2011-12 seasonal influenza vaccines effectiveness against confirmed A(H3N2) influenza hospitalisation: pooled analysis from a European network of hospitals. A pilot study

PLoS One. 2013;8(4):e59681. doi: 10.1371/journal.pone.0059681. Epub 2013 Apr 2.


Background: Influenza vaccination strategies aim at protecting high-risk population from severe outcomes. Estimating the effectiveness of seasonal vaccines against influenza related hospitalisation is important to guide these strategies. Large sample size is needed to have precise estimate of influenza vaccine effectiveness (IVE) against severe outcomes. We assessed the feasibility of measuring seasonal IVE against hospitalisation with laboratory confirmed influenza through a network of 21 hospitals in the European Union.

Methods: We conducted a multicentre study in France (seven hospitals), Italy (one hospital), and Navarra (four hospitals) and Valencia (nine hospitals) regions in Spain. All ≥18 years hospitalised patients presenting an influenza-like illness within seven days were swabbed. Cases were patients RT-PCR positive for influenza A (H3N2); controls were patients negative for any influenza virus. Using logistic regression with study site as a fixed effect we calculated IVE adjusted for potential confounders. We restricted the analyses to those swabbed within four days.

Results: We included, 375 A(H3N2) cases and 770 controls. The overall adjusted IVE was 24.9% (95%CI-1.8;44.6). Among the target group for vaccination (N = 1058) the adjusted IVE was 28.8% (95%CI:2.8;47.9); it was respectively 36.8% (95%CI:-48.8; 73.1), 42.6% (95%CI:-16.5;71.7), 17.8%(95%CI:-40.8; 52.1) and 37.5% (95%CI:-22.8;68.2) in the age groups 18-64, 65-74, 75-84 and more than 84 years.

Discussion: Estimation of IVE based on the pooling of data obtained through a European network of hospitals was feasible. Our results suggest a low IVE against hospitalised confirmed influenza in 2011-12. The low IVE may be explained by a poor immune response in the high-risk population, imperfect match between vaccine and circulating strain or waning immunity due to a late season. Increased sample size within this network would allow more precise estimates and stratification of the IVE by time since vaccination and vaccine types or brands.

Publication types

  • Historical Article
  • Multicenter Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Aged, 80 and over
  • Case-Control Studies
  • European Union
  • Female
  • History, 21st Century
  • Hospitalization / statistics & numerical data*
  • Humans
  • Influenza A Virus, H3N2 Subtype* / isolation & purification
  • Influenza Vaccines / immunology*
  • Influenza, Human / epidemiology
  • Influenza, Human / history
  • Influenza, Human / prevention & control*
  • Male
  • Middle Aged
  • Pilot Projects
  • Seasons
  • Young Adult


  • Influenza Vaccines

Grants and funding

The French study site was funded by Sanofi Pasteur and Inserm (Institut national de la santé et de la recherche médicale). The Italian study was partially supported by GlaxoSmithKline. The Valencia study site was funded by grants from Sanofi Pasteur and Sanofi Pasteur MSD. The study of Navarre was supported by the Navarra Health Service, by the European Centre for Disease Prevention and Control (I-MOVE programme), by the Carlos III Institute of Health (PS09/01179), and by the Spanish Ministry of Health (EC11-302). The pooled analysis was co-funded by the Sanofi Pasteur, Sanofi Pasteur MSD, GlaxoSmithKline and EpiConcept. Vaccine producers had no role in the pooled study design, data collection and analysis, decision to publish, or preparation of the manuscript.