Influenza vaccine effectiveness against hospitalization in children aged 6-72 months: a test-negative study in the Southern Italy, 2022-2026.

Abstract

Background: Seasonal influenza is an important cause of hospitalization in young children, yet real-world evidence on vaccine effectiveness (VE) against severe outcomes remains limited in Italy. We estimated influenza VE against influenza-associated hospitalization among children aged 6-72 months in the district of Foggia, Southern Italy.

Methods: We conducted a test-negative case-control study among children hospitalized for acute respiratory infection and tested for influenza during the 2022-2023 to 2025-2026 influenza seasons. Cases were influenza-positive children and controls were influenza-negative children. VE was estimated as (1 - odds ratio) x 100; adjusted VE was derived from multivariable logistic regression models.

Results: The study included 935 children, of whom 207 were influenza-positive cases and 728 were influenza-negative controls. Adjusted VE was 62.5% (95% CI: 34.4%-78.5%) for receipt of at least one influenza vaccine dose and 61.9% (95% CI: 29.3%-79.4%) for full vaccination. By vaccine type, adjusted VE was 80.3% (95% CI: 43.8%-93.1%) for at least one dose of LAIV and 50.9% (95% CI: 6.9%-74.2%) for at least one dose of IIV. Season-specific adjusted VE was highest in 2024-2025, reaching 92.2% (95% CI: 65.6%-98.2%) for at least one dose and 90.7% (95% CI: 58.5%-97.9%) for full vaccination.

Conclusions: Influenza vaccination was associated with a reduced risk of influenza-associated hospitalization among children aged 6-72 months, with the strongest protection observed during the 2024-2025 season. These findings support continued efforts to improve pediatric influenza vaccination uptake.