The influenza vaccine for the 2024/2025 season provided moderate protection against influenza A and strong protection against influenza B, according to an assessment of interim influenza vaccine effectiveness just published on Eurosurveillance. 

795px-A_woman_receives_the_seasonal_influenza_vaccine_(flu_shot)._Credit-_NIAID_(31028518766)

Source: NIAID

A woman receives the seasonal influenza vaccine (flu shot).

Ahead of the World Health Organization (WHO) consultation meeting on the composition of influenza virus vaccines for use in the 2025/26 northern hemisphere influenza season, this interim assessment helps inform decision making for the coming season and public health response.  

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The article reports interim results from five single- and three multi-country studies that analysed data from both primary care and hospital settings. These studies were conducted in Denmark, the United Kingdom (UK), and through the European Union’s Vaccine Effectiveness, Burden, and Impact Studies (VEBIS) networks, which included data from Belgium, Croatia, France, Germany, Hungary, Ireland, Lithuania, Malta, the Netherlands, Portugal, Romania and Spain. All studies used a test-negative design.  

Key strains

In Europe, the dominant virus strain for the 2024/2025 influenza season was influenza A(H1N1)pdm09, with influenza A(H3N2) and influenza B/Victoria lineage also circulating. The studies estimated study-specific vaccine effectiveness against any influenza, influenza A overall, and influenza A(H1N1)pdm09, A(H3N2) and B.  

Vaccine offers strong protection against influenza B, but is less effective against influenza A strains predominant in 2024/25  

Overall, influenza vaccine effectiveness in primary care for all ages ranged from 40% to 53%. In this setting, it was least effective among adults 65 years of age and over, providing no protection in EU studies, and 38% effectiveness in the UK study. In hospital settings, the vaccine effectiveness for all ages overall ranged from 34 to 52%.

Primary care settings

In primary care settings, vaccine effectiveness for all ages against influenza A(H1N1)pdm09 ranged between 30% and 72%. Effectiveness was lower among children and adults aged 65 and over in the UK study (42% for both age groups). In hospital settings, the vaccine effectiveness ranged from 46 to 53% against influenza A(H1N1)pdm09 overall. With a vaccine effectiveness from 38 to 45% in adults older than 65 years and 52 to 61% in children and adolescents aged 2–17 years it protected over one in three and more than one in two people vaccinated from severe disease. 

Due to low circulation of influenza A(H3N2) in Europe to date, vaccine effectiveness results varied considerably by age groups, ranging between 29% and 47% among all ages in primary care settings and between 31% and 49% among all ages in hospital settings.

All studies reported a high vaccine effectiveness against influenza B, with estimates across all ages ranging between 58% and 74% in primary care and 73 and88% in hospital settings. Among specific age groups, almost all estimates were at 50% or above. 

Moderately effective

Additional studies in Eurosurveillance from France and Beijing, China, presenting influenza interim vaccine effectiveness results for the 2024/24 season, also reported the vaccine was moderately effective overall, offering strong protection against influenza B. Estimates in the community in France also indicated lower effectiveness (VE: 22%) among adults ≥ 65-years old. In the Chinese study, researchers suggested that the close match of the vaccine strain with the dominant virus strain circulating in Beijing contributed to its effectiveness. 

Additional measures needed

Influenza vaccination prevented from one-third to more than three-quarters of influenza infections in the primary care or hospital settings among the vaccinated, although protection varied by age group and study. Moreover, as the vaccine protected one-third to over one-half of participants against infection with influenza A, the main circulating subtype, Rose et al. recommend continuing to promote vaccination among target groups where the season is ongoing.

Given the lower vaccine effectiveness for specific virus subtypes and among older adults in certain studies, other prevention measures should also be strengthened. End-of-season studies with greater sample sizes and additional information on virus characteristics may clarify the variations in effectiveness seen in this study. 

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