Abstract
Within the Influenza Monitoring Vaccine Effectiveness in Europe (I-MOVE) project we conducted a multicentre case–control study in eight European Union (EU) Member States to estimate the 2011/12 influenza vaccine effectiveness against medically attended influenza-like illness (ILI) laboratory-confirmed as influenza A(H3) among the vaccination target groups. Practitioners systematically selected ILI / acute respiratory infection patients to swab within seven days of symptom onset. We restricted the study population to those meeting the EU ILI case definition and compared influenza A(H3) positive to influenza laboratory-negative patients. We used logistic regression with study site as fixed effect and calculated adjusted influenza vaccine effectiveness (IVE), controlling for potential confounders (age group, sex, month of symptom onset, chronic diseases and related hospitalisations, number of practitioner visits in the previous year). Adjusted IVE was 25% (95% confidence intervals (CI): -6 to 47) among all ages (n=1,014), 63% (95% CI: 26 to 82) in adults aged between 15 and 59 years and 15% (95% CI: -33 to 46) among those aged 60 years and above. Adjusted IVE was 38% (95%CI: -8 to 65) in the early influenza season (up to week 6 of 2012) and -1% (95% CI: -60 to 37) in the late phase. The results suggested a low adjusted IVE in 2011/12. The lower IVE in the late season could be due to virus changes through the season or waning immunity. Virological surveillance should be enhanced to quantify change over time and understand its relation with duration of immunological protection. Seasonal influenza vaccines should be improved to achieve acceptable levels of protection.
Highlights
Unlike the formulation of other vaccines, the formulation of seasonal influenza vaccines is reviewed annually by the World Health Organization (WHO) and frequently adapted to the constantly evolving nature of influenza viruses.How the vaccine performs in target group populations cannot be anticipated by pre-authorisation efficacy trials in healthy young adults, immunogenicity studies or the relatedness of vaccine and circulating viruses
In the eight participating countries, influenza peaked at different times – from week 5 in Italy and Poland to week 10 in Portugal (Figure 1)
After exclusion of one individual who had received antivirals prior to swabbing, 21 individuals for whom laboratory results were missing, 10 individuals who received vaccination prior to the begin of the country’s national vaccination campaign, 170 individuals who did not adhere to the European Union (EU) influenza-like illness (ILI) case definition, 19 individuals who were swabbed more than seven days after symptom onset and 163 individuals who presented before or after the week of onset of the first and last influenza case respectively, 4,362 individuals met the study inclusion criteria
Summary
Unlike the formulation of other vaccines, the formulation of seasonal influenza vaccines is reviewed annually by the World Health Organization (WHO) and frequently adapted to the constantly evolving nature of influenza viruses.How the vaccine performs in target group populations cannot be anticipated by pre-authorisation efficacy trials in healthy young adults, immunogenicity studies or the relatedness of vaccine and circulating viruses. Unlike the formulation of other vaccines, the formulation of seasonal influenza vaccines is reviewed annually by the World Health Organization (WHO) and frequently adapted to the constantly evolving nature of influenza viruses. Field influenza vaccine effectiveness (IVE) studies provide essential additional information to advise stakeholders on the performance of the vaccine, to contribute to vaccine strain selection process and to inform when additional measures, such as antivirals, are needed given a low observed effectiveness early in the season. In 2007, the European Centre for Disease Prevention and Control (ECDC) and a network of 18 public health institutes established the Influenza Monitoring Vaccine. 20 public health institutes from the EU and EEA are part of the I-MOVE network, which is coordinated by EpiConcept under the umbrella of ECDC [3]. One component of I-MOVE is a multicentre case–control study, which has provided IVE estimates each season since the pilot season in 2008/09 [4,5,6,7,8]
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