Background:
Respiratory Syncytial Virus (RSV) causes a significant burden of illness for children under 2 years of age. Nirsevimab, a long-acting monoclonal antibody, was registered for RSV prevention in Australia in 2023. In April 2024, Western Australia (WA) launched the country’s first state-wide universal nirsevimab program for infants entering their first RSV season and high-risk children entering their second RSV season. This study describes the effectiveness of nirsevimab against laboratory-confirmed RSV hospitalisation over a single epidemic season.
Methods:
Between April 2024 and October 2024, children hospitalised with laboratory-confirmed RSV-associated acute respiratory infection (ARI; cases) and contemporaneous test-negative ARI controls were enrolled. Demographic variables, medical risk factors, symptoms and outcomes were assessed. Nirsevimab effectiveness in preventing RSV-associated hospitalisation was estimated using a test negative design.
Results:
Over 7 months, 284 children eligible for nirsevimab were enrolled including 184 RSV positive (cases) and 100 test negative controls. Coverage of nirsevimab in RSV cases and controls was 22.1% and 64.0% respectively. Of RSV positive cases, 153 (83.2%) were born at term and only 30 (16.3%) had medical risk factors. The overall adjusted estimate of nirsevimab effectiveness against RSV-associated ARI hospitalisation was 88.2% (95% CI: 73.5, 94.7). Effectiveness against RSV admission requiring oxygen/respiratory support was 61.8% (95% CI:16.4, 82.5). There were no significant differences in demographic, medical risk factors and outcomes observed among those that were immunised compared to the unimmunised RSV positive cases.
Conclusion:
This study is the first to provide estimate of nirsevimab effectiveness against hospitalisation in Australia and demonstrates that a single dose of nirsevimab was highly effective against RSV-associated hospitalisation in infants. In 2025, state funded nirsevimab and National Immunisation Program-funded Maternal RSV vaccine is available. This study will provide longitudinal effectiveness of hybrid programs in the coming years.

Background: In March 2024, Argentina became the first country to implement a national maternal RSVpreF vaccination program for pregnant individuals as the primary strategy to prevent respiratory syncytial virus (RSV) disease among infants. We evaluated RSVpreF vaccine effectiveness (VE) against RSV-associated lower respiratory tract disease (LRTD) and severe RSV-LRTD leading to hospitalisation among infants ≤6 months of age.

Methods: A multicentre, retrospective, test-negative case-control study was conducted during the 2024 RSV season in 12 hospitals across Argentina. Infants ≤6 months (≤180 days) hospitalised with LRTD and tested for RSV were included—those testing positive were cases and those testing negative were controls. Infants were considered born to an RSVpreF-vaccinated mother if RSVpreF was received between 320/7 and 366/7 weeks’ gestation and ≥14 days before delivery. We estimated VE by comparing the odds of maternal RSVpreF vaccination among infant cases versus controls, using multilevel logistic regression with several approaches to address confounding.

Results: Of 505 infants, 17.8% of cases (51/286) and 49.8% of controls (109/219) were born to RSVpreF-vaccinated mothers. Median age in days at hospitalisation among cases and controls was 71 and 69, respectively. VE against RSV-associated LRTD leading to hospitalisation from birth through 3 months was 78.6% (95% confidence interval: 62.1-87.9) and from birth through 6 months was 71.3% (53.3-82.3). VE against RSV-associated severe LRTD hospitalisation was 76.9% (45.0-90.3) from birth through 6 months. Three RSV-LRTD in-hospital deaths occurred, all among infants whose mothers did not receive RSVpreF during pregnancy.

Conclusions: These real-world estimates demonstrate high RSVpreF effectiveness against hospitalisation due to RSV-LRTD and severe RSV-LRTD from birth and sustained through 6 months of life. These results corroborate phase 3 efficacy findings and expand on trial results by evaluating hospitalised RSV-LRTD endpoints, a more diverse and clinically complex population of infants, and a different RSVpreF administration window.

Background

Respiratory syncytial virus (RSV) is a leading cause of hospitalisations for acute lower respiratory tract infections in Australian infants. In April 2024 nirsevimab, a long-acting RSV-specific monoclonal antibody, was made available free of charge to all newborn infants in Queensland. Here we explore the community-level effects of nirsevimab introduction on RSV incidence at the population level.

Methods

A retrospective analysis was conducted of laboratory data collected prospectively by Sullivan Nicolaides Pathology (SNP) as part of routine surveillance. SNP is a network of private referral diagnostic laboratories; it undertakes about 40% of community laboratory tests in Queensland. We descriptively analysed monthly counts of RSV for children aged <1-year from January 2022 to October 2024.

Results

In total 21,921 tests were analysed (2022: 6,183; 2023: 5,618; 2024: 10,120), with 3,221 (14.7%) RSV-positive. The percentage of RSV positive tests declined from 15.8% in 2022 and 16.0% in 2023 to 13.3% in 2024. RSV detections were more common in males (1,713, 53.2%), with similar distributions across years.

Positive test percentages were compared across years during the peak RSV season (May to October). Of 15,574 tests in this period 2,216 (14.2%) were positive. Following the introduction of nirsevimab, monthly RSV-positive tests decreased among children aged 0–6-months (2022: 337/1,809 (18.6%); 2023: 207/1,287 (16.1%); 2024: 308/2,972 (10.4%)). A similar decline was observed in children aged 7–11-months (2022: 518/2,623 (19.8%); 2023 321/2,107 (15.2%); 2024: 525/4,776 (11.0%)).

Discussion

Nirsevimab introduction in Queensland coincided with a notable decline in RSV test positivity among infants in the first year of life. An interrupted time series analysis is underway to better understand the impact of nirsevimab and observed changes in disease metrics data.

Acknowledgment

This study is funded by a grant from Sanofi Pasteur S.A. (#VAS00012).

Background
Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory infections globally in children under the age of five years. With the development and implementation of RSV prevention strategies, it is important to identify children at high risk of hospitalisation due to RSV in order to ensure public health interventions are targeted to those most in need.
Methods
We used a probabilistically linked population cohort born in Western Australia between 2010 and 2020 and hospitalised before the age of 5 years. The primary outcome was the first laboratory-confirmed RSV-hospitalisation. Risk factor exposures included a range of perinatal, socio-demographic, household, environmental, congenital, and comorbid conditions antecedent to RSV-hospitalisation. Incidence rates, adjusted hazard ratios, and population attributable fractions (PAFs) were calculated using survival analysis techniques and Cox regression.
Results
From a cohort of 365,582 births, there were 8,316 RSV-hospitalisations before age 5 years. Risk factors for RSV-hospitalisation included demographic (male sex, Aboriginal ethnicity, maternal history of asthma), perinatal (younger maternal age, prematurity, maternal smoking during pregnancy), household/environmental (increasing number of siblings, season of birth), and comorbid and congenital conditions (cardiovascular defects, Trisomy 21 and cerebral palsy). The presence of two or more older siblings in the household and being born moderate to late preterm (32-36 weeks) had the highest PAFs (28.87% [95% CI: 26.25%, 31.39%] and 7.40% [95% CI: 6.75%, 8.04%]). While the risk of hospitalisation for children with some comorbid and congenital conditions was high (immunological conditions, aHR 3.94 [95% CI: 2.98, 5.23], respiratory system defects, aHR 3.13 [95% CI: 1.87,5.25]), the PAFs were relatively small (1.70% [95% CI: 1.53%, 1.86%] and 0.40% [95% CI: 0.30%, 0.49%]).
Conclusions
Our analysis has provided information on RSV risk factors that could assist public health authorities to identify high risk populations and prioritise the allocation of RSV prevention strategies.

Introduction: Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infection in children and a leading cause of infant hospitalisation in Australia. Nirsevimab, a monoclonal antibody, provides protection against RSV-associated lower respiratory tract disease and was registered for use in Australia in November 2023. The Western Australia (WA) Department of Health commenced roll-out of the most age-inclusive universal program in Australia on 2 April 2024; all WA infants born from 1 October 2023 through 30 September 2024 were eligible nirsevimab to receive immunisation. This evaluation assesses the real-world impact of nirsevimab on RSV-associated hospitalisations among a cohort of infants entering their first RSV season.
Methods: The number of births during 2022-2024 were obtained from WA Registry of Births and the number of infants who received nirsevimab prior to 12 months of age were extracted from the Australian Immunisation Register. Laboratory-confirmed RSV infection is notifiable in WA and patient admissions are matched weekly to notifications. An RSV-associated hospitalisation was defined as a specimen positive for RSV collected 10 days before through 7 after days the individual’s admission date during. The mean number of RSV-associated hospitalisations in 2022 and 2023 was defined as the ‘expected’ number for 2024.
Results: Of 30,920 eligible infants, 21,922 (71%) received nirsevimab. In 2024 there were 505 fewer RSV-associated hospitalisations than expected among infants aged <1 year (378 observed vs 883 expected). There was no reduction in RSV-associated hospitalisations observed in cohorts aged 1 to < 5 years (1,011 observed vs 847 expected), suggesting the decrease in infants was attributable to nirsevimab immunisation rather than lower overall RSV transmission in 2024.
Discussion: With approximately 70% of all nirsevimab-eligible infants immunised WA observed a 57% reduction in the expected number of RSV-associated hospitalisations. This equates to one hospitalisation prevented for every 43 infants immunised.

Background: In 2024, immunisations strategies against Respiratory Syncytial Virus (RSV) became available for children <2years and adults aged >60 years. Some states introduced RSV-prevention programs for young children including whole-of-population nirsevimab (long-acting RSV Mab) programs in Western Australia and Queensland. Vaccines for those >60 years were available on the private market. Hospital-based surveillance for laboratory-proven RSV, undertaken by the inFLUenza Complications Alert Network (FluCAN) and Paediatric Active Enhanced Disease Surveillance (PAEDS), commenced nationwide in April 2024.

Methods: Hospitalised children and adults with laboratory-proven RSV were prospectively recruited from 20 hospitals (including six tertiary paediatric hospitals). In addition, controls hospitalised with acute respiratory illness who tested negative for RSV were recruited contemporaneously. Risk factors, outcomes and immunisation status was determined from the medical records and Australian Immunisation Register. Immunisation effectiveness was estimated using the test-negative design.

Results: 3998 hospitalised RSV cases were reported from April-December 2024 (3416 children; 582 adults). Overall, 41.5% had risk factors for severe RSV (children: 33.9%; adults: 86.6%). Admission to ICU/HDU was required in 6.6% (children: 6.1%; adults: 9.4%). Median stay was longer in adults (4 days [IQR:3-9]) compared with children (2 [1-3]). Twenty eight in-hospital deaths occurred (children: 4 [0.12%); adults: 24 [4.12%]). 113 children aged <1 years (64 in WA; 31 in Qld; 18 in other states) received nirsevimab (40/1208 RSV-positive cases [2.61%]; 73/442 RSV-negative controls [16.51%]). Nirsevimab effectiveness against RSV-hospitalisation in infants <1 year was 83.15% (95%CI: 67.43,91.28). Only eight adults >60 years were vaccinated (3/221 RSV-positive cases; 5/622 RSV-negative controls).

Conclusions: Surveillance from 20 FluCAN/PAEDS hospitals in 2024 demonstrated capacity to assess the impact of evolving RSV-prevention strategies. Analysis of 2025 data will be critical to inform future prevention policies, given recent commencement of National Immunisation Program-funded maternal RSV vaccination in addition to state and territory-funded nirsevimab and RSV vaccine programs for older Australians.

Background: Western Australia (WA) experiences multiple climatic zones, influencing the epidemiology of respiratory viruses. We aimed to estimate the true incidence and under-ascertainment fraction of respiratory syncytial virus (RSV) and influenza hospitalisations from routine but variable microbiological testing across different climatic regions using predictive modelling.

Methods: We conducted a population-based cohort study using linked perinatal, hospitalisation and routine microbiological testing data for children aged <5 years, born in WA between 1 January 2010 and 31 December 2021. We used multivariable logistic regression to develop and validate predictive models for RSV- and influenza hospitalisations in southern temperate, northern tropical, and central desert regions. Model accuracy was assessed using cross-validated receiver operating characteristic curves. We compared laboratory-confirmed hospitalisation rates with model-predicted rates and determined the under-ascertainment fractions.

Results: Our cohort included 466,037 admissions (257,960 children), of whom, 33,106 were tested for RSV (23.4% positive) and 33,511 were tested for influenza (3.6% positive). We found higher hospitalisation rates in the regional central desert areas compared to predominantly metropolitan southern temperate areas for both RSV (36.71 versus 20.00 per 1,000 child-years for infants) and influenza (444.59 versus 144.40 per 100,000 child-years). Routine laboratory testing significantly underestimated the true burden of RSV (by 45–69%) and influenza (by 34–52%) hospitalisations.

Conclusions: The study highlights high under-ascertainment of RSV and influenza hospitalisations from routine viral testing data that varied across different climate regions. The findings suggest RSV and influenza vaccine programs should consider viral circulation in different climatic regions. Prediction models demonstrated reliability in estimating the true burden of RSV and influenza across the different climatic zones to support localised decision-making beyond Australia.

Acknowledgements: We thank the staff at WA Data Linkage Services and relevant Data Custodians. This study was supported by Sanofi–Aventis Australia (project# FLU00200).