Influenza Vaccine Effectiveness Against Influenza A(H1N1)pdm09-Associated Hospitalizations With SARI in Beijing,...

In the 2024-2025 season, influenza activity returned to pre-COVID-19 pandemic levels, with A(H1N1)pdm09 as the predominant subtype. Reassessment vaccine effectiveness (VE) in hospitalized patients is critical to inform potential adjustments to vaccination policy.

Using a test-negative design, we evaluated the VE of the seasonal influenza vaccine during the 2024-2025 season against A(H1N1)pdm09-associated hospitalizations with severe acute respiratory infections (SARI) in Beijing, China, from December 9, 2024 to February 9, 2025. VE was estimated by comparing the odds of influenza vaccination between case-patients (those testing positive for A(H1N1)pdm09) and controls (influenza test-negative patients), applying inverse-propensity-to-be-vaccinated weights.

The analysis included 1608 hospitalized SARI patients (17.4% tested positive for A(H1N1)pdm09; 13.8% were vaccinated overall). The adjusted VE against A(H1N1)pdm09-associated hospitalizations was 18.8% (95% CI: 0.9% to 33.6%) overall. Specifically, the VE was 45.4% (95% CI: 10.0% to 67.1%) in children aged 0.5-17 years, 43.8% (95% CI: -20.8% to 76.1%) in adults aged 18-59 years, and 8.6% (95% CI: -15.8% to 27.9%) in older adults ≥ 60 years. Among individuals with chronic conditions, the VE was 18.5% (95% CI: -2.3% to 35.2%) and was higher among those with chronic respiratory conditions (40.6%, 95% CI: -1.7% to 65.7%).

During the 2024-2025 season in Beijing, the overall VE against A(H1N1)pdm09-associated SARI hospitalizations was suboptimal. However, moderate VE was observed among children and younger adults, while limited effectiveness was seen in the elderly. Optimizing influenza vaccination strategies, including the introduction of high-dose or adjuvanted vaccines to enhance immune response in older adults, is crucial to alleviate influenza-associated hospitalizations.