Influenza Vaccine Effectiveness During the 2023/2024 Season: A Test-Negative Case-Control Study Among Emergency Hospital Admissions With Respiratory Conditions in Northern Ireland.

Background: We evaluated the effectiveness of the influenza vaccine programme against infection among emergency hospital admissions with respiratory conditions in Northern Ireland during the 2023/2024 influenza season.

Methods: Using a test-negative design, we compared the odds of vaccination between patients who tested positive (cases) and negative (controls) for laboratory-confirmed influenza, adjusting for confounders. VE was stratified by age group, sex and time since vaccination.

Results: We included 2368 hospitalised patients, of whom 1740 (73.5%) were influenza positive. Among these, 1703 (97.9%) were influenza A and 37 (2.1%) were influenza B. Of the influenza A-positive specimens, 84 were A(H1), 268 A(H3) and 1351 were untyped influenza A. VE against all laboratory-confirmed influenza was 47.5% (95% CI: 31.3%-60.1%), including 65.2% (95% CI: 44.2%-78.6%) in children aged 2-17, 46% (95% CI: 7.8%-68.2%) in adults 18-64 and 39.5% (95% CI: 4.8%-62.1%) in adults aged 65 and over. VE against infection for influenza A was 45.8% (95% CI: 25.1%-61%) in all age groups, but 64.7% (95% CI: 42.6%-78.6%) among children aged 2-17, 43.9% (95% CI: 3.7%-67.1%) among adults aged 18-64 years old and 39.6% (95% CI: 5%-62.1%) in adults aged ≥ 65 years. Being vaccinated was associated with 44.2% (95% CI: -13.3%-73.1%) and 37.9% (95% CI: 5.5%-59.5%) reduced odds of influenza A(H1) and A(H3)-associated community-acquired emergency admissions. VE against infection for influenza B was 87.2% (95% CI: 43.1%-98.3%). VE was highest within 2-8 weeks of vaccination at 67.5% (95% CI: 42.7%-81.7%) and declined to 41.2% (95% CI: 14.8%-59.5%) at 9-16 weeks.

Conclusions: Influenza vaccines provided protection against influenza-associated illness across age groups during the 2023/2024 influenza season.