The seasonality of respiratory syncytial virus in Western Australia prior to implementation of SARS‐CoV‐2 non‐pharmaceutical interventions

Estimated Incidence of Hospitalisations and Deaths Attributable to Respiratory Syncytial Virus Infections in Adults in Australia Between 2010 and 2019

BACKGROUND

Respiratory syncytial virus (RSV) morbidity and mortality in adults are often underestimated due to nonspecific symptoms, limited standard-of-care testing and lower diagnostic testing sensitivity compared with children. To accurately evaluate the RSV disease burden among adults in Australia, we conducted a model-based study to estimate RSV-attributable cardiorespiratory hospitalisation incidence and mortality rate.

METHODS

A quasi-Poisson regression model was used to estimate RSV-attributable cardiorespiratory, respiratory and cardiovascular events, using weekly hospitalisation and mortality data from 2010 to 2019, accounting for periodic and aperiodic time trends and viral activity and allowing for potential overdispersion. The time-series model compared the variability in confirmed RSV events alongside variability in all-cause cardiorespiratory events identified from ICD-10-AM codes to estimate the number of RSV-attributable events, including undiagnosed RSV-related events.

RESULTS

RSV-attributable incidence of cardiorespiratory hospitalisations increased with age and was highest among adults ≥ 65 years (329.5-386.6 cases per 100,000 person-years), nine times higher than in adults 18-64 years. The estimated incidence of RSV-attributable respiratory hospitalisations in adults ≥65 years (219.7-247.8 cases per 100,000 person-years) was 35-fold higher than in adults 18-64 years. RSV-attributable deaths accounted for 4% to 6% of cardiorespiratory deaths in adults ≥ 65 years, with RSV-attributable mortality rates ranging from 65.6 to 77.6 deaths per 100,000 person-years and respiratory mortality rates ranging from 20.3 to 24.0 deaths per 100,000 person-years, both 70-fold higher than in adults 18-64 years.

CONCLUSIONS

This study identified substantial RSV-associated morbidity and mortality among Australian adults and is the first study to report RSV-attributable mortality rates for Australia that account for untested events.

Effectiveness of nirsevimab in preventing RSV-hospitalisation among young children in Western Australia 2024

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 nirsevimab program for all infants and high-risk children entering their second RSV season. This study describes the effectiveness of nirsevimab against RSV hospitalisation over a single epidemic season.

METHODS

Between April and October 2024, children hospitalised with laboratory-confirmed RSV-associated acute respiratory infection (ARI) and test-negative controls were enrolled from three hospitals in WA. Demographic variables, medical risk factors, symptoms and outcomes were assessed. Nirsevimab effectiveness in preventing RSV-associated hospitalisation was estimated.

RESULTS

Over 7 months, 284 children eligible for nirsevimab were enrolled including 184 RSV positive cases and 100 controls. Coverage of nirsevimab in RSV cases was 22.8% and 60.0% in controls. The overall adjusted effectiveness of nirsevimab against RSV-associated ARI hospitalisation was 88.2% (95% CI: 73.5, 94.7). RSV infection occurred in 42 (22.8%) children who had received nirsevimab; there was no significant difference in RSV illness severity among those immunised and unimmunised.

CONCLUSION

Nirsevimab was highly effective at preventing RSV-associated ARI hospitalisation in young children in the southern hemisphere.

SUMMARY

This study is the first Australian study to provide nirsevimab effectiveness estimate against RSV hospitalisation over a single epidemic season. The adjusted estimate of nirsevimab effectiveness against RSV-associated ARI hospitalisation was 88.2%, similar to those reported from Northern Hemisphere countries.

Study of Children Aged Under 2 Years Admitted With RSV at Four Australian Hospitals [2021-2022]

AIMS

Primary aim was to review severe acute respiratory infections (SARI) hospitalisations caused by respiratory syncytial virus (RSV) in children aged < 2 years in paediatric hospitals in Australia. Secondary aims included RSV subtyping, assessing RSV seasonality and contributing to the World Health Organisation's RSV surveillance programme.

METHODS

We prospectively reviewed the medical records of children (< 2 years of age) with a confirmed SARI who were admitted to one of four major Australian paediatric hospitals and had a respiratory sample analysed by Polymerase Chain Reaction (PCR). A detailed dataset was completed for RSV positive cases.

RESULTS

Between 1 January 2021 and 31 December 2022, 2290 RSV (laboratory-confirmed) admissions were identified (53.4% of all SARI admissions). Approximately 50% of all RSV cases were aged 0-6 months. RSV-A predominated in 2021 with peak infections observed in summer while in 2022 RSV-B predominated with peak infections in the more traditional winter months. The median total length of stay (LOS) for RSV positive admissions was 46 h (IQR: 22-82 h). 9% of these children required an ICU admission with a prolonged median LOS 68 h (IQR: 40-112 h). Respiratory support utilisation was consistent over the 2 years. 1.8% required mechanical ventilation; 4.6% continuous positive airway pressure; 23.3% high flow oxygen; and 50.8% low flow oxygen.

CONCLUSIONS

RSV in children continues to cause a significant disease burden at Australian tertiary paediatric centres. Ongoing hospital surveillance is required to document the impact of RSV preventative therapies that have become available in 2024.

Estimating the cost-effectiveness of maternal respiratory syncytial virus (RSV) vaccination in Australia: A dynamic and economic modelling analysis

BACKGROUND

Respiratory syncytial virus (RSV) is a major cause of respiratory illness, with younger infants at greatest risk of hospitalisation. With the recent approval of a maternal RSV vaccine in Australia, it is timely to evaluate its potential costs and health benefits in Australia.

METHODS

We applied an integrated dynamic and economic evaluation model to estimate specific outcomes of RSV disease and the cost-effectiveness of a year-round maternal RSV vaccination program in Australia. Cost-effectiveness was estimated using the incremental cost-effectiveness ratio (ICER) per quality-adjusted life year (QALY) gained.

RESULTS

From a healthcare system perspective, the maternal vaccination program was estimated to be cost-effective at a vaccine price less than approximately 120 Australian dollars ($AU), assuming a willingness-to-pay (WTP) threshold of $AU 50,000/QALY gained. Most of the estimated cost-savings were from preventing RSV hospitalisations in infants aged <6 months. However, while 82% of the cost-savings were from preventing RSV hospitalisations in infants aged <6 months, only about 25% of the QALY gains were in this age group. The majority of the other QALY gains came via herd effects from prevention of death in older adults and to a lesser extent, prevention of nonmedically-attended illness in older teens and adults. When predicted cost-savings and QALY gains in those ≥6 months of age were excluded, the vaccine price required to meet the assumed WTP threshold fell to $AU 63.

CONCLUSIONS

A maternal RSV vaccination program in Australia could provide value for money by reducing hospitalisations and associated costs among infants aged <6 months, depending on the vaccine price. We have provided evidence that herd effects beyond the target population may be an important consideration in assessing cost-effectiveness of maternal RSV vaccination.

Understanding the local-level variations in seasonality of human respiratory syncytial virus infection: a systematic analysis

BACKGROUND

While previous reports characterised global and regional variations in RSV seasonality, less is known about local variations in RSV seasonal characteristics. This study aimed to understand the local-level variations in RSV seasonality and to explore the role of geographical, meteorological, and socio-demographic factors in explaining these variations.

METHODS

We conducted a systematic literature review to identify published studies reporting data on local-level RSV season onset, offset, or duration for at least two local sites. In addition, we included three datasets of RSV activity from Japan, Spain, and Scotland with available site-specific data. RSV season onset, offset, and duration were defined using the annual cumulative proportion method. We estimated between-site variations within a region using the earliest onset, the earliest offset, and the shortest duration of RSV season of that region as the references and synthesised the variations across regions by a multi-level mixed-effects meta-analysis. Using the three datasets from Japan, Spain and Scotland, we applied linear regression models with clustered standard errors to explore the association of geographical, meteorological, and socio-demographic factors with the season onset and offset, respectively.

RESULTS

We included 7 published studies identified from the systematic literature search. With the additional 3 datasets, these data sources covered 888,447 RSV-positive cases from 101 local study sites during 1995 to 2020. Local-level variations in RSV season within a region were estimated to be 6 weeks (41 days, 95% CI: 25-57) for season onset, 5 weeks (32 days, 13-50) for season offset, and 6 weeks (40 days, 20-59) for season duration, with substantial differences across years. Multiple factors, such as temperature, relative humidity, wind speed, annual household income, population size, latitude, and longitude, could jointly explain 66% to 84% and 35% to 49% of the variations in season onset and offset, respectively, although their individual effects varied by individual regions.

CONCLUSIONS

Local-level variations in RSV season onset could be as much as 6 weeks, which could be influenced by meteorological, geographical, and socio-demographic factors. The reported variations in this study could have important implications for local-level healthcare resources planning and immunisation strategy.

TRIAL REGISTRATION

PROSPERO CRD42023482432.

Postpandemic fluctuations of regional respiratory syncytial virus hospitalization epidemiology: potential impact on an immunization program in Switzerland

RSV hospitalization epidemiology is subject to rapid changes brought about by the COVID-19 pandemic and the prospect of vaccine prevention. The purpose of this report is to characterize recent epidemiologic and clinical fluctuations and to analyze their potential impact on an immunization program with nirsevimab. This is a 2018-2024 retrospective analysis of all hospitalizations caused by RSV in patients below 16 years of age occurring at an academic Children's Hospital that serves a defined population. We simulated the vaccine impact against RSV hospitalization by applying the expected effects of the infant immunization program with nirsevimab proposed in Switzerland to observed case counts. We analyzed 1339 hospitalizations. The consecutive occurrence of two major epidemics in 2022-2023 and 2023-2024 had never been recorded previously. The 2023-2024 season witnessed a major shift to older age. Only 61% of patients were below 12 months of age, while prepandemic long-term surveillance since 1997 found a range between 64 and 85% (median, 73%). Age below 3 months, prematurity, airway anomalies, congenital heart disease, and neuromuscular disorders were independently associated with ICU admission. Simulation of the vaccine impact using two scenarios of coverage and efficacy (scenario 1, 50% and 62%, respectively; scenario 2, 90% and 90%) and three different age distributions resulted in an infant vaccine impact of 31.0% (scenario 1) and 81.0% (scenario 2), respectively. Vaccine impact for all patients below 16 years ranged from 22.7 to 24.9% (scenario 1) and 54.2 to 68.8% (scenario 2).

CONCLUSION

RSV hospitalization epidemiology was characterized by substantial variability in patient age on admission. As the proposed RSV immunization program primarily targets infants, year-to-year fluctuation of cases among older children will cause a variability of vaccine impact of approximately 15%. This information may be useful for physicians and hospital administrators when they anticipate the resources needed during the winter season.

WHAT IS KNOWN

• RSV hospitalization epidemiology was subject to massive disturbances during the COVID-19 pandemic. • Extended half-life monoclonal antibodies and active maternal immunization offer new means of passive protection of infants against severe RSV disease.

WHAT IS NEW

• We demonstrate substantial year-to-year fluctuation of the age distribution at the time of RSV hospitalization. • Up to 40% of annual RSV hospitalizations in a given season occur in children above 12 months of age who do not benefit from maternal RSV immunization and may not be eligible for receipt of a monoclonal antibody.

Nirsevimab: Alleviating the burden of RSV morbidity in young children

Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections (LRTIs) and hospital admissions in early childhood. Recent advancements in novel preventive therapies, including extended half-life monoclonal antibodies and antenatal vaccination, have afforded new opportunities to significantly reduce the burden of this infection. Nirsevimab is a novel monoclonal antibody that provides sustained protection against RSV for at least 5 months among newborns and young children. It has received regulatory approval in numerous countries and is being implemented across various settings. Two pivotal Phase 3 trials (MELODY, HARMONIE) demonstrated significant reductions in RSV-associated LRTI hospitalisations following nirsevimab administration, with treatment efficacy of 62.1% and 83.2%. Emerging real-world data from early adopters of nirsevimab corroborates these findings. Studies from Spain, Luxembourg, France and the USA report effectiveness rates between 82% and 90% in preventing RSV-associated hospitalisations among infants entering their first RSV season. Current implementation strategies for nirsevimab have primarily focused on seasonal administration for all infants, aligned to local RSV seasons, and often include catch-up doses for those born before the season begins. Available cost-effectiveness analyses indicate that while nirsevimab offers significant potential public health benefits, its adoption must carefully consider economic factors such as treatment costs, implementation strategies tailored to local viral epidemiology, and logistics for vaccine delivery. Overall, nirsevimab presents a promising opportunity to alleviate the burden of severe RSV infections in young children. However, ongoing surveillance and refinements in implementation strategies are crucial to optimise its impact and ensure sustainability across diverse health-care settings.

The Changing Detection Rate of Respiratory Syncytial Virus in Adults in Western Australia between 2017 and 2023

The incidence of respiratory syncytial virus (RSV) in adults is inadequately defined and the impact of SARS-CoV-2-related non-pharmaceutical interventions (NPIs) is underexplored. Using laboratory data, we described the detection rate of RSV in adults ≥16 years in Western Australia (WA) between 2017 and 2023. With the exception of 2020, RSV detections rose annually between 2017 and 2023, reaching 50.7 per 100,000 in 2023 (95% confidence interval [CI], 47.9-53.8). RSV testing expanded considerably across the study period, with the testing in 2023 more than five times the 2017 total. The detection rate was highest in adults ≥60 years between 2017 and 2019, particularly those ≥75 years. Following 2020, the detections in all age groups increased, with the highest detection rate in 2023 in those ≥75-years (199.5 per 100,000; 95% CI, 180.5-220). NPIs significantly impacted RSV seasonality; the preceding winter pattern was disrupted, resulting in an absent 2020 winter season and two major summer seasons in 2020/21 and 2021/22. The RSV season began to realign in 2022, reverting to a winter seasonal pattern in 2023 and the largest season in the study period. Ongoing surveillance will be required to understand the stability of these increases and to delineate the impact of new immunisation strategies.

Head-to-Head Comparison Between Respiratory Syncytial Virus and Human Metapneumovirus Bronchiolitis in the Setting of Increased Viral Testing

We compared the epidemiology, severity and management of hospitalized respiratory syncytial virus (n = 305) and human metapneumovirus (n = 39) bronchiolitis in a setting with high respiratory virus testing (95% of admissions tested). Respiratory syncytial virus-positive infants were younger and tended to require more hydration support and longer hospital stays compared to human metapneumovirus-positive infants. Respiratory support requirements were similar between groups despite significant age differences.

Respiratory Syncytial Virus Reinfections in Children in Western Australia

Respiratory syncytial virus (RSV) reinfection in children is poorly understood. We examined the incidence, characteristics, and outcomes of hospital-attended RSV reinfections in children <16 years in Western Australia between 2012 and 2022. Individuals with repeat RSV detections ≥56 days apart were identified using laboratory data. The incidence of reinfection in the first five years of life was estimated using the total birth population from 2012 to 2017. Clinical data on a subset of reinfection episodes were obtained from two metropolitan pediatric centers. A total of 466 children with hospital-attended reinfections were identified. The median interval between RSV detections was 460 days (interquartile range: 324, 812), with a reinfection rate of 95 per 100,000 individuals (95% confidence interval: 82, 109). Reinfection was most common in children who experienced their first RSV detection <6 months of age. Predisposing factors were identified in 56% of children; children with predisposing factors were older at first and second detections, were more likely to be admitted, and had a longer length of stay. This study highlights the significant burden of hospital-attended RSV reinfections in children with and without predisposing factors. Expanded surveillance with in-depth clinical data is required to further characterize the impact of RSV reinfection.