Effectiveness of influenza vaccines in children aged 6 to 59 months: a test-negative case-control study at primary care and hospital level, Spain 2023/24

Abstract

During 2023/24, all children aged 6 to 59 months were targeted for seasonal influenza vaccination in Spain nationally. Using a test-negative case-control design with sentinel surveillance data, we estimated adjusted influenza vaccine effectiveness (IVE) against any influenza type to be 70% (95% confidence interval (CI): 51 to 81%) for primary care patients with acute respiratory illness (ARI) and 77% (95% CI: 21 to 93%) for hospitalised patients with severe ARI. In primary care, where most subtyped viruses (61%; 145/237) were A(H1N1), adjusted IVE was 77% (95% CI: 56 to 88%) against A(H1N1)pdm09.

Keywords: children; influenza; test-negative design; vaccine effectiveness.

Figure 1

Flowchart of selection of paediatric cases and controls^a eligible for the sentinel surveillance, to estimate vaccine effectiveness against influenza virus-caused A) acute respiratory infections (ARI) in primary care and B) severe ARI (SARI) in inpatients, Spain, September 2023–June 2024 (n = 27 hospitals in 12 regions)

Figure 2

Number of paediatric cases and controls^a selected in the study by week and type/subtype of influenza among A) patients from primary care with acute respiratory infections (ARI), and B) hospitalised patients with severe ARI (SARI), and by age in months among C) ARI and D) SARI, Spain, September 2023–June 2024 (n = 27 hospitals in 12 regions)