Within influenza vaccine effectiveness (VE) studies at primary care level with a laboratory-confirmed
outcome, clinical case definitions for recruitment of patients can vary. We used the
2022-23 VEBIS primary care European multicentre study end-of-season data to evaluate
whether the clinical case definition affected IVE estimates.We estimated VE using
a multicentre test-negative case-control design. We measured VE against any influenza
and influenza (sub)types, by age group (0-14, 15-64, ≥65 years) and by influenza vaccine
target group, using logistic regression. We estimated IVE among patients meeting the
European Union (EU) acute respiratory infection (ARI) case definition and among those
meeting the EU influenza-like illness (ILI) case definition, including only sites
providing information on specific symptoms and recruiting patients using an ARI case
definition (as the EU ILI case definition is a subset of the EU ARI one).We included
24 319 patients meeting the EU ARI case definition, of whom 21 804 patients (90 %)
meet the EU ILI case definition, for the overall pooled VE analysis against any influenza.
The overall and influenza (sub)type-specific VE varied by ≤2 % between EU ILI and
EU ARI populations.Among all analyses, we found similar VE estimates between the EU
ILI and EU ARI populations, with few (10%) additional non-ILI ARI patients recruited.
These results indicate that VE in the 2022-23 influenza season was not affected by
use of a different clinical case definition for recruitment, although we recommend
investigating whether this holds true for next seasons.
