Case definition discrepancies influence VE outcomes

Different definitions of severe acute respiratory infection (SARI) alter vaccine effectiveness estimates by more than 10%. 

The analysis used data from the VEBIS hospital network for the 2023/24 season across 7 European sites. Case definitions included Severe Acute Respiratory Infection – case definition 1 (SARI-CD1) (ECDC-recommended: hospital admission ≥1 of the following symptoms: cough, fever, shortness of breath, anosmia, ageusia, or dysgeusia) and SARI-CD2 (WHO-recommended: hospital admission with cough and fever). Participants were included if aged ≥65 years with quantitative reverse transcription polymerase chain reaction (RT-qPCR)-confirmed influenza A(H1N1)pdm09, A(H3N2), or SARS-CoV-2. Controls were RT-qPCR negative for these viruses or confirmed positive for other pathogens. 

Mr Diogo Marques (EpiConcept, France) presented the results. Estimated vaccine effectiveness against influenza A(H1N1)pdm09 was 34% (95% CI 14-49) using SARI-CD1 and 44% (95% CI 20-61) with SARI-CD2 [1]. For COVID-19, estimates were 29% (95% CI 11-43) with SARI-CD1 and 40% (95% CI 17-57) with SARI-CD2. Differences between definitions were also observed across subgroups stratified by age and time since vaccination. 

The authors concluded: “Our analysis identified vaccine effectiveness differences of more than 10% between SARI case definitions for influenza A(H1N1)pdm09 and SARS-CoV-2, particularly in patients aged ≥80 years and in those ≥60 days post-vaccination.” These discrepancies may reflect the fact that SARI-CD1 captures more cases. Therefore, comparing vaccine effectiveness across studies using different SARI definitions should be interpreted with caution.” 

1. Marques D, et al. Effect of severe acute respiratory infection case definitions on influenza and COVID-19 vaccine effectiveness studies: VEBIS hospital network, Europe, 2023-24. 6th ESCMID Vaccines, 10–13 September 2025, Lisbon, Portugal. 

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Posted on 21 October 202521 October 2025