Influenza vaccine effectiveness among adults aged ≥60 years in northeastern Zhejiang Province, China, 2021-2024

Influenza poses a particularly severe threat to older adults, yet vaccination coverage among this vulnerable population remains suboptimal in China. To address this public health challenge, Zhejiang Province initiated a free influenza vaccination program for older residents starting in 2020. This study evaluated the effectiveness of influenza vaccination in reducing outpatient visits among adults aged ≥60 years during three consecutive influenza seasons (2021-2024).

We employed a test-negative design (TND) among adults aged ≥60 years presenting with influenza-like illness (ILI) at sentinel surveillance hospitals in two cities in Zhejiang Province from October 2021 to April 2024. Standardized questionnaires were administered to collect demographic and clinical data. Respiratory specimens were tested for influenza virus types and subtypes using RT-PCR. Multivariable logistic regression models were employed to assess factors associated with vaccination status and influenza virus detection, with subsequent estimation of influenza vaccine effectiveness (VE).

A total of 3,796 ILI cases were enrolled, with 644 testing positive for influenza, yielding a positivity rate of 16.97%. The results of multivariable logistic regression analysis showed that age, whether vaccinated in the current year, and whether vaccinated in the previous year were the influencing factors for influenza-positive ILI cases (p < 0.05). The influenza vaccination coverage in the current season was 33.14%. The overall VE was 47.21% (95% CI: 35.38 to 56.88%). Subtype-specific VE was 55.81% (95% CI: 34.83 to 70.03%) for H1N1, 40.72% (95% CI: 23.30 to 54.18%) for H3N2, and 55.16% (95% CI: 21.77 to 74.30%) for B/Victoria. Age-stratified VE analysis showed effectiveness of 70.34% (95% CI: 41.47 to 84.98%) among those aged 60-69 years, 49.48% (95% CI: 34.41 to 61.09%) in the 70-79 age group, and 38.34% (95% CI: 10.35 to 57.60%) among individuals aged 80 years and older.

Influenza vaccination provides moderate protection for adults aged ≥60 years, with effectiveness varying by subtype, age, and season, particularly limited in the older population aged ≥80 years.