Background: In Spain, the influenza vaccine effectiveness (VE) was estimated in the last three seasons using an observational study (cycEVA) conducted in the frame of the Spanish Influenza Sentinel Surveillance System. We aimed to measure the effectiveness of the seasonal trivalent vaccine in preventing influenza like illness (ILI) laboratory-confirmed influenza infection at the end of the season 2010-11. Methods: We conducted a test-negative case-control study between weeks 50/2010 and 12/2011. Cases were ILI laboratory-confirmed influenza infection and controls were those testing negative. Sentinel physicians collected data on demographic and clinical characteristics, vaccination status, and on covariates related to confounding factors associating with influenza VE. We calculated adjusted odds ratios (OR), using logistic regression and computed influenza VE as (1 - OR) × 100. Results: The adjusted influenza VE against A(H1N1)pdm09 infection was 46% (95% confidence interval (95%CI): 0; 72). In A(H1N1)pdm09 infected patients who had received both 2010-11 trivalent influenza seasonal and 2009 monovalent pandemic vaccines, influenza VE was 74% (95%CI: 13; 93). The adjusted influenza VE against B infection was 23% (95%CI: -180; 79). Conclusion: The trivalent influenza vaccine 2010-11 showed a moderate VE for preventing ILI laboratory confirmed influenza infections. Influenza VE estimates were higher in patients who had received both 2010-11 seasonal trivalent and 2009 monovalent pandemic vaccines.
Bibliographical noteFunding Information:
We are grateful to all sentinel general practitioners, paediatrician and virologist participating in the cycEVA study, as well as to all professionals participating in the Spanish Sentinel Influenza Surveillance System. We also thank Esther Kissling (EpiConcept), for her fruitful discussions and comments on the cycEVA study analyses and Marta Valenciano (EpiConcept), for her helpful ideas and suggestions of the drafting. This work was supported by the ECDC through the I-MOVE (Influenza Monitoring Vaccine Effectiveness in Europe) project and by the Carlos III Institute of Health (Influenza A(H1N1)pdm09 Programme ( GR09/0017 ).Competing interest: The authors declare no competing interests.
- Case-control studies
- Influenza vaccine
- Sentinel system
- Vaccine effectiveness