ABSTRACT
Aim
To assess changes in Respiratory Syncytial Virus (RSV) epidemiology and hospitalisations following implementation of a limited‐eligibility nirsevimab program.
Methods
This observational cohort study included all infants aged < 2 years residing in our jurisdiction with laboratory‐confirmed RSV and/or receipt of an RSV monoclonal antibody between April 2022 and March 2025. Cases were followed for emergency department (ED) presentations and hospital admissions. Primary outcomes were RSV incidence, ED presentation, and hospitalisation rates.
Results
We identified 2355 RSV cases, with 1625 infants presenting to ED and 701 hospitalised. Among 308 infants receiving nirsevimab, the most common indication was prematurity; 2.4% experienced breakthrough RSV infection. RSV incidence increased post‐nirsevimab (April 2024–March 2025: 89.06/1000 infant‐years) compared with the pre‐nirsevimab period (April 2022–March 2024: 67.99/1000 infant‐years), consistent with local and national trends. Characteristics of hospitalised infants changed in the post‐nirsevimab period: age increased (from 9.4 to 10.7 months, p = 0.019), the proportion aged < 3 months decreased (from 27.6% to 19.7%, p = 0.013) and prematurity prevalence nearly halved (from 13.3% to 7.3%, p = 0.041). Clinical characteristics also changed, with an 8‐h reduction in length of stay (p = 0.010), and a one fifth reduction in the need for respiratory support (p < 0.001). No significant differences were observed for Aboriginal and Torres Strait Islander infants.
Conclusions
Hospitalisations among infants at greatest risk for severe RSV disease reduced following introduction of the limited‐eligibility nirsevimab program. However, Aboriginal and Torres Strait Islander infants remained over‐represented in hospital admissions and under‐served by the program in its first year, warranting further investigation.
Key Points
- What is already known on this topic?
-
○Respiratory Syncytial Virus (RSV) infection has a high health burden for children worldwide.
-
○Nirsevimab, a long‐acting monoclonal antibody against RSV, became available in several Northern Hemisphere countries in 2023 and in the Southern Hemisphere in 2024.
-
○Real‐world studies of universal‐eligibility programs have established nirsevimab effectiveness; however, evidence in the context of limited‐eligibility programs is lacking.
-
○
- What this paper adds?
-
○We present the first report of RSV disease in infants < 2 years of age in the Australian Capital Territory following the implementation of a limited‐eligibility nirsevimab program. The program targeted infants born < 37 weeks gestation, Aboriginal and Torres Strait Islander infants, and those with other risk factors for severe illness.
-
○Premature infants were well served by the program, and breakthrough RSV infection was infrequent. Following nirsevimab introduction, hospitalised infants were older, less likely to be premature, had shorter length of stay, less need for respiratory support, and less use of high flow nasal cannula oxygen therapy.
-
○A limited‐eligibility nirsevimab program may ease RSV burden in contexts where universal access is not possible.
-
○
1. Introduction
Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection and hospitalisation in children worldwide [1], disproportionately affecting infants, with 90% infected in the first 2 years of life [2]. In Australia, RSV accounted for 86 937 notifications in children under 4 years in 2024, representing half of the national burden [3] and leading to approximately 12 000 infant hospitalisations annually [4]. Severe disease typically presents as bronchiolitis [5]. Risk factors for severe bronchiolitis in the Australasian context include younger age, prematurity, multiple birth, comorbidity (such as chronic lung/neurological disease, congenital heart disease, genetic disorders), Indigenous ethnicity (Australian Aboriginal or Torres Strait Islander peoples), economic disadvantage, and tobacco smoke exposure [6].
Prior to 2024, the only RSV immunoprophylaxis available in Australia was the short‐acting monoclonal antibody (mAb) palivizumab (Synagis) [7]. There were several drawbacks, including narrow eligibility criteria, high cost, and need for monthly injections [8]. In 2024, nirsevimab (Beyfortus), a long‐acting mAb requiring a single dose per RSV season, became available [7, 9]. In Australia, nirsevimab is funded through individual jurisdictional programs, with varying eligibility. In the Australian Capital Territory (ACT), eligibility in 2024 was limited to vulnerable infants: those entering their first season who were born < 37 weeks' gestation, Aboriginal or Torres Strait Islander infants, and those with chronic lung disease, congenital heart disease, immunodeficiency, or other conditions substantially affecting respiratory or immune function; and those entering their second season with chronic lung disease, Trisomy 21, or other conditions significantly impairing respiratory function (if < 12 months of age), or significant congenital heart disease or severe immunodeficiency (if < 24 months of age). The program ran from April to September 2024.
Evidence from the Northern Hemisphere [10, 11, 12] demonstrates nirsevimab's effectiveness in preventing RSV infection and hospitalisation, and Australasian data are emerging. A Western Australian multi‐centre case–control study reported 88.2% effectiveness (95% CI: 73.5–94.7) against RSV‐related hospitalisation in infants < 2 years during a universal first‐season program [13]. In a National surveillance survey, effectiveness among infants < 12 months was 83.1% (95% CI: 67.4–91.3) across the two jurisdictions with universal programs [14]. The present study is intended to contribute to the emerging body of Australasian literature, and provide novel evidence in the context of a limited‐eligibility program. We aim to characterise changes in epidemiology and hospitalisation for RSV in infants < 2 years, following the introduction of nirsevimab for vulnerable infants in our jurisdiction.
2. Methods
This retrospective, observational cohort study was set in the ACT, Australia. The ACT is home to 486 200 people, comprising 1.75% of Australia's population [15]. There are approximately 5290 births annually, 7%–8% occurring prior to 37 weeks gestation, and around 2.1% of ACT residents are < 2 years of age [15, 16]. Canberra Health Services is the sole provider of public health services in the jurisdiction, and we obtained routinely collected data from the service's databases between December 2024 and July 2025 for the study period 1 April 2022 to 31 March 2025. A universal Digital Health Record (EPIC Care, Epic Systems Corporation) was adopted in November 2022; earlier records were sourced from legacy systems. Within the jurisdiction, two hospitals (Canberra and North Canberra Hospitals) provide emergency paediatric care, with one (Canberra Hospital) delivering all admitted paediatric acute care. The majority of RSV mAb is administered at the two hospitals, with the remaining provided at either a small private hospital or Aboriginal Health Service. Palivizumab was available prior to March 2024, and nirsevimab was introduced for the 2024 RSV season (April to September). Viral testing for RSV was performed by Canberra Health Services' pathology provider or private laboratories.
Ethics approval was granted by the ACT Health Human Research Ethics Committee (low‐risk subcommittee) on 3 December 2024 (reference 2024.LRE.00283), with a waiver of consent. The study was registered at https://clinicaltrials.gov/study/NCT07177508.
Eligible cases included all infants aged < 2 years residing in the ACT with laboratory‐confirmed RSV via nucleic acid amplification test and/or receipt of RSV mAb during the study period. Infants with multiple positive RSV results within 60 days were counted once at the first positive test date, unless a new illness episode was evident.
A full description of procedures for identifying RSV, ED, hospital admission and mAb cases, and a list of data extracted is included in the Supporting Information. In brief, cases were identified by reviewing positive RSV tests, RSV notifications, clinical coding, immunisation clinics, and chart reviews.
Population and socioeconomic data were obtained from publicly available sources on the Australian Bureau of Statistics website [15, 17]. Index of Relative Socio‐economic Disadvantage (IRSD) deciles for all of Australia; then the jurisdiction were assigned according to postcodes, then each collapsed into quintiles, where decile 1 and 2 formed quintile 1, and so on. A lower IRSD indicated greater disadvantage.
Primary outcomes were the incidence of RSV infection, RSV‐related emergency department (ED) presentations, and RSV‐related hospital admissions among infants aged < 2 years.
2.1. Statistical Analysis
Data were entered into Microsoft 365 Excel version 2506, then imported into SPSS v 30 (IBM) for analysis. Frequencies were estimated for all variables. Mean with standard deviation (SD) was calculated for age and gestation, as there were few outliers to skew the data. Conversely, median with interquartile range (IQR) was calculated for length of stay and time on the various therapies, as there were considerable outliers that appeared to artificially inflate the mean. Age‐adjusted rates were calculated for each outcome of interest using the following formula:
We used the 2‐tailed independent samples t‐test, independent samples Mann–Whitney U test, and Chi‐square test for independence to evaluate differences in mean, median, and proportions (respectively) of characteristics between the pre‐ and post‐nirsevimab periods. A p value of ≤ 0.05 was considered statistically significant, and Yates' correction for continuity was used for 2 × 2 Chi‐square tables.
3. Role of the Funding Source
XC received an ACT Health Directorate Vacation Scholarship to work on the project.
4. Results
Between April 2022 and March 2025, 2355 RSV cases were reported in infants < 2 years, with an age‐adjusted incidence of 74.78 per 1000 infant‐years (Figure 1, Table 1). Of these, 1736 cases had contact with the health service and therefore had complete data available for analysis (Table 1). There were 1625 infants presenting to the ED with RSV‐related illness (69.0% of all cases; 51.60 per 1000 infant‐years) (Table 2), and 701 infants hospitalised (43.1% of ED cases; 29.8% of all cases; 22.26 per 1000 infant‐years) (Table 3). The most common primary diagnosis among admitted infants was RSV bronchiolitis (ICD‐10‐AM 13th edition code J21.0, n = 515), followed by wheezing (R06.2, n = 45) and RSV pneumonia (J12.1, n = 45). Common secondary diagnoses included volume depletion (E86, n = 127) and RSV bronchiolitis (J21.0, n = 47).
FIGURE 1.
Number of cases in each group of interest.
TABLE 1.
Characteristics of reported respiratory syncytial virus cases.
| All reported RSV cases N = 2355 | Pre‐nirsevimab n = 1451 | Post‐nirsevimab n = 904 | p | |
|---|---|---|---|---|
| Age‐adjusted incidence/1000 infant years | 74.78 | 67.99 | 89.06 | |
| Age in months | ||||
| Mean (SD) | 11.13 (6.42) | 10.72 (6.50) | 11.78 (6.25) | < 0.001 a |
| Age group n (%) | ||||
| ≤ 3 months | 365 (15.5%) | 255 (17.6%) | 110 (12.2%) | < 0.001 b |
| > 3–6 months | 308 (13.1%) | 200 (13.8%) | 108 (11.9%) | |
| > 6–24 months | 1682 (71.4%) | 996 (68.6%) | 686 (75.9%) | |
| RSV cases with complete data N = 1736 | Pre‐nirsevimab n = 1181 | Post‐nirsevimab n = 555 | p | |
|---|---|---|---|---|
| Sex n (%) | ||||
| Male | 1012 (58.3%) | 685 (57.9%) | 328 (59.1%) | |
| Female | 724 (41.7%) | 497 (42.1%) | 227 (40.9%) | |
| IRSD quintile in jurisdiction n (%) | ||||
| 1 | 242 (15.6%) | 163 (15.4%) | 79 (15.9%) | 0.950 b |
| 2 | 445 (28.6%) | 301 (28.5%) | 144 (28.9%) | |
| 3 | 510 (32.8%) | 349 (33.0%) | 161 (32.3%) | |
| 4 | 260 (16.7%) | 180 (17.0) | 80 (16.1%) | |
| 5 | 97 (6.2%) | 63 (6.0) | 34 (6.8%) | |
| Aboriginal and/or Torres Strait Islander n (%) | 113 (6.5%) | 78 (6.6%) | 35 (6.3%) | 0.896 b |
|
Gestation at birth in weeks. days Mean (SD) |
38.6 (2.0) | 38.5 (2.1) | 39.0 (1.5) | 0.003 a |
| Prematurity n (%) | ||||
| 32– < 37 weeks | 131 (7.5%) | 98 (8.3%) | 33 (5.9%) | 0.038 b |
| < 32 weeks | 30 (1.7%) | 25 (2.1%) | 5 (0.9%) | |
Abbreviations: IRSD, Index of Relative Socio‐Economic Disadvantage; RSV, Respiratory Syncytial Virus.
Independent samples t‐test, 2‐tailed.
Chi‐square test for independence.
TABLE 2.
Characteristics of infants presenting to the emergency department with respiratory syncytial virus‐related illness.
| N = 1625 | Pre‐nirsevimab n = 1083 | Post‐nirsevimab n = 542 | p | |
|---|---|---|---|---|
| Age‐adjusted incidence/1000 infant years | 51.60 | 50.75 | 53.39 | |
| Proportion of all reported cases | 69.0% | 74.6% | 60.0% | |
|
Age in months Mean (SD) |
11.24 (6.77) | 10.94 (6.80) | 11.85 (6.67) | 0.010 a |
| Age group n (%) | ||||
| ≤ 3 months | 235 (14.5%) | 169 (15.6%) | 66 (12.2%) | 0.016 b |
| > 3–6 months | 238 (14.6%) | 171 (15.8%) | 67 (12.4%) | |
| > 6–24 months | 1152 (70.9%) | 743 (68.6%) | 409 (75.5%) | |
| Sex n (%) | ||||
| Male | 944 (58.1%) | 622 (57.4%) | 322 (59.4%) | |
| Female | 681 (41.9%) | 461 (42.6%) | 220 (40.6%) | |
| IRSD quintile in jurisdiction n (%) | ||||
| 1 | 227 (15.6%) | 149 (15.4%) | 78 (16.0%) | 0.893 b |
| 2 | 421 (28.9%) | 282 (29.1%) | 139 (28.5%) | |
| 3 | 478 (32.8%) | 318 (32.8%) | 160 (32.9%) | |
| 4 | 243 (16.7%) | 166 (17.1%) | 77 (15.8%) | |
| 5 | 88 (6.0%) | 55 (5.7%) | 33 (6.8%) | |
| Aboriginal and/or Torres Strait Islander n (%) | 102 (6.3%) | 70 (6.5%) | 32 (5.9%) | 0.741 b |
|
Gestation at birth in weeks. days Mean (SD) |
38.6 (2.0) | 38.5 (2.1) | 39.0 (1.5) | 0.002 a |
| Prematurity n (%) | ||||
| 32– < 37 weeks | 124 (7.6%) | 92 (8.5%) | 32 (5.9%) | 0.028 b |
| < 32 weeks | 29 (1.8%) | 24 (2.2%) | 5 (0.9%) | |
| Length of ED stay in hours Median (IQR) | 4.17 (2.98–5.65) | 4.41 (3.13–6.21) | 3.73 (2.74–4.73) | < 0.001 c |
| Number of ED presentations n (%) | ||||
| 1 | 1434 (88.3%) | 966 (89.2%) | 468 (86.3%) | 0.072 b |
| 2 | 172 (10.6%) | 109 (10.1%) | 63 (11.6%) | |
| 3 | 18 (1.1%) | 8 (0.7%) | 10 (1.8%) | |
| 4 | 1 (0.1%) | 0 (0%) | 1 (0.2%) | |
Abbreviations: ED, Emergency Department; IRSD, Index of Relative Socio‐Economic Disadvantage.
Independent samples t‐test, 2‐tailed.
Chi‐square test for independence.
Independent samples Mann–Whitney U test.
TABLE 3.
Characteristics of infants admitted to hospital with respiratory syncytial virus‐related illness.
| N = 701 | Pre‐nirsevimab n = 457 | Post‐nirsevimab n = 244 | p | |
|---|---|---|---|---|
| Age‐adjusted incidence/1000 infant years | 22.26 | 21.46 | 23.94 | |
| Proportion of all reported cases | 29.8% | 31.5% | 27.0% | |
| Proportion of ED presentations | 43.1% | 42.2% | 45.0% | |
|
Age in months Mean (SD) |
9.86 (7.04) | 9.40 (7.05) | 10.71 (6.95) | 0.019 a |
| Age group n (%) | ||||
| ≤ 3 months | 174 (24.8%) | 126 (27.6%) | 48 (19.7%) | 0.013 b |
| > 3–6 months | 108 (15.4%) | 76 (16.6%) | 32 (13.1%) | |
| > 6–24 months | 419 (59.8%) | 255 (55.8%) | 164 (67.2%) | |
| Sex n (%) | ||||
| Male | 414 (59.1%) | 258 (56.5%) | 156 (63.9%) | |
| Female | 287 (40.9%) | 199 (43.5%) | 36.1 (30.7%) | |
| IRSD quintile in jurisdiction n (%) | ||||
| 1 | 96 (15.4%) | 61 (15.1%) | 35 (16.1%) | 0.564 b |
| 2 | 179 (28.8%) | 114 (28.2%) | 65 (29.8%) | |
| 3 | 206 (33.1%) | 138 (34.2%) | 68 (31.2%) | |
| 4 | 109 (17.5%) | 74 (18.3%) | 35 (16.1%) | |
| 5 | 32 (5.1%) | 17 (4.2%) | 15 (6.9%) | |
| Aboriginal and/or Torres Strait Islander n (%) | 47 (6.7%) | 33 (7.2%) | 14 (5.7%) | 0.454 b |
|
Gestation at birth in weeks days Mean (SD) |
38.4 (2.3) | 38.4 (2.4) | 38.6 (2.0) | 0.070 a |
| Prematurity n (%) | ||||
| 32– < 37 weeks | 59 (8.4%) | 44 (9.6%) | 15 (6.1%) | 0.041 b |
| < 32 weeks | 20 (2.9%) | 17 (3.7%) | 3 (1.2%) | |
|
Length of stay in days Median (IQR) |
2.15 (1.28–3.58) | 2.38 (1.36–3.71) | 2.01 (1.16–3.23) | 0.010 c |
| Required intensive care n (%) | 23 (3.3%) | 17 (3.7%) | 6 (2.5%) | 0.503 b |
| Required respiratory support n (%) | 449 (64.1%) | 314 (68.7%) | 135 (55.3%) | < 0.001 b |
| Highest level of respiratory support n (%) | ||||
| Low flow nasal cannula oxygen | 171 (24.4%) | 111 (24.3%) | 60 (24.6%) | < 0.001 b |
| HFNC oxygen < 2 L/kg/min | 102 (14.6%) | 84 (18.4%) | 18 (7.4%) | |
| HFNC ≥ 2 L/kg/min | 133 (19.0%) | 92 (20.1%) | 41 (16.8%) | |
| CPAP | 40 (5.7%) | 26 (5.7%) | 14 (5.7%) | |
| Mechanical Ventilation/NAVA | 4 (0.6%) | 2 (0.4%) | 2 (0.8%) | |
| Time on each therapy in hours Median (IQR) | ||||
| Low flow nasal cannula oxygen | 13.2 (2.5–27.1) | 14.0 (6.0–28.1) | 9.0 (4.4–23.7) | 0.043 c |
| HFNC oxygen < 2 L/kg/min | 33.0 (20.7–56.0) | 36.7 (22.5–59.0) | 25.7 (18.5–42.4) | 0.006 c |
| HFNC ≥ 2 L/kg/min | 17.1 (9.0–31.5) | 15.8 (7.9–31.5) | 20.7 (12.1–33.0) | 0.160 c |
| CPAP | 33.7 (16.1–55.3) | 33.6 (14.2–61.8) | 35.7 (20.4–49.6) | 0.951 c |
| Maximum fraction of inspired oxygen Mean (SD) | ||||
| HFNC any flow | 32.8 (9.8) | 33.3 (10.1) | 31.4 (8.9) | 0.160 a |
| CPAP | 38.1 (17.5) | 42.7 (19.7) | 29.9 (8.2) | 0.004 a |
| Maximum CPAP pressure in cm water Mean (SD) | 7.1 (1.2) | 7.3 (1.2) | 6.9 (1.0) | 0.302 a |
Abbreviations: CPAP, Continuous Positive Airway Pressure; ED, Emergency Department; HFNC, High Flow Nasal Cannula; IRSD, Index of Relative Socio‐economic Disadvantage; NAVA, Neurally Adjusted Ventilatory Assist.
Independent samples T‐test, 2‐tailed.
Chi‐square test for independence.
Independent samples Mann–Whitney U test.
Regarding immunoprophylaxis, 21 infants received palivizumab between April 2022 and March 2024 (the pre‐nirsevimab period), and 308 infants received nirsevimab from April 2024 to March 2025 (the post‐nirsevimab period) (Table 4). Prematurity, defined as birth before 37 weeks' gestation, was the most common indication (79%), followed by Aboriginal or Torres Strait Islander status (16%). Of the 290 nirsevimab recipients linked to RSV data, 2.4% tested positive for RSV, 2.4% presented to ED, and 1.7% were hospitalised (all with bronchiolitis); none required intensive care, and there were no deaths.
TABLE 4.
Characteristics of infants receiving monoclonal antibody immunoprophylaxis against respiratory syncytial virus.
| Palivizumab n = 21 | Nirsevimab n = 308 | |
|---|---|---|
|
Age at first dose in months Mean (SD) |
6.48 (4.13) | 2.75 (3.46) |
| Indication n (%) a | ||
| Prematurity < 37 weeks gestation | 243 (79%) | |
| Prematurity ≤ 26 weeks gestation | 4 (19%) | |
| Chronic lung disease | 16 (76%) | 16 (5%) |
| Congenital heart disease | 5 (24%) | 10 (3%) |
| Aboriginal or Torres Strait Islander | 49 (16%) | |
| Infants receiving monoclonal antibody with complete data | Palivizumab n = 21 | Nirsevimab n = 290 |
|---|---|---|
| RSV breakthrough b outcomes n (%) | ||
| Positive RSV test | 3 (14%) | 7 (2.4%) |
| ED presentation | 2 (10%) | 7 (2.4%) |
| Hospital admission | 2 (10%) | 5 (1.7%) |
Abbreviations: ED, Emergency Department; RSV, Respiratory Syncytial Virus.
Percentages total more than 100% as some individuals had more than one indication.
Includes any RSV infection occurring from the day following palivizumab/nirsevimab administration.
Reported RSV incidence was higher in the post‐nirsevimab period (89.06 per 1000 infant‐years) than the pre‐nirsevimab period (67.99 per 1000 infant‐years) (Figure 2). Post‐nirsevimab, infants were older (mean 11.78 months, SD 6.25 vs. 10.72 months, SD 6.50; p < 0.001) and less commonly premature (6.8% vs. 10.4%; p = 0.038), with higher mean gestational age (39.0 weeks, SD 1.5 vs. 38.5 weeks, SD 2.10; p = 0.003) (Table 1). Similar trends were observed among ED presentations.
FIGURE 2.
Incidence of reported Respiratory Syncytial Virus infection, Emergency Department presentation and hospital admission in infants under 2 years of age from April 2022 to March 2025.
Hospitalisation incidence was modestly higher in the post‐nirsevimab period (23.94 per 1000 infant years) compared to the pre‐nirsevimab period (21.46 per 1000 infant years). Admitted infants were older post‐nirsevimab (mean age 10.71 months, SD 6.95), compared to the pre‐nirsevimab period (mean age 9.40 months, SD 7.05, p = 0.019) and less often premature (7.3% vs. 13.3%; p = 0.041). Mean gestational age was marginally higher (38.6 vs. 38.4 weeks; p = 0.070). Clinically, in the post‐nirsevimab period, length of stay shortened by approximately 8 h (p = 0.010), respiratory support use fell by 20% (p < 0.001), and fewer infants required High Flow Nasal Cannula (HFNC) therapy (p < 0.001) (Table 3). There were no significant differences in the proportion of infants identifying as Aboriginal or Torres Strait Islander, or in indices of disadvantage between the two periods.
5. Discussion
This study characterises RSV epidemiology and burden during the first year of a limited‐eligibility nirsevimab program for vulnerable infants in a small Australian jurisdiction.
Despite the program introduction, reported RSV incidence was higher in 2024 (89.06 per 1000 infant years) than in the two preceding years (67.99 per 1000 infant years). It is possible that increased testing may have contributed to our finding. We suspect this is unlikely though, as routine testing became common at our health service during the SARS‐CoV‐2 pandemic and practice is now moving to align with bronchiolitis guidelines that recommend against routine viral testing [18]. Indeed, the 2024 season appeared more severe nationally, with an increase in all‐age cases of around 37% Australia‐wide [3], and of around one third locally [19]. Despite the substantial increase in reported RSV, the coinciding increase in RSV‐related ED presentations (from 50.75 to 53.39 per 1000 infant years) and hospital admissions (from 21.46 to 23.94 per 1000 infant years) was modest, with a smaller proportion of cases presenting to ED or being admitted.
The shift in characteristics of the infants in the post‐nirsevimab period signals some effect of the limited‐eligibility program at the population level, particularly among the hospitalised cohort. Post‐nirsevimab, hospitalised infants were older (mean 10.7 vs. 9.4 months; p = 0.019), aligning with reports from European settings with universal eligibility that observed a shift towards older age at first RSV infection, hospitalisation, or PICU admission, albeit to a greater magnitude [20, 21, 22, 23, 24, 25]. Our observation, however, may not be solely attributed to nirsevimab, as a recent Australian tertiary‐centre cohort study reported rising age among children hospitalised with RSV, without a clear cause [26]. Importantly, the proportion of hospitalised infants in our study at highest age‐risk for severe disease (< 3 months) decreased significantly (27.6% to 19.7%; p = 0.013). Equally, the proportion born prematurely almost halved (13.3% to 7.3%; p = 0.041), a finding not shared in studies where infants of all gestational ages had access to nirsevimab [13, 20, 21, 24, 25], however anticipated in our study given infants of < 37 weeks gestation were particularly targeted. We were unable to accurately determine coverage among premature infants because birth data from a small private maternity service in our jurisdiction were not available. However, we estimate reasonable reach within this high‐risk cohort, as prematurity was an indication for nirsevimab in 243 of 308 recipients, and 285 infants were born < 37 weeks' gestation within our health service during the eligibility period.
By contrast, we did not observe a substantial effect among Aboriginal and Torres Strait Islander infants, despite prioritisation within program eligibility criteria. They remained disproportionately represented among hospitalisations (5.7% of admissions vs. 3.6% of the local 0–4‐year‐old population) [27]. This aligns crudely with Western Australian population‐level estimates of increased risk of RSV‐related hospitalisation among Aboriginal infants (adjusted hazard ratio 1.72 (95% CI: 1.57, 1.88) for < 12 months, 1.59 (95% CI: 1.34, 1.89) for 12–24 months), after adjustment for multiple confounders [28]. Furthermore, coverage in our jurisdiction was limited: only 50 nirsevimab recipients identified as Aboriginal or Torres Strait Islander, compared with 363 births in the previous year [29]. Although coverage for routine childhood vaccinations in Aboriginal and Torres Strait Islander children has previously met national targets, rates have declined since 2020 [30], with complex and multi‐factorial drivers. A recent qualitative Yarning study highlighted several key themes, including knowledge gaps, mistrust, access challenges, and the need for culturally safe, nuanced approaches [31]. Our findings indicate ongoing vulnerability and suboptimal mAb coverage among this group of infants in our jurisdiction, and future research should aim to explore these issues in depth, and in a culturally safe manner.
We observed a trend towards less severe disease among hospitalised infants in the post‐nirsevimab period. Length of stay reduced by around 8 h (2.38 days, IQR 1.36–3.71, to 2.01 days, IQR 1.16–3.23, p = 0.010), whereas several European studies with universal nirsevimab eligibility reported length of stay reduced by as much as two days [20, 21, 23]. We also saw a significant reduction in the proportion of infants requiring respiratory support (any oxygen or ventilation support) from 68.7% to 55.3% (p < 0.001), and a one‐third reduction in the proportion of infants treated with HFNC oxygen therapy (p < 0.001). Findings regarding respiratory support vary in the literature. Among two case controlled studies, Wadia et al. [13] reported reduced respiratory support in nirsevimab‐immunised infants, but not for ventilatory support (HFNC, non‐invasive or invasive ventilation), while Agüera et al. [32] found no differences. A further two pre/post studies each observed a decrease in the post‐nirsevimab period, however the patterns differed with Rius‐Peris et al. [25] reporting a decrease in both HFNC oxygen therapy and non‐invasive ventilation, whereas Bermúdez‐Barrezueta et al. [20] found a decrease only in non‐invasive ventilation. We also noted a non‐significant decrease in the need for intensive care, though small numbers limit interpretation. We suggest the apparent amelioration of disease severity in our sample largely reflects a shift in infants' characteristics away from the highest risk phenotype (young and premature), rather than a true reduction in individual severity. Reassuringly, breakthrough RSV infections among mAb recipients were very few, and none showed features suggestive of enhanced disease.
Changes in the median time on HFNC therapy in our study should be interpreted in the context of practice change. Local clinical guidelines recommending maintaining flow at 2 L/kg/min throughout therapy were introduced during the post‐nirsevimab period.
5.1. Strengths and Limitations
Our study has several strengths. Firstly, our whole of jurisdiction approach and census of RSV‐related ED presentations and hospital admissions presents a complete picture. Secondly, we minimised undercounting of cases by triangulating several methods of data collection. Thirdly, we provide comprehensive clinical data, which is often lacking in existing reports concerning the real‐world effectiveness of nirsevimab. And finally, we contribute evidence in the context of a limited‐eligibility program, also scarce in the current literature. Nevertheless, there are limitations. Our jurisdiction, census, and nirsevimab program are small, limiting our ability to detect meaningful changes in the post‐nirsevimab period, particularly for infrequent events such as need for intensive care. Our identification of RSV cases depended on laboratory confirmed RSV; however, current clinical guidelines recommend against routine viral testing [18]. Whilst viral testing became routine during the SARS‐CoV‐2 pandemic, practice is shifting locally. Additionally, RSV Rapid Antigen Tests (RAT) are widely available in the community; however, they are not centrally reported and we did not collect data on home or primary care RAT. Although the maternal RSV vaccine ‘Abrysvo’ was not publicly funded until the end of our study period (February 2025), a small number of doses were privately sourced by women from June 2024 [33], and we did not attempt to collect maternal data. Future research should evaluate the impact of both nirsevimab and Abrysvo on the burden of RSV disease.
6. Conclusion
Our study suggests that, despite the ongoing burden of RSV disease, the first year of nirsevimab immunoprophylaxis for vulnerable infants in our jurisdiction demonstrated a promising shift in the characteristics of hospitalised infants away from those at greatest risk for severe illness. Concerningly though, Aboriginal and Torres Strait Islander infants continue to be over‐represented in hospital and under‐served by current immunoprophylaxis programs, warranting further investigation. Our study presents a small but useful contribution to the growing body of literature describing the real‐world effectiveness of novel RSV immunoprophylaxis, particularly in the Australian and limited‐eligibility contexts.
Author Contributions
Nicola Irwin: conceptulization, methodology, project administration, investigation, formal analysis, supervision, writing – original draft and review and editing. Deepti Raina: conceptulization, methodology, investigation, supervision, writing – original draft and review and editing. Shevaun Ey: investigation, formal analysis and writing – original draft. Xiyu Chen: investigation, writing – original draft. Drew Richardson: methodology, supervision, writing – review and editing. All authors had full access to all the data in the study and had final responsibility for the decision to submit for publication.
Funding
X.C. received an ACT Health Directorate Vacation Scholarship to work on the project.
Conflicts of Interest
The authors declare no conflicts of interest.
Supporting information
Data S1: Support Information.
Acknowledgements
The authors wish to thank officers from the Population Health Division of the ACT Health Directorate for their guidance and assistance with data concerning nirsevimab doses and RSV notifications. The authors are grateful to Associate Professor Karina Kennedy for her assistance with identifying RSV cases, and to Natalie Blore for her assistance with clinical coding. The authors wish to thank Dr. Blessy Charles for her support and advice during the project.
Data Availability Statement
Deidentified individual participant data may be shared with researchers with a methodologically sound proposal, upon reasonable request to the corresponding author, and subject to ethics and governance processes. Data may be shared from 3 months after publication to 3 years. A data sharing agreement will be required.
References
- 1. Li Y., Wang X., Blau D. M., et al., “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, no. 10340 (2022): 2047–2064. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2. Kieffer A., Beuvelet M., Sardesai A., et al., “Expected Impact of Universal Immunization With Nirsevimab Against RSV‐Related Outcomes and Costs Among All US Infants in Their First RSV Season: A Static Model,” Journal of Infectious Diseases 226, no. Suppl 2 (2022): S282–S292. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3. Australian Centre for Disease Control , “2024 Annual Australian Respiratory Surveillance Report,” Canberra, Australia, (2024).
- 4. The Hon Mark Butler MP , Protection Against RSV for Mums and Bubs With Free Vaccine Available From 3 February (Australian Government Department of Health and Ageing, 2025). [Google Scholar]
- 5. Mazur N. I., Caballero M. T., and Nunes M. C., “Severe Respiratory Syncytial Virus Infection in Children: Burden, Management, and Emerging Therapies,” Lancet 404, no. 10458 (2024): 1143–1156. [DOI] [PubMed] [Google Scholar]
- 6. Loveys K., Borland M. L., Oakley E., et al., “Risk Factors for Severe Bronchiolitis in Australian and Aotearoa New Zealand Infants: A Systematic Review,” Journal of Paediatrics and Child Health 61, no. 10 (2025): 1549–1565. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7. Australian Technical Advisory Group on Immunisation (ATAGI) , Australian Immunisation Handbook (Australian Government Department of Health and Aged Care, 2024). [Google Scholar]
- 8. Barnett S. T., Tuckerman J., Barr I. G., Crawford N. W., and Wurzel D. F., “Respiratory Syncytial Virus Preventives for Children in Australia: Current Landscape and Future Directions,” Medical Journal of Australia 222 (2025): 579–586. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9. Munro A. P. S., Drysdale S. B., Cathie K., et al., “180‐Day Efficacy of Nirsevimab Against Hospitalisation for Respiratory Syncytial Virus Lower Respiratory Tract Infections in Infants (HARMONIE): A Randomised, Controlled, Phase 3b Trial,” Lancet Child Adolesc Health 9, no. 6 (2025): 404–412. [DOI] [PubMed] [Google Scholar]
- 10. Riccò M., Cascio A., Corrado S., et al., “Impact of Nirsevimab Immunization on Pediatric Hospitalization Rates: A Systematic Review and Meta‐Analysis,” Vaccine 12, no. 6 (2024): 640. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11. Sumsuzzman D. M., Wang Z., Langley J. M., and Moghadas S. M., “Real‐World Effectiveness of Nirsevimab Against Respiratory Syncytial Virus Disease in Infants: A Systematic Review and Meta‐Analysis,” Lancet Child Adolescent Health 9, no. 6 (2025): 393–403. [DOI] [PubMed] [Google Scholar]
- 12. Wang X., Kong L., Liu X., Wu P., Zhang L., and Ding F., “Effectiveness of Nirsevimab Immunization Against RSV Infection in Preterm Infants: A Systematic Review and Meta‐Analysis,” Frontiers in Immunology 16 (2025): 1581970. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13. Wadia U., Moore H. C., Richmond P. C., et al., “Effectiveness of Nirsevimab in Preventing RSV‐Hospitalisation Among Young Children in Western Australia,” Journal of Infection 90, no. 4 (2025): 106466. [DOI] [PubMed] [Google Scholar]
- 14. Blyth C. C., Wadia U., Britton P. N., et al., “Respiratory Syncytial Virus Epidemiology and Effectiveness of Infant Nirsevimab: 2024 Results From the Australian Sentinel Hospital Network (FluCAN‐PAEDS),” Euro Surveillance: Bulletin Europeen Sur Les Maladies Transmissibles = European Communicable Disease Bulletin 31, no. 2 (2026): 2500275. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15. Australian Bureau of Statistics , National, State and Territory Population (ABS, 2024). [Google Scholar]
- 16. ACT Health and Community Services Directorate , Mothers & Babies Dashboard Data. HealthStats ACT (ACT Government, 2025). [Google Scholar]
- 17. Australian Bureau of Statistics , Socio‐Economic Indexes for Areas (SEIFA), Australia (ABS, 2023). [Google Scholar]
- 18. Borland M. L., Loveys K., Babl F. E., et al., “Australiasian Bronchiolitis Guideline: 2025 Update,” Journal of Paediatrics and Child Health 61, no. 8 (2025): 1197–1215. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19. ACT Health , ACT Respiratory Illness Surveillance Report 2024 (ACT Health, 2024). [Google Scholar]
- 20. Bermúdez‐Barrezueta L., Matías Del Pozo V., Marugán‐Miguelsanz J. M., et al., “Universal Administration of Nirsevimab in Infants: An Analysis of Hospitalisations and Paediatric Intensive Care Unit Admissions for RSV‐Associated Lower Respiratory Tract Infections,” European Journal of Pediatrics 184, no. 6 (2025): 345. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21. Alejandre C., Penela‐Sánchez D., Alsina J., et al., “Impact of Universal Immunization Program With Monoclonal Antibody Nirsevimab on Reducing the Burden of Serious Bronchiolitis That Need Pediatric Intensive Care,” European Journal of Pediatrics 183, no. 9 (2024): 3897–3904. [DOI] [PubMed] [Google Scholar]
- 22. Cantais A., Annino N., Thuiller C., et al., “First RSV Epidemic With Nirsevimab. Older Children Than Previous Epidemics, Even When Hospitalized,” Journal of Medical Virology 96, no. 2 (2024): e29483. [DOI] [PubMed] [Google Scholar]
- 23. Ernst C., Bejko D., Gaasch L., et al., “Impact of Nirsevimab Prophylaxis on Paediatric Respiratory Syncytial Virus (RSV)‐Related Hospitalisations During the Initial 2023/24 Season in Luxembourg,” Euro Surveillance 29, no. 4 (2024): 2400033. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24. Jimeno Ruiz S., Peláez A., Labourt A., et al., “Evaluating the Effectiveness of Nirsevimab in Reducing Pediatric RSV Hospitalizations in Spain,” Vaccine 12, no. 10 (2024): 1160. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25. Rius‐Peris J. M., Palomo‐Atance E., Muro‐Díaz E., Llorente‐Ruiz C., Murcia‐Clemente L., and Alcaraz R., “Nirsevimab Immunisation Significantly Reduces Respiratory Syncytial Virus‐Associated Bronchiolitis Hospitalisations and Alters Seasonal Patterns,” Acta Paediatrica 114 (2025): 1963–1976. [DOI] [PubMed] [Google Scholar]
- 26. Tuckerman J., Anderson J., Alafaci A., et al., “Epidemiology and Clinical Profile of Paediatric RSV Infections: Insights From a 6‐Year Australian Cohort Study,” Archives of Disease in Childhood 110, no. 9 (2025): 759–760. [DOI] [PubMed] [Google Scholar]
- 27. Australian Bureau of Statistics , Estimates of Aboriginal and Torres Strait Islander Australians (ABS, 2021). [Google Scholar]
- 28. Sarna M., Blyth C. C., Taye B. W., et al., “Who Is at Risk of a Respiratory Syncytial Virus Hospitalisation? A Linked, Population‐Based Birth Cohort Analysis in Children Aged Less Than 5 Years,” Lancet Regional Health ‐ Western Pacific 61 (2025): 101654. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29. Australian Bureau of Statistics , Births, Australia (ABS, 2024). [Google Scholar]
- 30. National Centre for Immunisation Research and Surveillance Australia , Annual Immunisation Coverage Report 2024 – Interim Data Release (Kids Research, Sydney Children's Hospitals Network, 2025). [Google Scholar]
- 31. Harwood E., Taylor K., Clark K., et al., “Yarning About Vaccinations: Empowering Individuals to Have Supportive Conversations With Aboriginal Peoples About Vaccinations, Using a Community‐Engaged Approach,” Australian and New Zealand Journal of Public Health 49, no. 1 (2025): 100206. [DOI] [PubMed] [Google Scholar]
- 32. Agüera M., Soler‐Garcia A., Alejandre C., et al., “Nirsevimab Immunization's Real‐World Effectiveness in Preventing Severe Bronchiolitis: A Test‐Negative Case‐Control Study,” Pediatric Allergy and Immunology 35, no. 6 (2024): e14175. [DOI] [PubMed] [Google Scholar]
- 33. National Centre for Immunisation Research and Surveillance Australia , “Maternal RSV Vaccine Uptake Data,” Westmead, Australia, 2025.
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data S1: Support Information.
Data Availability Statement
Deidentified individual participant data may be shared with researchers with a methodologically sound proposal, upon reasonable request to the corresponding author, and subject to ethics and governance processes. Data may be shared from 3 months after publication to 3 years. A data sharing agreement will be required.