# Influenza vaccine effectiveness against detected infection in the community, France, October 2024 to February 2025

François Blanquart, Vincent Vieillefond, Benoit Visseaux, Claire Nour Abou Chakra, Marta C Nunes, Alexandra Jacques, Stephanie Haim-Boukobza, Laurence Josset, Valentin Wehrle, Guillaume Deleglise, et al.

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## ► To cite this version:

François Blanquart, Vincent Vieillefond, Benoit Visseaux, Claire Nour Abou Chakra, Marta C Nunes, et al.. Influenza vaccine effectiveness against detected infection in the community, France, October 2024 to February 2025. Eurosurveillance, 2025, 30 (7), pp.2500074. 10.2807/1560-7917.es.2025.30.7.2500074. hal-04991514

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Distributed under a Creative Commons Attribution 4.0 International License
Influenza vaccine effectiveness against detected
infection in the community, France, October 2024 to
February 2025

François Blanquart¹, Vincent Vieillefond², Benoit Visseaux³, Claire Nour Abou Chakra⁴, Marta C Nunes⁴,⁵, Alexandra Jacques⁶, Stephanie Haim-Boukobza⁷, Laurence Josset⁸,⁹, Valentin Wehrle¹⁰, Guillaume Deleglise¹¹, Thomas Duret¹¹, Marie Anne Rameix-Welti¹²,¹³, Bruno Lina⁸,⁹, Vincent Enouf¹³, on behalf of the RELAB study group¹⁴, Antonin Bal⁸,⁹

1. Center for Interdisciplinary Research in Biology (CIRB), Collège de France, CNRS, INSERM, PSL Research University, Paris, France

2. BPO-BIOEPINE-Biogroup, Levallois-Perret, France

3. Département d'infectiologie, Laboratoire Cerba, Cerba Healthcare, Frépillon, France

4. Center of Excellence in Respiratory Pathogens (CERP), Hospices Civils de Lyon (HCL), Lyon, France; Centre International de Recherche en Infectiologie (CIRI), Équipe Santé Publique, Epidémiologie et Écologie Évolutive des Maladies Infectieuses (PHE3ID), Inserm U1111, CNRS UMR5308, ENS de Lyon, Université Claude Bernard Lyon 1, Lyon, France

5. South African Medical Research Council, Vaccines & Infectious Diseases Analytics Research Unit, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa

6. Biogroup Lorraine, Grand Est, France

7. Laboratoires Cerballiance, Cerba Healthcare, Issy-les-Moulineaux, France

8. Hospices Civils de Lyon (HCL), Centre National de Référence des virus des infections respiratoires, Institut des Agents Infectieux, Laboratoire de Virologie, Lyon, France

9. Centre International de Recherche en Infectiologie (CIRI), Laboratoire VirPath, Inserm U1111, CNRS UMR5308, ENS de Lyon, Université Claude Bernard Lyon 1, Lyon, France

10. BioLBS, 3 Place Félix Faure, 76170 Lillebonne, France

11. INOVIE GEN-BIO, 8 rue Jacqueline Auriol, 63100 Clermont-Ferrand, France

12. Molecular Mechanisms of Multiplication of Pneumovirus, Université Versailles St-Quentin en Yvelines, Paris-Saclay INSERM UMR 1173 (21), Assistance Publique des Hôpitaux de Paris, Paris, France

13. National Reference Center for Respiratory Viruses, Molecular Mechanisms of Multiplication of Pneumovirus, Institut Pasteur, Université Paris Cité, Paris, France

14. Members of the RELAB study group are acknowledged at the end of the article

Correspondence: Antonin Bal (antonin.bal@chu-lyon.fr)

Citation style for this article:

Blanquart François, Vieillefond Vincent, Visseaux Benoit, Abou Chakra Claire Nour, Nunes Marta C, Jacques Alexandra, Haim-Boukobza Stephanie, Josset Laurence, Wehrle Valentin, Deleglise Guillaume, Duret Thomas, Rameix-Welti Marie Anne, Lina Bruno, Enouf Vincent, on behalf of the RELAB study group, Bal Antonin. Influenza vaccine effectiveness against detected infection in the community, France, October 2024 to February 2025. Euro Surveill. 2025;30(7):pii=2500074.
https://doi.org/10.2807/1560-7917.ES.2025.30.7.2500074

Article received on 24 Jan 2025 / Accepted on 20 Feb 2025 / Published on 20 Feb 2025

Influenza circulates at high levels in Europe since November 2024. Using a test-negative study based on data from French community laboratories between October 2024 and February 2025, we estimated vaccine effectiveness (VE) against PCR-detected influenza infection (44,420/15,052; positive/negative individuals). For all age groups, the overall VE was 42% (95% CI: 37–46%), with 26% (95% CI: 18–34%) against influenza A and 75% (95% CI: 66–82%) against influenza B. Among individuals ≥65-year-olds VE was 22% (95% CI: 13–30%) and among 0–64-year-olds, 60% (95% CI: 56–65%).

The incidence of influenza has steeply risen in France and in Europe since November 2024, with, at the end of 2024, a high PCR-test positivity rate for influenza virus and elevated numbers of primary care consultations, as well as increased hospital and intensive care unit (ICU) admissions due to this illness [1-5]. In the United Kingdom (UK), influenza hospital and ICU admissions were in early January 2025 twice as frequent as in the peak of the 2023/24 season for this disease [4]. This

interim report provides an overview of vaccine effec-
tiveness (VE) estimates in France based on data from
community laboratories. The VE overall and by virus
type is presented, as well as among older adults, a
high-priority group for vaccination.

Laboratories, procedures, data collection,
and study population

The VE study was conducted using data from RELAB,
a network of community-based laboratories located
at over 1,600 sites nationwide [6]. All samples from
patients presenting in RELAB laboratories for respira-
tory virus testing, are systematically subjected to a
triplex reverse-transcription PCR (RT-PCR) for severe
acute respiratory syndrome coronavirus 2 (SARS-
CoV-2), influenza, and respiratory syncytial virus. The
data collected include sex (male/female), age, pres-
ence of symptoms (fever≥ 38°C, respiratory symptoms),
self-reported vaccination status, PCR technique and
RT-PCR testing result.

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FIGURE 1

(A) Influenza test positivity per week, for non-vaccinated individuals and individuals vaccinated this season (15 days–3 months prior), (B, C, D) influenza positivity stratified by age group, France, October 2024–February 2025 (n = 59,472)

<figure><img src="image_4.png" /></figure>

Shaded areas represent 95% confidence intervals. The inset shows the proportion of type A influenza over time. The age group of 0–4 years is not shown because very few individuals from that age class were vaccinated.

Participating laboratories weekly transmit virological (RT-PCR results along with quantification cycle (Cq) values, influenza virus typing data when available) and clinical information on all tested patients to the national reference centre (NRC) for respiratory viruses. In this investigation, additional viral genomic sequencing was performed by the NRC on a random subset of study samples testing positive for influenza virus.

The study population included individuals presenting in a RELAB laboratory from week 44 of 2024 to week 5 of 2025 (28/10/2024 to 02/02/2025). Overall, 48,028 of 59,472 patients (81%) presented fever and/or respiratory symptoms.

## Influenza infection in the community

Based on analysis of the study samples, influenza circulation in the community in the 2024/25 season started in mid-November (week 46) in non-vaccinated patients (Figure 1). Positivity rate in non-vaccinated patients reached 49% (2,346/4,785) in week 4 of 2025 (vs 25% in vaccinated patients, 173/687). Patients testing negative (n=44,420), and positive (n=15,052) for influenza had a slight sex ratio difference, but fever

and respiratory symptoms were more common among those testing positive (Pearson's chi-squared test; p<0.001) (Table 1). The main age group was 18–64 years for both positive and negative patients. Influenza type A represented 72% of all typed viruses (n=5,841/8,151) with no significant variation since week 50 (Figure 1). Sequencing performed on a random subset of 423 influenza A samples found that A(H1N1)pdm09 was the main A sub-type detected in this population (212/423; 50%), mainly represented by 5a.2a clade with C.1.9.3 being the most detected subclade. A(H3N2) viruses predominantly belonged to 2a.3a.1 - J.2.a subclade while B viruses were mostly represented by V1.1.3a.2-C.5.1 and C.5.7 subclades. Of note, the vaccine composition for 2024/25 northern hemisphere includes 5a.2a.1, 2a.3a.1 and V1.1.3a.2 strains for A(H1N1)pdm09, A(H3N2) and B viruses, respectively [7].

## Vaccine effectiveness by influenza type

As the vaccination campaign in France started in mid-October, participants were unlikely to have received vaccination 3 to 6 months prior to the study period when they were tested (see Discussion). Study participants were considered vaccinated if they self-reported

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<footnote>a Study participants were considered vaccinated if they self-reported receiving an influenza vaccine at least 15 days before getting tested, but no longer than 3 months prior.</footnote>
TABLE 1

<table><caption>Characteristics of the study population by negative or positive influenza test result, France, October 2024–February 2025 (n = 59,472)</caption><thead><tr><th rowspan="3">Characteristic</th><th colspan="4">Influenza test result</th><th rowspan="3">pvalue<sup>a</sup></th></tr><tr><th colspan="2">Negative, n=44,420</th><th colspan="2">Positive, n=15,052</th></tr><tr><th>Number</th><th>%</th><th>Number</th><th>%</th></tr></thead><tbody><tr><td>Sex</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Female</td><td>26,754</td><td>60</td><td>8,702</td><td>58</td><td rowspan="2">p&lt;0.001</td></tr><tr><td>Male</td><td>17,666</td><td>40</td><td>6,350</td><td>42</td></tr><tr><td>Age group in years</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0-4</td><td>2,496</td><td>5.6</td><td>1,729</td><td>11</td><td rowspan="4">p&lt;0.001</td></tr><tr><td>5-17</td><td>3,597</td><td>7.9</td><td>2,522</td><td>17</td></tr><tr><td>18-64</td><td>25,126</td><td>57</td><td>8,727</td><td>58</td></tr><tr><td>≥65</td><td>13,291</td><td>30</td><td>2,074</td><td>14</td></tr><tr><td>Fever (body temperature ≥38°C)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>19,281</td><td>43</td><td>2,247</td><td>15</td><td rowspan="3">p&lt;0.001</td></tr><tr><td>Yes</td><td>19,546</td><td>44</td><td>11,407</td><td>76</td></tr><tr><td>Missing</td><td>5,593</td><td>13</td><td>1,398</td><td>9.3</td></tr><tr><td>Respiratory symptom</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>12,057</td><td>27</td><td>2,287</td><td>15</td><td rowspan="3">p&lt;0.001</td></tr><tr><td>Yes</td><td>29,073</td><td>65</td><td>11,584</td><td>77</td></tr><tr><td>Missing</td><td>3,290</td><td>7.4</td><td>1,181</td><td>7.8</td></tr><tr><td>Any symptom</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Any symptom</td><td>34,114</td><td>77</td><td>13,914</td><td>92</td><td>p&lt;0.001</td></tr><tr><td>Vaccination<sup>b</sup></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No vaccine</td><td>33,545</td><td>76</td><td>13,136</td><td>87</td><td rowspan="6">p&lt;0.001</td></tr><tr><td>Vaccine &lt;15 days</td><td>1,077</td><td>2.4</td><td>94</td><td>0.6</td></tr><tr><td>Vaccine 15 days-3 months<sup>c</sup></td><td>5,626</td><td>13</td><td>1,129</td><td>7.5</td></tr><tr><td>Vaccine 3-6 months<sup>c</sup></td><td>1,446</td><td>3.3</td><td>363</td><td>2.4</td></tr><tr><td>Vaccine more than 6 months<sup>c</sup></td><td>2,726</td><td>6.1</td><td>330</td><td>2.2</td></tr><tr><td>Year-week of testing</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2024-44</td><td>3,111</td><td>7</td><td>52</td><td>0.3</td><td rowspan="16">p&lt;0.001</td></tr><tr><td>2024-45</td><td>3,582</td><td>8.1</td><td>55</td><td>0.4</td></tr><tr><td>2024-46</td><td>2,669</td><td>6</td><td>79</td><td>0.5</td></tr><tr><td>2024-47</td><td>3,030</td><td>6.8</td><td>149</td><td>1</td></tr><tr><td>2024-48</td><td>3,003</td><td>6.8</td><td>285</td><td>1.9</td></tr><tr><td>2024-49</td><td>3,174</td><td>7.1</td><td>461</td><td>3.1</td></tr><tr><td>2024-50</td><td>3,609</td><td>8.1</td><td>963</td><td>6.4</td></tr><tr><td>2024-51</td><td>3,974</td><td>8.9</td><td>1,725</td><td>11</td></tr><tr><td>2024-52</td><td>1,414</td><td>3.2</td><td>836</td><td>5.6</td></tr><tr><td>2025-1</td><td>2,510</td><td>5.7</td><td>1,437</td><td>9.5</td></tr><tr><td>2025-2</td><td>4,273</td><td>9.6</td><td>2,134</td><td>14</td></tr><tr><td>2025-3</td><td>3,522</td><td>7.9</td><td>1,847</td><td>12</td></tr><tr><td>2025-4</td><td>3,285</td><td>7.4</td><td>2,637</td><td>18</td></tr><tr><td>2025-5</td><td>3,264</td><td>7.3</td><td>2,392</td><td>16</td></tr><tr><td>Influenza type</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>A</td><td>NA</td><td>NA</td><td>5,806</td><td>39</td><td>NA</td></tr><tr><td>A+B</td><td>NA</td><td>NA</td><td>35</td><td>0.2</td><td>NA</td></tr><tr><td>B</td><td>NA</td><td>NA</td><td>2,310</td><td>15</td><td>NA</td></tr><tr><td>Subtype-clade</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>A(H3N2)-3C.2a1b.2a.2a.3a.1</td><td>NA</td><td>NA</td><td>84</td><td>0.6</td><td>NA</td></tr><tr><td>A(H1N1)pdm09-6B.1A.5a.2a.1</td><td>NA</td><td>NA</td><td>17</td><td>0.1</td><td>NA</td></tr><tr><td>A(H1N1)pdm09-6B.1A.5a.2a</td><td>NA</td><td>NA</td><td>195</td><td>1.3</td><td>NA</td></tr><tr><td>B-V1A.3a.2</td><td>NA</td><td>NA</td><td>127</td><td>0.8</td><td>NA</td></tr><tr><td>Missing</td><td>NA</td><td>NA</td><td>14,629</td><td>97</td><td>NA</td></tr></tbody></table>

NA: not applicable.

<figure><img src="image_6.png" /></figure>

<footnote>a Differences between negative and positive tests were tested with Pearson's chi-squared test and considered significant at the <0.001 level</footnote>

<footnote>b Vaccination is self-reported.</footnote>

<footnote>c The time intervals presented in the table are not mutually exclusive and reflect the way participants were asked to self-report since when they were last vaccinated against influenza.</footnote>
<caption>TABLE 2</caption>

Vaccination status by age group in a subset of the study population, France, October 2024–February 2025 (n = 54,607)ª

<table><thead><tr><th rowspan="3">Vaccine</th><th colspan="8">Age groups in years</th></tr><tr><th colspan="2">18-64, n=31,856</th><th colspan="2">0-4<br>n=4,191</th><th colspan="2">5-17<br>n=5,929</th><th colspan="2">≥ 65<br>n=12,631</th></tr><tr><th>Number</th><th>%</th><th>Number</th><th>%</th><th>Number</th><th>%</th><th>Number</th><th>%</th></tr></thead><tbody><tr><td>No vaccine</td><td>29,359</td><td>92</td><td>4,148</td><td>99</td><td>5,839</td><td>98</td><td>7,335</td><td>58</td></tr><tr><td>Vaccine &lt; 15 days</td><td>416</td><td>1.3</td><td>9</td><td>0.2</td><td>19</td><td>0.3</td><td>727</td><td>5.8</td></tr><tr><td>Vaccine 15 days-3 months</td><td>2,081</td><td>6.5</td><td>34</td><td>0.8</td><td>71</td><td>1.2</td><td>4,569</td><td>36</td></tr></tbody></table>

receiving an influenza vaccine at least 15 days before getting tested, but no longer than 3 months prior. Over the whole period, vaccine coverage among PCR-tested individuals was 6.5% in 18-64-year-olds and 36% in ≥ 65-year-olds (Table 2), and increased over the season in the overall population from 1.8% (44/2,481) in week 44 to 19% (396/2,117) in week 52 of 2024, and up to 52% (266/515) in the ≥ 65-year-olds. We used a test-negative design to infer VE against detected influenza infection. We fitted a logistic (binomial) linear model to test result (negative/positive), as a function of sex, age category, PCR technique, week, and vaccination status, age and week being the most important confounders. We tested several models with week as a continuous or categorical factor, and including or not an interaction between week and vaccine status (which can be significant if effectiveness changes over time). We retained the best model in terms of the Akaike Information Criterion. The retained model included week as a categorical factor and no interaction, and we used this model in subsequent analyses. The VE was measured as the odds ratio (OR) of the vaccine effect on positivity (the exponential of the inferred coefficient corresponding to vaccine status in the linear model). When test positivity is small enough, ORs approximate the risk ratio [8].

The resulting VE estimates for individuals vaccinated 15 days-3 months before testing was 42% (95%CI: 37 to 46%) for influenza overall, 26% (95%CI: 18 to 34%) for influenza A, and 75% (95%CI: 66 to 82%) for influenza B infections (Figure 2, Table 3).

When focusing on results from a single type of PCR test (the most frequently used in RELAB laboratories, Eurobioplex FluCoSyn ebx-042, Eurobio Scientific, Les Ulys, France), the overall VE was 42% (95%CI: 36 to 49%) suggesting that overall results were robust. Regardless of the assay used, VE was estimated at 60% (95%CI: 56 to 64%) among the 0-64-year-olds and 22% (95%CI: 13 to 30%) among those aged ≥ 65 years. As type B influenza circulates preferentially in younger age groups, as shown in Supplementary Figure 1, we calculated effectiveness by age and type and found 33% (95%CI: 22 to 43%) vs 20% (95%CI: 6.9 to 32%) against influenza A in 0-64 and ≥ 65 years old, and 82%

(95%CI: 74 to 88%) vs 64% (95%CI: 27 to 82%) against influenza B in 0-64 and ≥ 65-year-olds. The VE did not change over time: it was comparable at the end of the period (weeks 1-5) at 40% (95%CI: 35 to 46%) over all influenza viruses, and 22% (95%CI: 11 to 31%) for influenza A. We examined whether effectiveness was particularly low during the end-of-year/school holiday festive period, as vaccination could have been associated with more contacts during that period (if individuals vaccinate when they plan to have more contacts). The VE was slightly lower among individuals ≥ 65 years in the weeks of school holiday (weeks 52-1), with largely overlapping CIs (VE: 15%; 95%CI: -6.7 to 33%). Finally, results were also robust when focusing on symptomatic individuals, with VE at 41% (95%CI: 36 to 46%) for the subpopulation presenting with respiratory symptoms and 41% (95%CI: 35 to 47%) for the subpopulation presenting with fever.

## Discussion

In France, the influenza vaccination campaign was launched in week 42 (mid-October) in targeted populations. The vaccines provided were quadrivalent inactivated vaccines, with standard dose, and no adjuvant. No enhanced vaccines were available. Early data presented herein suggest low to high vaccine effectiveness (VE) against influenza types A and B in the community, with estimates of 26% and 75% respectively.

These interim VE estimates by influenza type are consistent with trends observed during the 2022/23 season in Europe and during the 2023/24 season in the United States where VE against influenza B was higher than that against A viruses [9,10]. In both Europe and France, estimated VE against type A for 2023/24 was 51% in primary care, which is higher than in the current season [11,12]. In 2024/25, despite the mismatch between the clade of the predominant circulating A(H1N1)pdm09 strains (5a.2a) and the one in the vaccine strain (5a.2a.1), A(H1N1)pdm09 circulating strains were antigenically similar to the A/Victoria/4897/2022 virus included in the 2024/25 northern hemisphere vaccine, while some A(H3N2) isolates were antigenically distinct from the vaccine strain [3]. Studies focusing on VE by subtype are needed to assess the role of A(H3N2) viruses, which are known to undergo

<figure><img src="image_7.png" /></figure>

<footnote>ª People vaccinated over 3 months prior to testing (n=4,865) are not considered in the table.</footnote>
FIGURE 2

Vaccine effectiveness against influenza overall, and stratified by virus type and/or characteristics of individuals in the study, as well as overall vaccine effectiveness in a subset of participants tested with a single kind of PCR assayª, France, October 2024–February 2025 (n = 59,472)

<figure><img src="image_8.png" /></figure>

ª The PCR assay in question was Eurobioplex FluCoSyn ebx-042, Eurobio Scientific, Les Ulys, France.

Points are the maximum likelihood estimates, segments show the 95% confidence intervals. All numbers are presented in Table 3. From left to right in the graph, vaccine effectiveness is presented overall, against influenza A, influenza B, in the subset of tests done with the Eurobio PCR technique, in the o-64-year-olds, in the ≥65-year-olds, against influenza A in the o-64-year-olds, against influenza B in the o-64-year-olds, against influenza A in the ≥65-year-olds, against influenza B in the ≥65-year-olds, at weeks 1–5, against influenza A at weeks 1–5, in the subset of ≥65-year-olds during the school holidays (weeks 52–1), in the subset of patients presenting with respiratory symptoms, the subset of patients presenting with fever, the subset of ≥65-year-olds presenting with respiratory symptoms, and the subset of ≥65-year-olds presenting with fever.

substantial genetic drift and to present antigenic variation [13], on the reduced VE observed herein. A recent study conducted in medically-attended outpatients in the United States during the 2023/24 season showed a similar VE of about 30% for both A(H1N1)pdm09 and A(H3N2) subtypes [9] while a Canadian study conducted in 2024/25 season in outpatients found 53% and 54% VE for A(H1N1)pdm09 and A(H3N2), respectively [14].

In our current study in the 2024/25 season so far, the VE was inferred to be lower in ≥65-year-olds (22%) than in o-64-year-olds (60%), highlighting the need to consider targeted immunisation strategies, such as the high-dose (available during 2023/24 season in France) or adjuvanted vaccines which were not available this season in France [15]. In the 2024/25 Canadian study, where high-dose and adjuvanted vaccines were used, estimated VE against influenza at 59% in ≥65-year-old patients and at 54% in 1–64-year-olds [14]. Besides

vaccine formulation, other factors, such as the intensity of the epidemic, the vaccine coverage, the circulation of a viral escape variant, or childhood immune imprinting [16], might also contribute to the differences in VE observed between older adults in the two countries. These might, however, also be caused by behavioural differences between vaccinated and unvaccinated individuals. On the other hand, the reduced VE in our study in older adults is consistent with previous reports that have documented lower immune responses in this population group due to immunosenescence and comorbidities [17].

The current analysis is subject to several limitations. The reason for PCR testing and how it might depend on vaccination status was unknown, which may introduce selection bias. Selection bias happens in observational studies focusing on the population seeking a PCR test, for example when both vaccination and infection encourage test-seeking. Conditioning on the collider (test-seeking behaviour) creates a negative correlation between vaccination and test positivity which would bias effectiveness upwards [18-20]. In our data, the vaccine coverage at week 52 was 52% in ≥65-year-old individuals, comparable to the nationwide figure of 50%, suggesting that vaccination did not influence test-seeking [21]. Another concern is confounding by health status (co-morbidities) of vaccinated individuals, which can act in either direction depending on whether healthy individuals ('healthy vaccine bias') or conversely individuals with co-morbidities ('confounding by indication') are more likely to get vaccinated [22,23]. Such confounding may play a limited role in our study, as we focus on mild symptoms. Self-reported vaccination status is subject to recall bias, but this should not have impacted the present analysis. Notably, around 3% (1,809/59,472) of individuals reported receiving the influenza vaccine 3 to 6 months prior to testing for respiratory infection, 814 of them before mid-January, which was not possible given the timing of the vaccination campaign in France. This small subset, which we excluded from the main analysis, had an estimated VE of 44% (95% CI: 37 to 51%) comparable to that in participants vaccinated in the 15 days–3 months prior to testing. Furthermore, 54% (8,151/15,052) of samples were subtyped and only 3% (423/15,052) were sequenced, restricting our ability to analyse VE for specific subtypes or clades. End-of-season estimates incorporating sequencing data will provide a more comprehensive assessment of VE and identify potential escape variants. The study focused on VE against infection rather than severe disease outcomes, which limits the generalisability to hospitalisation and mortality rates. Given the high mortality observed this season in France, additional hospital-based studies would provide complementary insights and are particularly needed given the inferred low effectiveness in older adults [3]. The low VE observed herein for type A notably in people aged ≥65 years could contribute to the high rate of hospitalisations observed in France this season in addition to the low vaccine coverage [3].

<figure><img src="image_9.png" /></figure>
TABLE 3

Vaccine effectiveness against influenza overall, and stratified by virus type and/or characteristics of individuals in the study, as well as overall vaccine effectiveness in a subset of participants tested with a single kind of PCR assayª, France, October 2024–February 2025 (n = 59,472)

<table><thead><tr><th rowspan="2">Analysis</th><th colspan="2">Cases</th><th colspan="2">Controls</th><th rowspan="2">VE in % (95% CI)</th></tr><tr><th>All</th><th>Vaccinated</th><th>All</th><th>Vaccinated</th></tr></thead><tbody><tr><td>Overall</td><td>15,052</td><td>1,129</td><td>44,420</td><td>5,626</td><td>42 (37 to 46)</td></tr><tr><td>Influenza A</td><td>5,806</td><td>555</td><td>46,730</td><td>5,666</td><td>26 (18 to 34)</td></tr><tr><td>Influenza B</td><td>2,310</td><td>40</td><td>50,226</td><td>6,181</td><td>75 (66 to 82)</td></tr><tr><td>Eurobio PCR</td><td>6,775</td><td>503</td><td>23,149</td><td>2,912</td><td>42 (36 to 49)</td></tr><tr><td>0–64-year-olds</td><td>12,978</td><td>418</td><td>31,129</td><td>1,768</td><td>60 (56 to 64)</td></tr><tr><td>≥ 65-year-olds</td><td>2,074</td><td>711</td><td>13,291</td><td>3,858</td><td>22 (13 to 30)</td></tr><tr><td>Influenza A 0–64-year-olds</td><td>4,801</td><td>208</td><td>33,391</td><td>1,797</td><td>33 (22 to 43)</td></tr><tr><td>Influenza B 0–64-year-olds</td><td>2,262</td><td>29</td><td>35,930</td><td>1,976</td><td>82 (74 to 88)</td></tr><tr><td>Influenza A ≥ 65-year-olds</td><td>1,005</td><td>347</td><td>13,339</td><td>3,869</td><td>20 (6.9 to 32)</td></tr><tr><td>Influenza B ≥ 65-year-olds</td><td>48</td><td>11</td><td>14,296</td><td>4,205</td><td>64 (27 to 82)</td></tr><tr><td>Weeks 1–5</td><td>10,447</td><td>842</td><td>16,854</td><td>2,937</td><td>40 (35 to 46)</td></tr><tr><td>Influenza A weeks 1–5</td><td>4,020</td><td>429</td><td>18,527</td><td>2,969</td><td>22 (11 to 31)</td></tr><tr><td>≥ 65-year-olds school holiday</td><td>466</td><td>192</td><td>1,321</td><td>589</td><td>15 (–6.7 to 33)</td></tr><tr><td>Influenza with respiratory symptoms</td><td>11,584</td><td>944</td><td>29,073</td><td>3,822</td><td>41 (36 to 46)</td></tr><tr><td>Influenza with fever</td><td>11,407</td><td>745</td><td>19,546</td><td>2,006</td><td>41 (35 to 47)</td></tr><tr><td>Influenza with respiratory symptoms ≥ 65-year-olds</td><td>1,657</td><td>600</td><td>8,546</td><td>2,620</td><td>23 (13 to 32)</td></tr><tr><td>Influenza with fever ≥ 65-year-olds</td><td>1,260</td><td>445</td><td>4,278</td><td>1,267</td><td>22 (9.1 to 33)</td></tr></tbody></table>

## Conclusion

Preliminary findings for the 2024/25 influenza season highlight the continued value of vaccination in reducing influenza burden, with effectiveness varying by virus type and age group. Strengthened public health efforts to increase vaccine uptake and potentially to improve vaccine formulations remain critical to reducing the impact of seasonal influenza.

## Ethical statement

The study protocol received approval from the institutional review board of the Hospices Civils de Lyon (Comité Scientifique et Éthique des Hospices Civils de Lyon) in June 2023 (reference 23-5039). All patients or the parents of minors were provided with an information notice explaining the nature and purpose of the collected data. In cases where a patient or parent objected to data usage, the corresponding data were excluded from the research database. To ensure confidentiality, unique anonymised alphanumeric identifiers were assigned, and data transfers to the research team were conducted via a secure portal, accessible exclusively to authorised research personnel.

## Funding statement

None.

## Use of artificial intelligence tools

None declared.

## Data availability

All RELAB sequences are available on GISAID.

## Acknowledgements

We would like to acknowledge all members of RELAB laboratories.

## Conflict of interest

None declared.

## Authors' contributions

FB performed statistical analyses and drafted the manuscript.

AB conceptualised the study, supervised data collection, provided feedback on analyses and drafted the manuscript.

VV, BV, SHB, AJ, VW, GD, TD, VE, LJ, BL, MARW supervised data collection within community laboratories, participated to the study conceptualisation and implementation.

MCN and CNABC contributed to methodological expertise and analyses.

All authors contributed to the interpretation of the results, commenting and critical revision of the manuscript, and approved the final version for submission.

## Members of the RELAB Study Group

Arnaud François (Bioesterel, Biogroup PACA)–Alexandre Vignola (Oriade Noviale, Biogroup AURA)–Vincent Garcia (Alphabio, Biogroup PACA)–Alexandra Jacques (Biogroup

<figure><img src="image_10.png" /></figure>

<footnote>ª The PCR assay in question was Eurobioplex FluCoSyn ebx-042, Eurobio Scientific, Les Ulys, France.</footnote>
Lorraine, Grand Est)-Jonas Amzalag (Biolam, Biogroup IDF)-Nadège Gourgouillon (CAB Biogroup, Grand Est)-Remi Labetouille (Laborizon, Biogroup Nouvelle Aquitaine)-Frederique Roumanet (Unilians, Biogroup AURA)-Arthur Denoel (Laborizon, Biogroup Bretagne)-Hilel Mehamha (CBM25, Biogroup Bourgogne Franche-Comté)-Thierry Guffond (Diagnovic, Biogroup Hauts de France)-Magali HYPOLITE (2a2B, Biogroup Corse)-Yanis Chaib (Biolam, Biogroup IDF)-Timothée Goetschy (Biogroup national, équipe Data)-Elodie Ostermann (Biogroup national, équipe Data)-Anne Holstein (Laborizon Biogroup Centre Val de Loire)-Vincent Vieillefond (BPO-Bioépine Biogroup IDF)-Jean Marc Giannoli (Biogroup national)-Julienne de Pontcharra (Bioesterel, Biogroup PACA)-Jean Francois Comes (Laborizon, Biogroup Bretagne)-Justine Gasnot (Bioesterel, Biogroup PACA)-Theo Corbet (Unilians, Biogroup AURA)-Laurent Kbaier (Bioesterel, Biogroup PACA)-Emmanuel Chanard (Cerballiance, Auvergne Rhône Alpes)-Arcadie Gioud (Cerba Xpert, Auvergne Rhône Alpes)-Stéphanie Arsene (Cerballiance, Normandie Bocage)-Maxime Sansot (Cerballiance, Pays de Loire)-Anne-Lise Gautier (Cerballiance, Portes de Bretagne)-Kariach Goldar (Cerballiance, Martinique)-Mahery Ramiandrisoa (Cerballiance, Réunion)-Aristide Nzeumi (Cerballiance, Réunion)-Pauline Jestin (Cerballiance, Charentes)-Gilles Abs (Cerballiance, Centre Val-de-Loire)-Guillemette Wandler (Cerballiance, Centre Valde-Loire)-Anne-Claire Strzelecki (Cerballiance, Occitanie)-Sarah Cerdan (Cerballiance, Occitanie)-Edouard Delaunay (Cerballiance, Alpes Durance)-Sandrine Barrieu-Moussat (Cerballiance, Côte d'azur)-Laurence Prots (Cerballiance, Côte d'azur)-Edouard Delaunay (Cerballiance, Provence)-Johanna Roux (Cerballiance, Ile-de-France Est)-Yasmina De Saint Salvy (Cerballiance, Ile-de-France Est)-Agnes Durand (Cerballiance, Ile-de-France Est)-Aude Lesenne (Cerballiance, Ile-de-France Sud)-Kader Merah (Cerballiance, Ile-de-France Sud)-Erwan Le Naour (Cerballiance, Aquitaine Nord)-David Robert (Cerballiance, Aquitaine Nord)-Sophie Zaffreya (Cerballiance, Aquitaine Nord)-Jean-Philippe Galhaud (Cerballiance, Aquitaine Sud)-Claire Felloni (Cerballiance, Artois)-Dominique Dyda (Cerballiance, Haut-de-France)-Aurelie Dupuis (Cerballiance, Bretagne)-Gwenole Prigent (Cerballiance, Bretagne)-Stephanie Arsene (Cerballiance, Normandie Ouest)-Antoine Prigent (Cerballiance, Normandie Ouest)-Natacha Tatoyan (Cerballiance, Nouvelle Calédonie)-Benoit Visseaux (Laboratoire Cerba, Pole Infectiologie)-Bénédicte Roquebert (Laboratoire Cerba, Pole Infectiologie)-Stéphanie Haim-Boukobza (Cerba Healthcare)-Odile Rousselet (Cerba Healthcare) and Michel Sala (Cerba Healthcare). Antoine Oblette (Hospices Civils de Lyon, Centre National de Référence des virus des infections respiratoires). Nathalie Bergaud (Hospices Civils de Lyon, Centre National de Référence des virus des infections respiratoires).

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