Flu epidemic: vaccine efficacy is reportedly low in those aged 65 and over… how can this be explained ?

While the flu virus is still circulating at very intense levels, Public Health France published the first estimates on Wednesday, January 29, 2025, concerning the effectiveness of the vaccine. It reaches only 31% among those aged 65 and over, who are at the highest risk of severe forms of the disease. How can this low level of effectiveness be explained ?

While on January 14, Health Minister Yannick Neuder announced a peak in the flu epidemic within ten days, it intensified further during the week of January 20 to 26. In its weekly bulletin, published on Wednesday, January 29, Santé publique France notes a sharp intensification of the epidemic in France, an increase in indicators in all age groups – in the city or in hospital – with exceptionally high activity among children. In addition, in hospitals, indicators are starting to rise again among adults. Regarding deaths, influenza is mentioned in 7.1% of electronic certificates, at a stable but very high level, above the peaks of previous years.

46% effectiveness among target audiences

While vaccination remains the best way to protect against the virus, particularly severe forms, vaccination coverage was estimated, as of December 31, at 49.8% among people aged 65 and over, and 22.7% among those aged under 65 at risk of severe influenza. These are levels lower than the 2023-2024 season.

What about the effectiveness of the vaccine used this winter season ? According to the first estimates produced by the Sentinelles network based on data from community medicine, the effectiveness of the vaccine is moderate, estimated at 46% on average for the risk groups targeted by the vaccination campaign. It is even lower among those aged 65 and over with an estimate of only 31%. However, this age group is particularly vulnerable to the virus. According to figures from Public Health France, those aged 65 and over accounted for 58% of hospitalizations after emergency visits for flu/flu-like illness during week 04 of 2025.

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To compensate for the low effectiveness of the vaccine, health authorities are stressing the importance of barrier gestures, which are essential “to help limit the circulation of respiratory viruses, including among those most at risk of severe forms: hand washing, ventilating rooms and wearing a mask in the event of symptoms (fever, sore throat or cough), in crowded places and in the presence of vulnerable people”.

Why is the vaccine so ineffective among those aged 65 and over ?

On April 23, 2024, in the midst of preparations for the 2024/2025 campaign against seasonal flu, we learned of the withdrawal from the market of the Efuelda® vaccine, marketed by Sanofi laboratories. In a press release, the Directorate General of Health wanted to be reassuring: “this decision will not affect the supply of flu vaccines, the laboratory having undertaken to completely replace pre-orders of the Efluelda® vaccine with the VaxigripTetra® vaccine”.

However, is the second worth the first? ? Efuelda® is a quadrivalent vaccine recommended in 2020 for those aged 60 and over and covered by Health Insurance from the age of 65. As we know, vaccination against seasonal flu reduces the risk of hospitalization for all causes and mortality by around 35%. “But its effectiveness varies according to the seasons and can decline in a few months, especially among the most vulnerable elderly people”, explains the National Academy of Medicine published on May 15. “To overcome the impairment of vaccine responses associated with immunosenescence (the age-related loss of effectiveness of the immune system), one method is to increase the doses of antigens administered in order to activate a greater number of antigen-presenting dendritic cells and thus lead to increased stimulation of T and B lymphocytes.” This is the method used to manufacture the Efluelda® vaccine.

A well-tolerated and more immunogenic vaccine

In concrete terms, this vaccine contains 60 µg of hemagglutinin from 4 viral strains, A/Victoria/4897/2022 (H1N1), A/Darwin/9/2021 (H3N2), B/Austria/1359417/2021 and B/Phuket/3073/2013 (Yamagata lineage) – only the first three are currently circulating. Compared to standard dose formulations, which contain 15 µg of hemagglutinin per viral strain, “the high-dose formulation is also well tolerated but more immunogenic in the elderly and the immunocompromised”. This vaccine offered better protection against the risks of severe form and hospitalization linked to influenza in these people.

Following the withdrawal of Efluelda® from the French market, the Academy of Medicine reacted as follows : “this regrettable announcement compromises the effectiveness of the next vaccination campaign in the elderly”.