Overview of respiratory virus epidemiology in the EU/EEA - weekly monitoring

Respiratory virus activity

• ILI and acute respiratory infection (ARI) consultation rates are continuing to decrease or remaining stable at low levels (15 reporting countries). Moving Epidemic Method (MEM) thresholds were available for 14 countries, two of which observed consultation rates above baseline levels. Short-term forecasts of ILI and ARI rates in EU/EEA countries are published on ECDC’s RespiCast.
• In primary care sentinel settings, the median test positivity at the EU/EEA level was highest for influenza, at 5% (pooled country data: 5%; interquartile range (IQR) of country values: 0−14%), with most countries reporting a decrease in test positivity. In week 17, two out of 11 countries reported a test positivity above 10%. Qualitative assessments of seasonal influenza activity from 17 countries indicate decreasing intensity in recent weeks (five baseline, nine low, three medium). Of 16 countries reporting geographical spread of seasonal influenza, one reported no activity, eight reported sporadic spread, one local, five regional, and one widespread.
• Among the 49 sentinel primary care detections of seasonal influenza, 42 (86%) were typed as influenza virus type B and 7 (14%) were typed as influenza virus type A. Of the influenza type B detections, 35 were further defined as B/Victoria lineage, while the remaining seven were of unknown lineage. Of the influenza type A detections that were further subtyped, two were A(H1)pdm09 and four were A(H3). The remaining influenza type A detection was of unknown subtype. It is worth noting that one country continues to account for most of the total number of reported influenza B detections (57% in week 17)
• The median sentinel primary care RSV positivity was 0% (pooled: 0.5%; IQR: 0–0%). Decreasing or stable trends were observed at the country level in both sentinel positivity and non-sentinel detections.
• The median sentinel primary care positivity for SARS-CoV-2 remained low at 0% (pooled: 2%; IQR: 0−2%). Decreasing or stable trends were observed at the country level in both sentinel positivity and non-sentinel detections.

Severe disease

• Rates of severe acute respiratory infection (SARI) from sentinel secondary sites were stable or decreasing and at levels comparable to the same time last year in all five countries reporting data up to week 17.
• The median SARI test positivity for seasonal influenza was 1% (pooled: 2%; IQR: 0.5–6%), with decreasing or stable trends observed in most countries reporting this indicator. All countries reporting non-sentinel hospital or ICU data observed decreasing or stable trends.
• The median SARI test positivity for RSV was 1% (pooled: 1%; IQR: 1−1%), with decreasing or stable trends observed in all countries reporting this indicator.
• The median SARI test positivity for SARS-CoV-2 was 1% (pooled: 2%; IQR: 0−3%). Both SARI positivity and non-sentinel indicators of severity remained at low levels in all countries.
• EuroMOMO pooled estimates of weekly excess all-cause mortality for the participating European countries were within expected levels.

Virus characterisation

Influenza

• WHO recommendations that trivalent vaccines for use during the 2023–2024 influenza season in the northern hemisphere contain the following (egg-based and cell culture or recombinant-based vaccines respectively): an A/Victoria/4897/2022 or A/Wisconsin/67/2022 (H1N1)pdm09-like virus (subclade 5a.2a.1); an A/Darwin/9/2021 or A/Darwin/6/2021 (H3N2)-like virus (clade 2a); and a B/Austria/1359417/2021 (B/Victoria lineage)-like virus (subclade V1A.3a.2).
• From week 40, 2023 to week 17, 2024, 3 088 A(H1)pdm09, 1 220 A(H3) and 180 B/Victoria viruses from sentinel and non-sentinel sources were genetically characterised. Of the A(H1)pdm09 viruses that have been assigned to a clade, 2 076 were reported as clade 5a.2a and 1 005 were subclade 5a.2a.1. Of the A(H3) viruses that have been assigned to a clade, 30 were reported as subclade 2a, 10 were subclade 2a.3a, 1 176 were subclade 2a.3a.1 and one was subclade 2a.3b. All B/Victoria viruses were reported as subclade V1A.3a.2
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• Antigenic characterisation data presented in WHO 2024-2025 northern hemisphere vaccine composition report indicate current northern hemisphere vaccine components are well matched to circulating 5a.2a and 5a.2a.1 A(H1N1)pdm09 subclades and V1A.3a.2 B/Victoria subclades. While components also appear well matched for 2a.3a A(H3) clade viruses, 2a.3a.1 clade viruses are less well matched. Based on human post-vaccination serology studies, haemagglutination inhibition and virus neutralisation against some recent 2a.3a.1 viruses were significantly reduced for some serum panels.
• ECDC has published interim influenza vaccine effectiveness (VE) estimates for the 2023–2024 season. Analysis of data submitted from multi-country primary care and hospital study sites between September 2023 and January 2024 indicated that up to 53% and 44% of vaccinated individuals in primary care or hospital settings, respectively, were protected against mild and severe influenza.

SARS-CoV-2 variants for weeks 15–16 (8 April to 21 April 2024)

• The estimated distribution (median and IQR of proportions from two countries) of variants of concern (VOCs) or variants of interest (VOIs) was 94% (90−97%) for BA.2.86 (which includes JN.1 isolates) and 2% (1—3%) for XBB.1.5-like (which now includes XBB.1.5+F456L). These estimates should be interpreted with caution as they are based on data from only three countries, a result of the very low number of sequences deposited in recent weeks during a period of low SARS-CoV-2 transmission.

Period overview (week 25, 2023 to week 17, 2024)

• Following relatively low respiratory illness transmission over the summer period, consultation rates increased in primary care settings from September 2023. Consultation rates were highest at different timepoints in each country during the winter period, with peak rates reached between week 50, 2023 and week 7, 2024. As of week 17, 2024, consultation rates continued to decrease and have returned to baseline levels in most EU/EEA countries. Transmission of SARS-CoV-2 began increasing in late summer, with clear increases observed at the EU/EEA level up to week 49 and decreases in activity thereafter. Activity is currently low in most EU/EEA countries. Similarly, a steady decrease in severe disease has been observed since week 50. COVID-19 has predominantly affected individuals aged 65 years and above. Week 50 marked the start of the seasonal influenza epidemic. A decreasing trend in influenza activity has been observed since week 4, 2024, with a mixed picture at the country level. Compared to trends observed in previous influenza epidemics, seasonal influenza activity decreased earlier this season. Severe disease due to influenza has affected all age groups. Since week 6, 2024, a decrease in the severe disease indicators for seasonal influenza has been observed in most EU/EEA countries. Both influenza type A and type B viruses have been detected, with a dominance of A(H1)pdm09 viruses during the first part of the season. As of week 13, B/Victoria lineage was the most detected virus, although the number of detections was low. RSV activity began increasing around week 41, reaching a peak in week 50, followed by a decreasing trend. RSV has had the greatest impact among children aged 0–4 years.

ECDC monitors rates of respiratory illness presentation and respiratory virus activity in the EU/EEA, presenting findings in the European Respiratory Virus Surveillance Summary (ERVISS.org). Updated weekly, ERVISS describes the epidemiological and virological situation for respiratory virus infections across the EU/EEA and follows the principles of integrated respiratory virus surveillance outlined in ‘Operational

considerations for respiratory virus surveillance in Europe’.