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Multicenter Study
. 2025 Jun;30(23):2500011.
doi: 10.2807/1560-7917.ES.2025.30.23.2500011.

Influenza vaccine effectiveness in Europe and the birth cohort effect against influenza A(H1N1)pdm09: VEBIS primary care multicentre study, 2023/24

Esther Kissling  1   2   3 , Marine Maurel  1   3 , Francisco Pozo  4   5 , Gloria Pérez-Gimeno  4   6 , Silke Buda  7 , Noémie Sève  8 , Lisa Domegan  9 , Mariëtte Hooiveld  10 , Beatrix Oroszi  11 , Iván Martínez-Baz  4   12 , Raquel Guiomar  13 , Neus Latorre-Margalef  14 , Ivan Mlinarić  15 , Mihaela Lazar  16 , Jaume Giménez Duran  17 , Ralf Dürrwald  7 , Vincent Enouf  18 , Adele McKenna  9 , Marit de Lange  19 , Gergő Túri  11 , Camino Trobajo-Sanmartín  4   12 , Verónica Gomez  20 , Tove Samuelsson Hagey  14 , Vesna Višekruna Vučina  15 , Maria Carmen Cherciu  16 , Miriam García Vazquez  21 , Annika Erdwiens  7 , Shirley Masse  22 , Charlene Bennett  23 , Adam Meijer  19 , Katalin Kristóf  24 , Jesús Castilla  4   12 , Ana Paula Rodrigues  20 , Sanja Kurečić Filipović  15 , Alina Elena Ivanciuc  16 , Sabrina Bacci  25 , Marlena Kaczmarek  25 ; European primary care VE group  26
Affiliations
Multicenter Study

Influenza vaccine effectiveness in Europe and the birth cohort effect against influenza A(H1N1)pdm09: VEBIS primary care multicentre study, 2023/24

Esther Kissling et al. Euro Surveill. 2025 Jun.

Abstract

IntroductionInfluenza A(H1N1)pdm09, A(H3N2) and B/Victoria viruses circulated in Europe in 2023/24, with A(H1N1)pdm09 dominance. First influenza infections in childhood may lead to different vaccine effectiveness (VE) in subsequent years.AimThe VEBIS primary care network estimated influenza VE in Europe using a multicentre test-negative study.MethodsPrimary care practitioners collected information and specimens from patients consulting with acute respiratory infection. We estimated VE against influenza (sub)type and clade, by age group and by year of age for A(H1N1)pdm09, using logistic regression.ResultsWe included 29,958 patients, with 3,054, 1,053 and 311 influenza A(H1N1)pdm09, A(H3N2) and B cases, respectively. All-age VE against influenza A(H1N1)pdm09 was 52% (95% CI: 44-59). By year of age, VE was 27% (95% CI: -2 to 47) at 44 years with peaks at 72% (95% CI: 52-84) and 54% (95% CI: 41-64) among children and those 65 years and older, respectively. All-age A(H1N1)pdm09 VE against clade 5a.2a was 41% (95% CI: 24-54) and -11% (95% CI: -69 to 26) against clade 5a.2a.1. The A(H3N2) VE was 35% (95% CI: 20-48) among all ages and ranged between 34% and 40% by age group. All-age VE against clade 2a.3a.1 was 38% (95% CI: 1-62). All-age VE against B/Victoria was 83% (95% CI: 65-94), ranging between 70 and 92% by age group.DiscussionThe 2023/24 VEBIS primary care VE against medically attended symptomatic influenza infection was high against influenza B/Victoria, but lower against influenza A(H1N1)pdm09 and A(H3N2). Clade- and age-specific effects may have played a role in the lower A(H1N1)pdm09 VE.

Keywords: Influenza; case-control study; imprinting; influenza vaccines; multicentre study; vaccine effectiveness.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

The line graph depicts the numbers of cases and controls during the 2023/24 influenza season in Europe, with 3,054 influenza A(H1N1)pdm09 cases peaking in week 3 2024, 1,054 influenza A(H3N2) cases peaking in week 2 2024, and with low levels of influenza B, without an obvious peak. Numbers of controls were higher each week than cases, with smaller ratios of cases to controls in the beginning and the end of the study period.
Figure 1
Number of influenza cases and test-negative controls by week of symptom onset, VEBIS primary care study, September 2023–June 2024 (n = 29,959)
Four bar charts depicting the numbers vaccinated and unvaccinated by 5-year age bands, for controls, A(H1N1)pdm09 cases and specifically for 5a.2a and 5a.2a.1 cases. The proportion vaccinated appears greater in the 45–49-year-old group for 5a.2a.1 cases, compared with 40–44- and 50–54-year-old groups: 36% (8/22), 13% (3/23) and 9% (2/23), respectively. Among controls the proportion vaccinated in the 45–49-, 40–44- and 50–54-year-old groups was: 11% (181/1,689), 9% (156/1,733) and 13% (206/1,577).
Figure 2
Number of patients by 5-year age band and vaccination status, and proportion of vaccinated cases, for influenza test-negative controls, A(H1N1)pdm09, clade 5a.2a and clade 5a.2a.1 cases, VEBIS primary care study, September 2023–June 2024 (n = 29,959)
The graph depicts vaccine effectiveness and its 95% confidence intervals by year of age as a continuous variable, with vaccine effectiveness by birth cohort group super-imposed. The curve is roughly u-shaped with higher VE among children and younger adults, with a dip among middle-aged adults (39–47-year-olds), with higher VE among older adults.
Figure 3
Vaccine effectiveness against influenza A(H1N1)pdm09 by birth cohort / year of age, VEBIS primary care multicentre study, September 2023–June 2024 (n = 29,959)
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