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During 2023/24, all children aged 6 to 59 months were targeted for seasonal influenza vaccination in Spain nationally. Using a test-negative case-control design with sentinel surveillance data, we estimated adjusted influenza vaccine effectiveness (IVE) against any influenza type to be 70% (95% confidence interval (CI): 51 to 81%) for primary care patients with acute respiratory illness (ARI) and 77% (95% CI: 21 to 93%) for hospitalised patients with severe ARI. In primary care, where most subtyped viruses (61%; 145/237) were A(H1N1), adjusted IVE was 77% (95% CI: 56 to 88%) against A(H1N1)pdm09.

BackgroundScarce European data in early 2021 suggested lower vaccine effectiveness (VE) against SARS-CoV-2 Omicron lineages than previous variants.AimWe aimed to estimate primary series (PS) and first booster VE against symptomatic BA.1/BA.2 infection and investigate potential biases.MethodsThis European test-negative multicentre study tested primary care patients with acute respiratory symptoms for SARS-CoV-2 in the BA.1/BA.2-dominant period. We estimated PS and booster VE among adults and adolescents (PS only) for all products combined and for Comirnaty alone, by time since vaccination, age and chronic condition. We investigated potential bias due to correlation between COVID-19 and influenza vaccination and explored effect modification and confounding by prior SARS-CoV-2 infection.ResultsAmong adults, PS VE was 37% (95% CI: 24-47%) overall and 60% (95% CI: 44-72%), 43% (95% CI: 26-55%) and 29% (95% CI: 13-43%) < 90, 90-179 and ≥ 180 days post vaccination, respectively. Booster VE was 42% (95% CI: 32-51%) overall and 56% (95% CI: 47-64%), 22% (95% CI: 2-38%) and 3% (95% CI: -78% to 48%), respectively. Primary series VE was similar among adolescents. Restricting analyses to Comirnaty had little impact. Vaccine effectiveness was higher among older adults. There was no signal of bias due to correlation between COVID-19 and influenza vaccination. Confounding by previous infection was low, but sample size precluded definite assessment of effect modification.ConclusionPrimary series and booster VE against symptomatic infection with BA.1/BA.2 ranged from 37% to 42%, with similar waning post vaccination. Comprehensive data on previous SARS-CoV-2 infection would help disentangle vaccine- and infection-induced immunity.

Influenza A viruses circulated in Europe from September 2023 to January 2024, with influenza A(H1N1)pdm09 predominance. We provide interim 2023/24 influenza vaccine effectiveness (IVE) estimates from two European studies, covering 10 countries across primary care (EU-PC) and hospital (EU-H) settings. Interim IVE was higher against A(H1N1)pdm09 than A(H3N2): EU-PC influenza A(H1N1)pdm09 IVE was 53% (95% CI: 41 to 63) and 30% (95% CI: -3 to 54) against influenza A(H3N2). For EU-H, these were 44% (95% CI: 30 to 55) and 14% (95% CI: -32 to 43), respectively.

Background: Influenza A(H3N2) viruses dominated early in the 2022-2023 influenza season in Europe, followed by higher circulation of influenza A(H1N1)pdm09 and B viruses. The VEBIS primary care network estimated the influenza vaccine effectiveness (VE) using a multicentre test-negative study., Materials and Methods: Primary care practitioners collected information and specimens from patients consulting with acute respiratory infection. We measured VE against any influenza, influenza (sub)type and clade, by age group, by influenza vaccine target group and by time since vaccination, using logistic regression., Results: We included 38 058 patients, of which 3786 were influenza A(H3N2), 1548 influenza A(H1N1)pdm09 and 3275 influenza B cases. Against influenza A(H3N2), VE was 36% (95% CI: 25-45) among all ages and ranged between 30% and 52% by age group and target group. VE against influenza A(H3N2) clade 2b was 38% (95% CI: 25-49). Overall, VE against influenza A(H1N1)pdm09 was 46% (95% CI: 35-56) and ranged between 29% and 59% by age group and target group. VE against influenza A(H1N1)pdm09 clade 5a.2a was 56% (95% CI: 46-65) and 79% (95% CI: 64-88) against clade 5a.2a.1. VE against influenza B was 76% (95% CI: 70-81); overall, 84%, 72% and 71% were among 0-14-year-olds, 15-64-year-olds and those in the influenza vaccination target group, respectively. VE against influenza B with a position 197 mutation of the hemagglutinin (HA) gene was 79% (95% CI: 73-85) and 90% (95% CI: 85-94) without this mutation., Conclusion: The 2022-2023 end-of-season results from the VEBIS network at primary care level showed high VE among children and against influenza B, with lower VE against influenza A(H1N1)pdm09 and A(H3N2)., Competing Interests: None., (© 2024 The Authors. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)