Abstract
Background: Continuous monitoring of seasonal influenza vaccine effectiveness (SIVE) is needed due to the changing nature of influenza viruses and it supports the decision on the annual update of vaccine composition. Age-specific SIVE was evaluated against different influenza subtypes in the hospitalized population in Lithuania during four influenza seasons. Methods: A test-negative case-control study design was used. SIVE and its 95% confidence intervals (95% CI) were calculated as (1 – odds ratio (OR)) × 100%. Results: Adjusted SIVE in 18–64-year-old individuals against influenza A, A(H1N1)pdm09 and B/Yamagata were 78.0% (95% CI: 1.7; 95.1%), 88.6% (95% CI: −47.4; 99.1%), and 76.8% (95% CI: −109.9; 97.4%), respectively. Adjusted SIVE in individuals aged 65 years and older against influenza A, influenza B, and B/Yamagata were 22.6% (95% CI: −36.5; 56.1%), 75.3% (95% CI: 12.2; 93.1%) and 73.1% (95% CI: 3.2; 92.5%), respectively. Unadjusted SIVE against influenza A(H3N2) among 18–64-year-old patients was 44.8% (95% CI: −171.0; 88.8%) and among those aged 65 years and older was 5.0% (95% CI: −74.5; 48.3%). Conclusions: Point estimates suggest high SIVE against influenza A in 18–64-year-old participants, and against influenza B and B/Yamagata in those 65 years old and older.
Highlights
According to the World Health Organization (WHO), the number of patients with severe acute respiratory infection (SARI), mainly caused by influenza viruses, increases during autumn and winter seasons in the Northern Hemisphere [1]
Based on the absolute number of vaccinated individuals of 65 years and older reported by the Center for Communicable Diseases and AIDS and the corresponding population of this age group based on Statistics Lithuania, influenza vaccination uptake in older people over the last decade increased from 9% during the influenza season 2010–2011 to 19% in 2019–2020
The highest statistically significant adjusted seasonal influenza vaccine effectiveness (SIVE) was found against influenza B and B/Yamagata among persons aged 65 and above (75% and 73%, respectively); similar but not statistically significant SIVE was found in 18–64-year-old individuals
Summary
According to the World Health Organization (WHO), the number of patients with severe acute respiratory infection (SARI), mainly caused by influenza viruses, increases during autumn and winter seasons in the Northern Hemisphere [1]. The National Program of Immunoprophylaxis in Lithuania recommends that all healthcare workers, pregnant women, institutionalized people, persons with underlying medical conditions, and aged over 65 years receive annual influenza vaccination [5]. Based on the absolute number of vaccinated individuals of 65 years and older reported by the Center for Communicable Diseases and AIDS and the corresponding population of this age group based on Statistics Lithuania, influenza vaccination uptake in older people over the last decade increased from 9% during the influenza season 2010–2011 to 19% in 2019–2020. Conclusions: Point estimates suggest high SIVE against influenza A in 18–64-year-old participants, and against influenza B and B/Yamagata in those 65 years old and older
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