Elsevier

Vaccine

Volume 28, Issue 16, 1 April 2010, Pages 2903-2907
Vaccine

Estimating the influenza vaccine effectiveness in elderly on a yearly basis using the Spanish influenza surveillance network—Pilot case–control studies using different control groups, 2008–2009 season, Spain

https://doi.org/10.1016/j.vaccine.2010.01.054Get rights and content

Abstract

We conducted a case–control and screening method studies to estimate influenza vaccine effectiveness (IVE) in the age group ≥65 years, based on the Spanish Influenza Sentinel Surveillance System (SISSS). Cases (influenza laboratory-confirmed) were compared to influenza-negative ILI patients (test-negative) and patients without ILI since the beginning of the season (non-ILI). For the screening method, cases’ vaccination coverage was compared to the vaccination coverage of the GPs’ catchment population. The results suggested a protective effect of the vaccine against laboratory-confirmed influenza in elderly in 2008–2009. The screening method and the test-negative control designs enable estimating IVE using exclusively SISSS data.

Introduction

Annual vaccination is the main intervention to prevent seasonal influenza related outcomes in elderly [1], [2]. All European Union (EU) and European Economic Area (EEA) member states recommend annual influenza vaccination in high-risk groups [3], [4]. Most estimates of the influenza vaccine effectiveness (IVE) in these countries are based on data provided by the national influenza surveillance systems [5]. It is the case of Spain, whose influenza surveillance system (Spanish Influenza Sentinel Surveillance System – SISSS) was established along with other European networks in 1996 [11], [12]. The objectives of the SISSS are to provide timely clinical and virological information on influenza activity and to identify circulating strains. In the season 2008–2009, SISSS included over 700 general practitioners (GPs) and paediatricians from 16 out of 18 Spanish regions, covering 2.08% of the Spanish population. GPs report cases of influenza like illness (ILI) on a weekly basis and take swabs for the virological surveillance. Since 2002–2003, IVE was estimated in the frame of SISSS using the screening method: the proportion of ILI cases vaccinated was compared to the vaccination coverage of the Spanish population. The source of vaccination coverage in the ≥65 years was the Ministry of Health estimates, available shortly after the end of the influenza vaccination campaign.

Published studies using different designs and outcomes provided a wide range of IVE estimates in elderly [5], [6], [7]. There are only few published studies that used laboratory-confirmed influenza as outcome in Europe [8], [9], [10]. Since most of the IVE studies are observational, they are prone to bias. Confounding factors such as underline illness severity or functional status can distort the estimates and different methods to adjust for these confounding factors have been proposed [13], [14].

To develop a system to monitor seasonal and pandemic IVE in the EU/EEA, the European Centre for Diseases Prevention and Control (ECDC) funded a project (I-MOVE) that piloted different study designs during the 2008–2009 season [5]. In the framework of I-MOVE, five case–control [15] and two cohort studies were conducted in six EU countries. Spain participated in I-MOVE project with a case–control study using two different control groups. We also estimated the IVE using the screening method. Both designs (case–control and screening method) were carried out in the frame of SISSS. We aimed at measuring IVE against laboratory-confirmed influenza in elderly ≥65 years and identifying a study design feasible and sustainable to monitor IVE in Spain.

Section snippets

Methods

We conducted a case–control study among elderly ≥65 years old. Seven out 16 Spanish regional networks included in the SISSS participated in the study. We invited to participate in the study all 219 sentinel GPs of these seven networks. The population covered by the participating GPs represented 1.74% of the elderly population in the seven Spanish regions.

Participating GPs swabbed all patients in the age group ≥65 years who were attended for influenza like illness from week 40/2008 to week

Results

In Spain, the influenza activity was moderate in the season 2008–2009. The predominant influenza circulating virus was A(H3N2). Influenza activity baseline levels were overpassed in week 50/2008 reaching the peak in week 01/2009 (218 ILI cases/100,000 inhabitants). The epidemic period lasted for nine weeks, from week 50/2008 to week 07/2009. The highest incidence rate was registered in under 15 years old, so that the cumulative incidence was 3365/100,000 in the age group under 5 years old and

Discussion

In the 2008–2009 season we estimated the IVE in Spain in the frame of the SISSS, using case–control and screening designs. The estimated crude IVEs in the case–control studies with the two control groups were similar. These estimates suggest a good protective effect of the 2008–2009 influenza vaccine against influenza confirmed medically attended ILI in elderly. The point IVE estimate using the screening method was lower than the case–control study estimates even though the confidence intervals

Acknowledgments

We are thankful to all participating sentinel GPs in the cycEVA Study. We also thank Esther Kissling (EpiConcept), Bruno Ciancio (ECDC), Johan Giesecke (ECDC) for their support during the study. We are especially grateful to Alain Moren (EpiConcept) for his guidance and support during the study. This study was partly funded by European Centre for Disease Prevention and Control.

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