness for prevention of respiratory syncytial virus–associated hospitalization among infants entering their first respiratory syncytial virus season — New vaccine surveillance network, October 2023–February 2024. Morbidity and Mortality Weekly Report (MMWR) 73(9), 209–214. https://doi.org/10.15585/mmwr.mm7309a4.CrossRefGoogle Scholar
Velali, K, et al Maternal RSVpreF vaccination and nirsevimab immunization in the first RSV season in Greece: A single-center study. 16o International Congress of Union of European Neonatal & Perinatal Societies, Bratislava, Slovakia, 26–30 May 2026. https://www.mcascientificevents.eu/neobestpractice (accessed 25 May 2026).Google Scholar
Sarna, M, et al. (2023) Factors predicting secondary respiratory morbidity following early-life respiratory syncytial virus infections: Population-based cohort study. Open Forum Infectious Diseases 10(10), ofad450. https://doi.org/10.1093/ofid/ofad450.CrossRefGoogle ScholarPubMed
Reeves, RM, et al. (2019) Burden of hospital admissions caused by respiratory syncytial virus (RSV) in infants in England: A data linkage modelling study. Journal of Infection 78(6), 468–475. https://doi.org/10.1016/j.jinf.2019.02.012.CrossRefGoogle Scholar
Tregoning, JS and Schwarze, J̈ (2010) Respiratory viral infections in infants: Causes, clinical symptoms, virology, and immunology. Clinical Microbiology Reviews 23(1), 74–98. https://doi.org/10.1128/CMR.00032-09.CrossRefGoogle ScholarPubMed
Drajac, C, et al. (2017) Pulmonary susceptibility of neonates to respiratory syncytial virus infection: A problem of innate immunity? Journal of Immunology Research 2017, 1–13. https://doi.org/10.1155/2017/8734504.CrossRefGoogle ScholarPubMed
Ciobanu, AM, et al. (2020) Benefits and risks of IgG transplacental transfer. Diagnostics (Basel) 10(8), 583. https://doi.org/10.3390/diagnostics10080583.CrossRefGoogle ScholarPubMed
Atwell, JE, et al. (2022) Biological factors that may impair transplacental transfer of RSV antibodies: Implications for maternal immunization policy and research priorities for low- and middle-income countries. Vaccine 40(32), 4361–4370. https://doi.org/10.1016/j.vaccine.2022.06.034.CrossRefGoogle ScholarPubMed
Mao, Z, et al. (2023) Economic burden and health-related quality-of-life among infants with respiratory syncytial virus infection: A multi-country prospective cohort study in Europe. Vaccine 41(16), 2707–2715. https://doi.org/10.1016/j.vaccine.2023.03.024.CrossRefGoogle ScholarPubMed
Díez-Gandía, E, et al. (2021) The impact of childhood RSV infection on children’s and parents’ quality of life: A prospective multicenter study in Spain. BMC Infectious Diseases 21(1), 924. https://doi.org/10.1186/s12879-021-06629-z.CrossRefGoogle ScholarPubMed
Bozzola, E, et al. (2026) Family costs for pediatric hospitalized respiratory syncytial virus lower respiratory tract infections: An Italian multicenter study. Italian Journal of Pediatrics 52(1), 2. https://doi.org/10.1186/s13052-025-02154-w.CrossRefGoogle Scholar
Brault, A, et al. (2024) Effect of nirsevimab on hospitalisations for respiratory syncytial virus bronchiolitis in France, 2023-24: A modelling study. The Lancet Child & Adolescent Health 8(10), 721–729. https://doi.org/10.1016/S2352-4642(24)00143-3.CrossRefGoogle ScholarPubMed
Boundy, EO, et al. (2025) Respiratory syncytial virus immunization coverage among infants through receipt of nirsevimab monoclonal antibody or maternal vaccination — United States, October 2023–March 2024. Morbidity and Mortality Weekly Report (MMWR) 74(31), 484–489. https://doi.org/10.15585/mmwr.mm7431a3.CrossRefGoogle ScholarPubMed
Razzini, JL, et al. (2026) Impact of universal nirsevimab prophylaxis in infants on hospital and primary care outcomes across two respiratory syncytial virus seasons in Galicia, Spain (NIRSE-GAL): A population-based prospective observational study. The Lancet Infectious Diseases 26(5), 522–534. https://doi.org/10.1016/S1473-3099(25)00742-X.CrossRefGoogle ScholarPubMed
Kiefer, A, et al. (2026) Primary care respiratory syncytial virus disease characteristics in infants and young children in Germany: Data from the prospective pediatric airway pathogen incidence surveillance network, 2021-2023. International Journal of Infectious Diseases 167, 108509. https://doi.org/10.1016/j.ijid.2026.108509.CrossRefGoogle Scholar
Ares-Gómez, S, et al. (2024) Effectiveness and impact of universal prophylaxis with nirsevimab in infants against hospitalisation for respiratory syncytial virus in Galicia, Spain: Initial results of a population-based longitudinal study. The Lancet Infectious Diseases 24(8), 817–828. https://doi.org/10.1016/S1473-3099(24)00215-9.CrossRefGoogle ScholarPubMed
Martinón-Torres, F, et al. (2023) Early lessons from the implementation of universal respiratory syncytial virus prophylaxis in infants with long-acting monoclonal antibodies, Galicia, Spain, September and October 2023. Eurosurveillance 28(49), 2300606. https://doi.org/10.2807/1560-7917.ES.2023.28.49.2300606.CrossRefGoogle ScholarPubMed
Figure 0

Table 1. Comparison between RSV-immunized and unimmunized infants with bronchiolitisTable 1. long description.

Figure 1

Figure 1. Rates of overall and specific complications (pneumonia, ΑΟΜ, other complications) in RSV-immunized and unimmunized infants with bronchiolitis. AOM: acute otitis media.

Figure 2

Table 2. Comparative analysis of infant bronchiolitis hospitalizations in the 2023–2024, 2024–2025, and 2025–2026 seasonsTable 2. long description.

Figure 3

Figure 2. Monthly incidence of bronchiolitis hospitalizations (per 1000 admissions) across the 2023–2024, 2024–2025, and 2025–2026 seasons.Figure 2. long description.

Figure 4

Figure 3. Monthly incidence of RSV bronchiolitis hospitalizations (per 1000 admissions) across the 2023–2024, 2024–2025, and 2025–2026 seasons.Figure 3. long description.