Main text

Introduction

In temperate regions, respiratory syncytial virus (RSV) typically circulates in the winter months, causing more severe illness particularly in infants and older adults which often results in hospitalisation, including admission to intensive care units1,2. There are often periods of co-circulation with influenza and other seasonal respiratory viruses. RSV is monitored through sentinel and/or non-sentinel surveillance in many countries, territories and areas (henceforth referred to as countries) in the World Health Organization (WHO) European Region, the European Union (EU) and European Economic Area (EEA) countries (hereafter referred to as Europe). These sentinel surveillance systems were originally established for influenza and previously described 3. A number of RSV vaccines are approaching possible licensure 4–6. Monoclonal immunoglobulins are available as prophylaxis during the typical period of RSV circulation to protect young infants with chronic underlying heart and lung disease at higher risk of severe disease7.
The emergence and spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in early 2020 and the subsequent public health and social measures (PHSM) implemented by Member states to reduce its transmission and anticipated morbidity and mortality have disrupted the spread of other respiratory viruses, including RSV, in both the southern 5,8,9 and northern hemispheres10–12.
This work aims to describe the epidemiology of RSV in Europe during the 2020/21 and 2021/22 winter seasons (weeks 40/2020 to 20/2021 and 40/2021 to 20/2022) and two inter-seasonal periods (weeks 21 to 39/2021 and 21 to 39/2022) through three surveillance systems (primary care sentinel, secondary care Severe Acute Respiratory Infection (SARI), and non-sentinel surveillance) in comparison to historical pre-COVID-19 pandemic data.

Methods

This retrospective epidemiological analysis of RSV used weekly data submitted to The European Surveillance System (TESSy) by the European Regional surveillance network jointly coordinated by ECDC and the WHO Regional Office for Europe. As RSV is not notifiable, reporting is voluntary. Weekly counts of detections and specimens tested in 48 reporting countries between week 40/2020 and week 39/2022 were downloaded on 14th October 2022. Countries were included in this analysis if at least one sample tested was reported.
Using specimens taken from selected cases of influenza-like illness (ILI) and acute respiratory infection (ARI) presenting toe primary care sentinel surveillance system (these systems have been described previously 3, we calculated the aggregated weekly counts of RSV detections and tests and the percentage positivity when at least ten tests were performed. These counts and percentages were compared to the four previous seasons (2016/17 to 2019/20; hereafter referred to as pre-COVID-19 pandemic seasons). Using weekly RSV percentage positivity from four pre-COVID-19 pandemic seasons of sentinel data, we used the Moving Epidemic Method (MEM) to calculate RSV-specific epidemic thresholds for each country (Supplementary table 2) and identify potential changes in seasonal patterns, as previously shown 13. It is important to note that these thresholds may differ from those used in country for surveillance purposes. Epidemic activity was determined to have started when percentage positivity was above the country-specific epidemic threshold for the first of at least two consecutive weeks. The duration of the epidemic period was defined as the total number of weeks above the threshold (including non-consecutive weeks).
Similarly, aggregated weekly counts of detections and specimens tested were calculated, along with percentage positivity, for SARI sentinel surveillance (hospital inpatients meeting the case definition). Only two countries reported data in 2020/21 and sixteen in 2021/22al periods; historical data regarding number of tests performed were unavailable for this surveillance system. Data included aggregated SARI cases by age group (≤4 years, 5 to 14 years, 15 to 64 years and ≥65 years).
For the non-sentinel surveillance system, only the weekly counts of detections were aggregated but percentage positivity could not always be calculated given the unavailability and/or uncertainty around some of the denominators. Specimens from this system were taken from patients that originated from hospitals, schools, primary care facilities not involved in sentinel surveillance, or nursing homes and other institutions.
For all three systems, data in the study period were compared to historical data. Some countries stopped surveillance or reporting data to TESSy over the summer months for some surveillance systems considered. All analyses were conducted in R version 4.0.514.

Results