Influenza Vaccine Shows Moderate Efficacy Against A Strains in Europe

The influenza vaccine for the 2024/2025 season has demonstrated moderate effectiveness against influenza A virus strains while providing robust protection against influenza B, according to a recent evaluation published in Eurosurveillance.

This interim assessment is particularly significant as it precedes the World Health Organization's (WHO) upcoming meeting to discuss the composition of influenza vaccines for the 2025/26 northern hemisphere season. The findings are based on data derived from five single-country studies and three multi-country studies conducted across various European nations including Denmark, the United Kingdom, and other EU member states. These were gathered through the European Union's Vaccine Effectiveness, Burden, and Impact Studies (VEBIS) networks.

The 2024/2025 season in Europe has been primarily characterized by the circulation of the influenza A(H1N1)pdm09 strain, alongside A(H3N2) and B/Victoria lineage strains. The studies assessed vaccine effectiveness against a range of influenza strains and reported varying levels of protection.

Overall, the effectiveness of the influenza vaccine in primary care settings across all age groups ranged from 40% to 53%. However, the vaccine was less effective among older adults, particularly those aged 65 and over, showing no effectiveness in some EU studies and approximately 38% effectiveness in the UK study. In hospital environments, the overall vaccine effectiveness ranged from 34% to 52%.

In primary care settings, the effectiveness against influenza A(H1N1)pdm09 varied significantly, with estimates ranging from 30% to 72%. The results were particularly lower for children and seniors, with the UK study indicating 42% effectiveness for both demographics. In hospital settings, the effectiveness against this strain ranged from 46% to 53%. For older adults, effectiveness was recorded between 38% and 45%, while it was higher for children and adolescents, ranging from 52% to 61%. This suggests that the vaccine successfully protected a substantial portion of these groups from severe illness.

Despite the lower prevalence of influenza A(H3N2) in Europe thus far, effectiveness estimates varied considerably, ranging from 29% to 47% in primary care and 31% to 49% in hospital settings.

On the other hand, all studies reported a high level of effectiveness against influenza B, with estimates for all ages ranging from 58% to 74% in primary care and 73% to 88% in hospital settings. Most age-specific estimates also indicated effectiveness rates of 50% or higher.

Additional studies published in Eurosurveillance from France and Beijing, China, corroborated these findings, indicating moderate overall effectiveness of the vaccine with strong protection against influenza B. In France, estimates indicated lower effectiveness of 22% among those aged 65 and older.

The variation in vaccine effectiveness across different age groups and virus subtypes underscores the need for continued promotion of vaccination within target populations. As the influenza season progresses, health authorities are advised to strengthen supplementary preventive measures.

Further investigations with larger sample sizes and additional information regarding the characteristics of circulating strains are expected to enhance understanding of the observed variations in vaccine effectiveness.