New data from six European countries reveals the true toll of respiratory syncytial virus on older adults, urging action on diagnostics, vaccine rollout, and better coordination between countries.
Study: Hospitalisation trends of respiratory syncytial virus (RSV) infection in adults, six European countries, before and during COVID-19, 2016 to 2023. Image credit: Akkalak Aiempradit/Shutterstock.com
Respiratory syncytial virus (RSV) infection often leads to hospitalizations and deaths. Its impact on adult health is still poorly understood. A recent study published in the journal Eurosurveillance explores the RSV-related hospitalization burden in some European countries during the coronavirus disease 2019 (COVID-19) pandemic.
Introduction
RSV has been primarily studied for its prevalence and health effects in young children. However, emerging evidence indicates that it accounts for unexpectedly high adult hospitalization rates. Immunocompromised, older adults, and those with underlying illnesses are particularly vulnerable. Concomitant RSV infection can worsen the condition of people who have asthma, chronic obstructive pulmonary disease (COPD), or heart failure, causing pneumonia or heart attacks, or even death.
Three RSV vaccines are now approved by the European Medicines Agency (EMA): Arexvy for those aged 60 years and above, Abrysvo for this group and pregnant women, and mRESVIA, an mRNA vaccine. Vaccination strategies require proper estimates of the RSV-related healthcare burden in adults. However, routine RSV testing and surveillance systems designed to pick up RSV cases are lacking.
The current study, which followed a retrospective design, focused on RSV-related hospitalization rates and their variation by season and year, beginning before the pandemic and continuing until 2023.
The authors excluded the 2019/20 and 2020/21 seasons from key comparisons due to disruptions from the pandemic and changes in viral circulation. They also included a prospective surveillance dataset from the Valencia region of Spain, allowing comparisons between routine registry data and systematic testing.
This dataset followed an influenza-like illness (ILI) case definition and was adjusted to the RSV circulation period, which may have affected comparability with national data.
The data came from national hospital registries containing routinely collected adult admission data covering all RSV-specific respiratory infections from five European countries. The sixth country (Spain-Valencia region) contributed prospective active surveillance network hospital-based data. RSV-related hospitalizations were stratified into those with an RSV diagnosis code and those with a laboratory-confirmed diagnosis.
Study findings
Routinely collected health records lack laboratory confirmation. Therefore, ICD codes are used to estimate RSV-related hospitalizations. The current study shows that “despite the availability of PCR laboratory-confirmed data; RSV incidence may still be underestimated by at least 2.2 times due to the underestimation of diagnostic tests based on RT-PCR.” Moreover, RSV-coded admissions underestimate actual cases by up to 4.3 times.
Again, RSV-confirmed admissions were twofold the estimate based on ICD codes. Despite this, in the absence of laboratory data, RSV-coded admissions are a valid substitute as they correlate strongly with the RSV-confirmed rates.
Strong correlations (Spearman r² values ranging from 0.96 to 0.99) were observed between RSV-coded and RSV-confirmed trends, reinforcing the value of ICD-coded data when testing is unavailable.
Overall picture
While 0.2% - 1.5% of adults had RSV-coded admissions over the study period, RSV-confirmed infection rates were 0.6% - 5.1%. Most admissions were due to pneumonia, the next most common being bronchitis, bronchiolitis, and unspecified lower respiratory tract infection. Upper respiratory tract infections were higher in the Netherlands and England than in other countries.
This difference was partly attributed to variation in the specificity of diagnostic codes recorded at discharge between countries.
Finland’s RSV-related admission rates were the highest in all years except 2016/7. Denmark and Spain-Valencia reported the lowest RSV-coded hospitalization rates, with fewer than 10 cases per 100,000 person-years across all age groups. However, the number of RSV-coded admissions in Spain-Valencia was fewer than 10 in each age group, limiting the robustness of those figures.
Rates by age
RSV admissions increased with age. Adults over the age of 85 years had the highest rate of hospitalizations in all countries. Hospitalization rates were lowest in people below 64 years before and during COVID-19.
Rates by year
Pre-and post-pandemic RSV-coded and RSV-confirmed rates were lowest among people younger than 64. For instance, before COVID-19, RSV-coded and RSV-confirmed rates in Denmark stood at 0.6 and 1.8 per 100,000 person-years, respectively. Thus, RSV-confirmed admissions were numerically higher but followed the same overall trends.
Hospitalizations peaked in the 85-plus age group before the pandemic. Finland registered as high as 445 RSV-confirmed hospitalizations per 100,000 person-years in this age group during 2017/18, reducing to zero during 2020/21. In Denmark and Spain-Valencia, they stood at 44 and 260 admissions per 100,000 person-years before the pandemic, remaining relatively stable during the pandemic.
Two-yearly peaks were seen in Finland and Spain-Valencia, corroborating prior research. The recurring pattern observed across multiple countries and age groups suggests cyclical RSV transmission dynamics. High correlation coefficients between national datasets supported strong consistency in year-over-year trends across countries.
In the pandemic years 2020/21, RSV admissions rapidly declined to less than two in all countries and age groups. In Finland, they fell to zero. This might be due to decreased transmission due to public health policies focusing on limiting transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
During the pandemic, RSV-coded admissions in the 85-plus group increased by 61% in England, but not in other age groups. Other countries did not show any change.
RSV-confirmed admissions fell by 48% and 43% in Finland's 75-84 and 85-plus age groups, respectively, but only in Scotland's 85-plus age group. In Spain-Valencia, a broad-based decline was observed. These reductions may reflect real declines in RSV transmissions, changes in testing behavior, or shifts in coding and healthcare-seeking practices.
ICU admissions and deaths
ICU admissions were higher among RSV patients up to the age of 75 years before the pandemic, but not thereafter. This pattern was most evident in England, the only country with available ICU data.
During the pandemic, the proportion of RSV patients admitted to the ICU decreased with age, irrespective of laboratory confirmation. The reasons remain to be clarified. This may relate to triage practices, ICU capacity constraints, or reduced illness severity.
Case fatality ratios were lowest in patients under 64, at <4%. In the over 85 age group, they peaked, at 6.2% to 17.6%. Finland had the most significant drop in fatality ratio across age groups during the pandemic, at 30% to 80%, vs 6% to 47% in England.
These opposing trends may reflect country-level differences in diagnostic coding, hospital practices, or testing policies after 2020. Hospitalizations for respiratory tract infections were associated with higher fatality rates than RSV-related admissions.
The impact of influenza and other vaccinations must be considered in all these assessments, as they reduce the severity of all these outcomes. During the study period, the RSV vaccine had not yet been rolled out, which may partly explain the relatively higher RSV-specific burden compared to influenza.
The study also noted that a lack of detailed comorbidity data limited the ability to assess risk in immunocompromised populations.
Conclusions
The study shows that RSV-coded hospitalizations were significantly different from RSV-confirmed rates. This emphasizes the need for better surveillance strategies and more sensitive and accurate diagnostic measures.
The average underestimate factor for RSV-coded admissions compared to laboratory-confirmed cases was 1.9, ranging from 1.1 to 4.3 across countries.
Despite this discrepancy, and regardless of the use of laboratory-confirmation vs ICD codes, the risk of severe RSV infection consistently increased in older adults in all six European countries and across all study years. This indicates that old age is a significant risk factor for RSV-related hospitalization.
The authors call for coordinated European surveillance systems, improved diagnostic testing, and consistent coding practices to support vaccine deployment and public health planning.
“This study lays the groundwork for future research and surveillance efforts aimed at mitigating the impact of RSV.”
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