Figure 1a illustrates the weekly incidence of medically attended RSV infections as reported by the AGI between October 2014 and September of 2023. The highest incidence per 100,000 inhabitants is observed in the age group of ‘0–4’, with lower incidences in the older age groups. The data indicates reduced RSV activity during the 2020/2021 COVID pandemic season and higher incidences in the subsequent 2021/2022 and 2022/2023 seasons.

Incidence per 100,000 inhabitants and absolute number of medically attended RSV infections (PCR tested) in the outpatient sector according to the sentinel surveillance system of the AGI at RKI by age group from the 2014/2015 to the 2022/2023 season

Recalculating the same data to absolute values highlights a different aspect of the age-specific BoD. Figure 1b shows the absolute number of RSV cases by age group, aggregated by season. On average, between the 2014/2015 and 2022/2023 seasons (excluding the 2020/2021 pandemic season), the sentinel surveillance estimates 2,546,000 RSV-related outpatient consultations. Of these, 43.5% occur in the ‘0–4’ age group, 8.4% in ‘5–14’, 13.4% in ‘15–34’, 21.7% in ‘35–59’ and 13.0% in the ‘60+’ age group. The between-season variability shows a total of 1,313,100–3,911,800 cases overall [standard deviation (SD) 918,825] with cases in the ‘60+’ age group varying between 88,500 and 624,100 and averaging 331,000, with the proportion ranging between 6% and 17%.

Figure 2 depicts the absolute number of hospitalisations (main diagnosis) per year between 2000 and 2022 for different age groups, stratified by ICD code, as available from the hospital statistics [19]. The absolute number of annual cases increases over the study period for all age groups and ICD codes. RSV-related bronchitis (ICD-code J20.5) and bronchiolitis (J21.0) are predominantly found in the youngest age group, ‘0–4’. This age group also shows the highest number of hospitalisations, reaching up to 30,000 in 2021. For the older age groups, pneumonia (J12.1) is the predominant hospital diagnosis.

Yearly number of hospitalisations in Germany due to RSV as main diagnosis for different age groups and ICD codes between 2014 and 2022

While the annual case numbers steadily increase across all age groups, a significant and abrupt increase in case numbers can be observed in the age group 60+ from 257 cases in 2016 to 1317 cases in 2017. Another abrupt increase in 2019 is followed by 2 years of reduced but still overall high activity before another increase in the latest available year, 2022. The data is only available by calendar year, which distorts the generally low RSV activity during the early phase of the COVID-19 pandemic.

The data from the hospital statistics also allow for the analysis of the length of stay (LoS) as depicted in Table A3-1 in Appendix 3 in the Supplementary Material. Patients with pneumonia (J12.1) show a longer LoS than patients with bronchitis (J20.5) or bronchiolitis (J21.0). The longest average LoS can be observed for pneumonia patients in the age group ‘35–59’ (11.5 days), followed by the age group 60+ (10.4 days, both groups median 8 days; 6/10 days 25%/75% percentile).

Figure 3 shows the results of the outcomes per season from the DRG statistic (numbers can be found in Table A4-1 in Appendix 4 in the Supplementary Material). Across all seasons, age groups and ICD codes, 17.0% of all RSV hospitalisations are treated in the ICU. The overall ICU rate was affected by the pandemic, with ICU rates of 7.5% for the 2019/2020 season, 6.5% for the 2020/2021 season, 30.8% for the 2021/2022 season and 9.5% for the 2022/2023 season. Excluding the two pandemic seasons (2020/2021 and 2021/2022), ICU rates are highest in the age group ‘40–59’ with 18.8% (followed by 16.1% in 60+) and lowest in the age group ‘1–2’ with 4.8%. ICU rates are generally higher for RSV-related pneumonia (13.3%) and B97.4-diagnoses (14.8%) compared with bronchitis and bronchiolitis (5.3%).

Number of hospitalisations for different age groups for the various RSV ICD codes and the seasons between 2019/2020 and 2022/2023, stratified by ICU stay (darker colour) and in-hospital mortality (brown colour)

In-hospital mortality reveals a similar pattern: the overall mortality rate is 1.39% over all age groups and seasons. For the four seasons from the pre-pandemic 2019/2020 season to the post-pandemic 2022/2023 season, it is 0.82%, 0.09%, 0.32% and 2.76%, respectively. In-hospital mortality is consistently highest in the 60+ age group, at 9.4%.

The DRG statistic for seasons 2019/2020 to 2022/2023 reveals similar patterns as the outpatient sentinel surveillance (see Fig. 3). The time trend shows the effects of the measures taken during the initial phase of the COVID-19 pandemic and the generally low activity of ARIs in the 2020/2021 season. It also shows the rebound effect in the 2021/2022 season, especially in the younger age groups and for RSV-related bronchitis and bronchiolitis. In the 2022/2023 season, the case numbers appear similar to the pre-pandemic 2019/2020 season. An exception is the case numbers in the 60+ age group, showing a sevenfold increase between pre- and post-pandemic season. This increase can be observed for all outcomes, i.e. 6.9-fold increase for patients who survived RSV hospitalisation and were not admitted to the ICU, a 6.0-fold increase in patients who survived RSV hospitalisation and were admitted to the ICU, a 7.5-fold increase in in-hospital mortality of patients admitted to the ICU and an 11.1-fold increase in in-hospital mortality in patients not admitted to the ICU (see Table A4-1 in Appendix 4 in the Supplementary Material).

Stratifying the data by the hospital size in which the RSV patients were treated shows that the factor of increase varies between hospital sizes. Figure 4 shows the factor of post-pandemic (2022/2023) cases divided by pre-pandemic (2019/2020) cases across age groups per hospital size. A strong age pattern is visible, with an increasing ratio with increasing age after the age of 2 years. The data also show that the ratio increases more in smaller hospitals with less than 400 beds.

Factor comparing pre- and post-pandemic RSV cases by age groups across different hospital sizes

The mortality statistic [23] in Germany reports 214 deaths due to the RSV ICD codes J12.1, J20.5 and J21.0 between 1998 and 2022. Age information is available for 142 cases, with 96.5% of these occurring in individuals aged 60 years and over. No deaths are reported in the periods between 1998–2012 and 2014–2016. There are 5 cases reported in 2013, between 4 and 15 deaths annually between 2017 and 2021, and 90 deaths in 2022, all occurring in the 60+ age group. This figure is ten times lower than the corresponding numbers for in-hospital mortality derived from the DRG-statistic which shows a 10.5% in-hospital mortality rate.